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EE2403 Special Electrical Machines
Department of EEE
2012-2013
UNIT – I SYNCHRONOUS RELUCTANCE MOTORS
PART - A
1. What are the different types of reluctance machines? Compare them.
The synchronous reluctance motor is used in industry, where motors are required to operate in
synchronism or where highly regulated speed control is required using a variable-frequency drive. The motors
provide constant, unvarying (or highly controlled) speed from no load to full load with no slip.
The switched reluctance motor (SRM) is excited by current pulses applied to each phase. The current
pulses are applied based on precise rotor position. The inductance profile is triangular, with maximum
inductance when it is in an aligned position. When voltage is applied to the stator phase, the motor creates
torque in the direction of increasing inductance. Both synchronous and SRMs require a position sensor unless
new, sensor less technology is employed. Position sensors are a source of increased cost and reduced reliability.
2. What is the principle of operation of reluctance machine?
In reluctance machines, torque is produced by the tendency of the rotor to move to a position where
the inductance of the excited stator winding is maximized (i.e., rotor tooth aligns with active stator phase to
minimize reluctance). The rotor is typically constructed of soft magnetic iron shaped so as to maximize the
variation of inductance with rotor position. Opposite poles form a phase and the phases are magnetically
independent of one another. The machines tend to be noisy; a characteristic that has limited their applications in
the past and has also limited their use currently in vehicles. Research has been on-going for years in an attempt
to address the noise issue, but little has been accomplished in actual noise mitigation. Reluctance machines are
relatively low-cost machines, and they generally do not contain PMs.
3. What are the properties of Reluctance motor?
Combined reluctance and magnet alignment torque, Field weakening capability, under excited
operation for most loaded condition, High inductance, High speed capability, High temperature capability
4. What are the various stator current modes used in synchronous reluctance motor?
Unipolar current modes, bipolar current modes
5. Mention the applications of distributed anisotropy cage rotor of synchronous reluctance motor?
These rotors are used for line – start (constant voltage and frequency) applications.
6. Mention the applications of distributed anisotropy Cageless rotor of synchronous reluctance motor?
These rotors are used for variable speed applications.
7. What is Synchronous reluctance motor?
Synchronous reluctance motor is similar to three – phase Synchronous motor except the rotor are
demagnetized and made with saliency to increase the reluctance power. It is a motor which develops torque due
to the difference in reluctance of the two axes, namely quadrature and direct axis.
8. What is meant by reluctance torque?
The torque which is exhibited on the rotor due to the difference in Reluctance in the air gap (or) a
function of angular position of rotor with respect to the stator coil is known as reluctance torque.
9. What are the advantages of Synchronous reluctance motor?
Rotor is simple in construction i.e. very low inertia, Robust, Low torque, ripple, Can be operated from
standard PWM AC Inverters, It can be also built with a standard induction motor, stator and windings.
10. What are the disadvantages of Synchronous reluctance motor?
It has poor power factor performance and therefore the efficiency is not as high as permanent magnet motor,
The converter kVA requirement is high, The pull – in and pull – out torque of the motor are weak.
11. What are the characteristics of Synchronous reluctance motor?
It has combined reluctance and magnetic alignment torque, It has field weakening capability ( to get higher
synchronous speed), Under excited operation for most load conditions, High inductance, High speed
capability, High temperature capability
12. State the applications of Synchronous reluctance motor?
It is used for constant speed applications i.e. timing devices, signaling devices, recording instruments and
phonograph, it is used in automatic processors such as in food processing and packaging industries.
Used in high speed applications, Synthetic fiber manufacturing equipment, Wrapping and folding machines,
synchronized conveyors.
13. What are the types of Synchronous reluctance motor?
Synchronous reluctance motor is classified into three types depending upon the construction of rotor.
They are Salient type or Radial type rotor, Flat type or axial type rotor, Flux Barrier type or Laminated type
rotor.
14. Write the torque equation of Synchronous reluctance motor?
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EE2403 Special Electrical Machines
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T = (U2/ 2ωs) (1/Xq - 1/Xd) sin 2δ, U = Supply Voltage, Is be the supply current which has two
components Id and Iq, Id = Direct axis current, Iq = Quadrature axis current,  s = Synchronous speed in rad /
sec, Xd = Direct axis reactance, Xq = Quadrature axis reactance
15. Skewing is required for Synchronous reluctance motor. Justify?
At the time of starting, reluctance motor are subjected to logging due to the saliency of motor. This can
be minimized by the skewing of the rotor parts.
16. What are the design considerations in Synchronous reluctance motor?
Power factor, Copper loss and core loss, Cost, Efficiency
17. What are the advantages of increasing Ld / Lq ratio in Synchronous reluctance motor?
Motor power factor increases, I2R losses reduced, reduced volt – ampere ratings of the inverter driving the
machine.
18. Compare Synchronous reluctance motor and Induction motor.
S.No.
Synchronous reluctance motor
Induction motor
1.
Better efficiency
Efficiency is low
2.
High cost
Low cost
3.
Low power factor
High power factor
4.
Used for low and medium power application
Used for high power application
19. Define : Magnetic flux
The amount of magnetic lines of force setup in a magnetic circuit is called magnetic flux. It is
analogous to electric current in electric circuit.
20. Define : Reluctance
The opposition offered to the magnetic flux by a magnetic circuit is called its reluctance.
21. Define : Permeance
It is a measure of the ease with which flux can be setup in a material. It is the reciprocal of the
reluctance of the material.
22. List out any four features of synchronous reluctance motors.
Better efficiency, high cost, low power factor, used for low and medium power application
23. What are the applications of linear induction motors?
Used for high power application, efficiency is low compared with synchronous reluctance.
PART B
1. Explain the principle of operation and constructional features of Synchronous reluctance motor.
2. Explain the torque – speed and torque – angle characteristics of Synchronous reluctance motor.
3. Draw and explain the steady state phasor diagram of Synchronous reluctance motor.
4. List the advantages and disadvantages of a Synchronous reluctance motor.
5. Derive the expression for torque equation of a Synchronous reluctance motor.
6. Explain the various stator current modes in a Synchronous reluctance motor.
7. Explain the various types of Synchronous reluctance motor based on rotor construction with neat sketch.
8. Explain the Cage and Cageless rotors of Synchronous reluctance motor with neat sketch.
9. Compare a reluctance motor with an equivalent induction motor and list out the merits and demerits of
reluctance motor over induction motor.
10. A 10 HP, 4 poles, 240V, 60Hz, reluctance motor operating under rated load condition has a torque angle of
30°. Determine (a) Load torque on shaft (b) Torque angle if the voltage drops to 224V (c) For the above
torque angle, will the rotor pullout of synchronism.
11. With neat sketches, describe the constructional features and principle of operation of different types of
synchronous reluctance motor.
12. (i) Draw and explain the steady state pharos diagram of a Synchronous reluctance motor.
(ii) Explain the following characteristics of synchronous Reluctance motor. Torque – angle characteristics,
Torque – speed characteristics.
UNIT – II STEPPING MOTORS
PART - A
1. Define: Stepper motor?
Stepper motor is a motor which rotates step by step and not continuous rotation. When the stator is
excited using a DC supply the rotor poles align with the stator poles in opposition such that reluctance is less.
2. What are the advantages of Stepper motor?
No feedback is normally required for either position control or speed control, Positional control is non –
cumulative, Stepping motor are compatible with modern digital equipment
3. Mention the different types of stepper motor?
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EE2403 Special Electrical Machines
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Variable Reluctance stepper motor (Single stack, Multi stack), Permanent magnet stepper motor, Hybrid
stepper motor, Outer rotor stepper motor
4. What are the different modes of excitation?
Single phase excitation, two phase excitation, Half step mode, Mini-step drive
5. Mention the features of stepper motor?
Small step angle, High positioning accuracy, High torque inertia ratio, Stepping rate, Pulse frequency
6. Define: Step Angle of stepper motor?
A stepping motor rotates through a fixed angle for every pulse. The rated value of this angle is called
the step angle and expressed in degrees.
7. Define: Holding torque of stepper motor?
Holding torque is defined as the maximum static torque that can be applied to the shaft of an excited
motor without causing continuous rotation.
8. Define: Resolution of stepper motor?
It is defined as the accuracy of positioning of the rotor pole at a particular step angle with respect to
stator pole.
9. Define: Detent torque of stepper motor?
It is defined as the maximum static torque that can be applied to the shaft of an unexcited motor without
causing continuous rotation.
10. Define: Pull – in torque of stepper motor?
These are alternatively called the starting characteristics and refer to the range of frictional load torque
at which the motor can start and stop without loosing steps for various frequencies in a pulse strain.
11. Define: Pull – out torque of stepper motor?
These are alternatively called the slewing characteristics. After the test, motor is started by a specified
driver in the specified excitation mode in the self starting range; the pulse frequency is gradually increased; the
motor will eventually run out of synchronism. The relation between the frictional load torque and the minimum
pulse frequency with which the motor can synchronize is called pull – out characteristics.
12. Define: Slewing frequency of stepper motor?
This is defined as the maximum frequency (stepping rate) at which the loaded motor can run without
losing steps is alternatively called the maximum slewing frequency.
13. Define: Stepping frequency of stepper motor?
The speed of rotation of a stepping motor is given in terms of the number of steps per second and the
term stepping rate is often used to indicate speed.
14. Define: Maximum starting torque of stepper motor?
This is alternatively called as maximum pull – in torque and is defined as the maximum frictional load
with which the motor can start and synchronize with the pulse train of frequency as low as 10 Hz.
15. Define: Maximum starting frequency of stepper motor?
It is the maximum control frequency at which the unloaded motor can start and stop without losing
steps.
16. Define: Maximum starting frequency of stepper motor?
It is the maximum control frequency at which the unloaded motor can start and stop without losing
steps.
17. What is rest position or equilibrium position of stepper motor?
It is defined as the position at which as excited motor comes to rest at no load.
18. What is detent position of stepper motor?
It is defined as the position at which a motor having a permanent magnet in its rotor or stator comes to
rest without excitation at no load.
19. Define: Maxwell’s stress
It is defined as curving of magnetic lines of force at the end of the poles of the stator.
20. Why interleaving is done in a stepper motor?
Interleaving is done in the stepper motor to decrease the step angle and thus increasing the resolution.
21. Explain: VR type stepper motor?
It is a basic type of stepping motor in which the motor step by step rotation is achieved when the rotor
teeth and stator teeth are in alignment such that the magnetic reluctance is minimized and this state provides a
rest or equilibrium position.
22. Explain: PM type stepper motor?
A stepping motor using permanent magnet in the rotor for step movement is called a permanent magnet
motor.
23. Explain: Hybrid type stepper motor?
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EE2403 Special Electrical Machines
Department of EEE
2012-2013
A hybrid motor has permanent magnet in its rotor. The term hybrid derives from the fact that the motor
is operated under the combined principles of permanent magnet and variable reluctance motor.
24. What is micro stepping and how is it achieved?
It is possible to subdivide on natural step into many small steps by means of electronics. This method is
known as mini step or micro step.
25. What is the effect of magnetic saturation in stepper motor?
The efficiency is improved, and the losses are reduced.
26. What are the classifications of drive system of stepping motor?
Open loop system, closed loop system
27. What is the relationship between the step number and the step angle in a stepper motor?
Step number (S) = 360° / θs
θs = step angle
28. What is the relationship between the step number and the rotational speed in a stepper motor?
Rotational speed in rpm (n) = 60f / S
Where, S = Step number, f = stepping rate in Hz.
29. What is Cyclonome?
A stepping motor manufactured by Sigma instruments Inc., are called Cyclonome.
30. What are the various bipolar drives used for stepper motor?
Basic Bipolar drives, Bipolar L/R drives, bipolar chopper drives
31. What are the different modes of excitation used in variable reluctance stepping motors?
1 phase on or full step operation, 2 phase on mode, alternate 1 phase on and 2 phase on mode half step
operation, micro stepping operation.
32. Mention the limitations of open loop operation of stepper motors.
The open loop drive is attractive and widely accepted in applications of speed and position controls.
However the performance of stepper motor driven in open loop mode may fail to follow a pulse command when
the frequency of the pulse train is too high or the inertial load is too heavy.
PART B
1. Explain the construction and principle of operation of Variable Reluctance Stepping motor?
The variable reluctance motor does not use a permanent magnet. As a result, the motor rotor
can move without constraint or "detent" torque. This type of construction is good in non industrial
applications that do not require a high degree of motor torque, such as the positioning of a micro slide.
The variable reluctance motor in the above illustration has four "stator pole sets" (A, B, C) set
15 degrees apart. Current applied to pole A through the motor winding causes a magnetic attraction
that aligns the rotor (tooth) to pole A. Energizing stator pole B causes the rotor to rotate 15 degrees in
alignment with pole B. This process will continue with pole C and back to A in a clockwise direction.
Reversing the procedure (C to A) would result in a counterclockwise rotation.
If your motor has three windings, typically connected as shown in the schematic diagram in
Figure , with one terminal common to all windings, it is most likely a variable reluctance stepping
motor. In use, the common wire typically goes to the positive supply and the windings are energized in
sequence.
The cross section shown in Figure is of 30 degree per step variable reluctance motor. The rotor
in this motor has 4 teeth and the stator has 6 poles, with each winding wrapped around two opposite
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EE2403 Special Electrical Machines
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poles. With winding number 1 energized, the rotor teeth marked X are attracted to this winding's
poles. If the current through winding 1 is turned off and winding 2 is turned on, the rotor will rotate 30
degrees clockwise so that the poles marked Y line up with the poles marked 2.
To rotate this motor continuously, we just apply power to the 3 windings in sequence.
Assuming positive logic, where a 1 means turning on the current through a motor winding, the
following control sequence will spin the motor illustrated in Figure 2.6 clockwise 24 steps or 2
revolutions:
Winding 1 1001001001001001001001001
Winding 2 0100100100100100100100100
Winding 3 0010010010010010010010010
time --->
There are also variable reluctance stepping motors with 4 and 5 windings, requiring 5 or 6
wires. The principle for driving these motors is same as that for the three winding motors, but it
becomes important to work out the correct order to energize the windings to make the motor step
nicely.
2. Explain in detail the various applications of stepper motor?
3. Discuss the various advantages and disadvantages of stepper motor?
Stepper Motor Advantages and Disadvantages
Advantages
Stepper motors have the following advantages:











5
Low cost
Ruggedness
Simplicity in construction
High reliability since there is no contact brushes in the motor. Therefore the life of the motor
is simply dependant on the life of the bearing.
No maintenance
Wide acceptance
No feedback components are needed
They work in just about any environment
Inherently more failsafe than servo motors.
Excellent response to starting/ stopping/reversing.
It is possible to achieve very low speed synchronous rotation with a load that is directly
coupled to the shaft.
EE2403 Special Electrical Machines
Department of EEE
2012-2013
 There is virtually no conceivable failure within the stepper drive module that could cause the
motor to run away. Stepper motors are simple to drive and control in an open-loop
configuration. They only require four leads. They provide excellent torque at
low speeds, up to 5 times the continuous torque of a brush motor of the same frame size or
double the torque of the equivalent brushless motor. This often eliminates the need for a
gearbox. A stepper-driven system is inherently stiff, with known limits to the dynamic
position error.
Disadvantages
Stepper motors have the following disadvantages:






Resonance effects and relatively long settling times
Rough performance at low speed unless a microstep drive is used
Liability to undetected position loss as a result of operating open-loop
They consume current regardless of load conditions and therefore tend to run hot
Losses at speed are relatively high and can cause excessive heating, and they are frequently
noisy (especially at high speeds).
They can exhibit lag-lead oscillation, which is difficult to damp. There is a limit to their
available size, and positioning accuracy relies on the mechanics (e.g., ball screw accuracy).
Many of these drawbacks can be overcome by the use of a closed-loop control scheme.
4. Explain the construction and operation of permanent magnet Stepping motor?
The permanent magnet motor, also referred to as a "canstack" motor, has, as the name implies,
a permanent magnet rotor. It is a relatively low speed, low torque device with large step angles of
either 45 or 90 degrees. It's simple construction and low cost make it an ideal choice for non industrial
applications, such as a line printer print wheel positioner. The permanent-magnet stepper motor
operates on the reaction between a permanent-magnet rotor and an electromagnetic field. Figure 2.7
shows a basic two-pole PM stepper motor. The rotor shown in Figure has a permanent magnet
mounted at each end. The stator is illustrated in Figure . Both the stator and rotor are shown as having
teeth. The teeth on the rotor surface and the stator pole faces are offset so that there will be only a
limited number of rotor teeth aligning themselves with an energized stator pole. The number of teeth
on the rotor and stator determine the step angle that will occur each time the polarity of the winding is
reversed. The greater is the number of teeth, the smaller is the step angle.
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5. Explain the construction and operation of hybrid Stepping motor?
Hybrid motors combine the best characteristics of the variable reluctance and permanent
magnet motors. They are constructed with multi-toothed stator poles and a permanent magnet rotor.
Standard hybrid motors have 200 rotor teeth and rotate at 1.80 step angles. Other hybrid motors are
available in 0.9ºand 3.6º step angle configurations. Because they exhibit high static and dynamic
torque and run at very high step rates, hybrid motors are used in a wide variety of industrial
applications.
6. Discuss the static characteristics of stepper motor with neat sketch?
Torque versus Speed
An important consideration in designing high-speed stepping motor controllers is the effect of
the inductance of the motor windings. As with the torque versus angular position information, this is
frequently poorly documented in motor data sheets, and indeed, for variable reluctance stepping
motors, it is not a constant! The inductance of the motor winding determines the rise and fall time of
the current through the windings. While we might hope for a square-wave plot of current versus time,
the inductance forces an exponential, as in Figure
The details of the current-versus-time function through each winding depend as much on the
drive circuitry as they do on the motor itself! It is quite common for the time constants of these
exponentials to differ. The rise time is determined by the drive voltage and drive circuitry, while the
fall time depends on the circuitry used to dissipate the stored energy in the motor winding.
At low stepping rates, the rise and fall times of the current through the motor windings has
little effect on the motor's performance, but at higher speeds, the effect of the inductance of the motor
windings is to reduce the available torque, as shown in Figure .
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Torque
Normal
mode
Slewing
mode
Speed
•
•
•
•
An ideal torque-speed characteristic for a stepper motor
2 distinct modes of operation:
– Locked-step (normal) mode
– Slewing mode
In the first, the rotor comes to rest between steps (mode commonly used to achieve a given
rotor position); rotor can be started, stopped, reversed
Slewing mode does not allow stopping or reversal of the rotor, although it advances in
synchronism with the stepping sequence (e.g. rewinding a tape drive)
Torque
Curve A: Pull-out torque
Curve B: Pull-in torque
Curve B
Curve A
Normal
mode
Slewing
mode
Pull-in rate
Max pull-in rate
8
Pull-out rate
Speed
Max pull-out rate
EE2403 Special Electrical Machines
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The motor's maximum speed is defined as the speed at which the available torque falls to zero.
Measuring maximum speed can be difficult when there are resonance problems, because these cause
the torque to drop to zero prematurely. The cutoff speed is the speed above which the torque begins to
fall. When the motor is operating below its cutoff speed, the rise and fall times of the current through
the motor windings occupy an insignificant fraction of each step, while at the cutoff speed, the step
duration is comparable to the sum of the rise and fall times. Note that a sharp cutoff is rare, and
therefore, statements of a motor's cutoff speed are, of necessity, approximate.
The details of the torque versus speed relationship depend on the details of the rise and fall
times in the motor windings, and these depend on the motor control system as well as the motor.
Therefore, the cutoff speed and maximum speed for any particular motor depend, in part, on the
control system! The torque versus speed curves published in motor data sheets occasionally come with
documentation of the motor controller used to obtain that curve, but this is far from universal practice!
Similarly, the resonant speed depends on the moment of inertia of the entire rotating system,
not just the motor rotor, and the extent to which the torque drops at resonance depends on the presence
of mechanical damping and on the nature of the control system. Some published torque versus speed
curves show very clear resonances without documenting the moment of inertia of the hardware that
may have been attached to the motor shaft in order to make torque measurements.
The torque versus speed curve shown in Figure typical of the simplest of control systems.
More complex control systems sometimes introduce electronic resonances that act to increase the
available torque above the motor's low-speed torque. A common result of this is a peak in the available
torque near the cutoff speed.
Characteristic Parameters
 Holding torque: the maximum torque which can be applied to an energized stationary motor
without causing spindle rotation
 Pull-out torque: the maximum torque which can be applied to a motor, running at a given
stepping rate, without losing synchronism
 Pull-in torque: the maximum torque against which a motor will start, at a given pulse rate, and
reach synchronism without losing a step.
 Pull-out rate: the maximum switching rate at which a motor will remain in synchronism while the
switching rate is gradually increased.
 Pull-in rate: the maximum switching rate at which a loaded motor can start without losing steps
 Slew range: the range of switching rates between pull-in and pull-out in which a motor will run in
synchronism but cannot start or reverse.
7. Discuss the dynamic characteristics of stepper motor with neat sketch?
At low stepping rates, the rise and fall times of the current through the motor windings has
little effect on the motor's performance, but at higher speeds, the effect of the inductance of the motor
windings is to reduce the available torque, as shown in Figure
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Torque
Normal
mode
Slewing
mode
Speed
•
•
•
•
An ideal torque-speed characteristic for a stepper motor
2 distinct modes of operation:
– Locked-step (normal) mode
– Slewing mode
In the first, the rotor comes to rest between steps (mode commonly used to achieve a given
rotor position); rotor can be started, stopped, reversed
Slewing mode does not allow stopping or reversal of the rotor, although it advances in
synchronism with the stepping sequence (e.g. rewinding a tape drive)
Torque
Curve A: Pull-out torque
Curve B: Pull-in torque
Curve B
Curve A
Normal
mode
Slewing
mode
Pull-in rate
Max pull-in rate
10
Pull-out rate
Speed
Max pull-out rate
EE2403 Special Electrical Machines
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2012-2013
The motor's maximum speed is defined as the speed at which the available torque falls to zero.
Measuring maximum speed can be difficult when there are resonance problems, because these cause
the torque to drop to zero prematurely. The cutoff speed is the speed above which the torque begins to
fall. When the motor is operating below its cutoff speed, the rise and fall times of the current through
the motor windings occupy an insignificant fraction of each step, while at the cutoff speed, the step
duration is comparable to the sum of the rise and fall times. Note that a sharp cutoff is rare, and
therefore, statements of a motor's cutoff speed are, of necessity, approximate.
The details of the torque versus speed relationship depend on the details of the rise and fall
times in the motor windings, and these depend on the motor control system as well as the motor.
Therefore, the cutoff speed and maximum speed for any particular motor depend, in part, on the
control system! The torque versus speed curves published in motor data sheets occasionally come with
documentation of the motor controller used to obtain that curve, but this is far from universal practice!
Similarly, the resonant speed depends on the moment of inertia of the entire rotating system,
not just the motor rotor, and the extent to which the torque drops at resonance depends on the presence
of mechanical damping and on the nature of the control system. Some published torque versus speed
curves show very clear resonances without documenting the moment of inertia of the hardware that
may have been attached to the motor shaft in order to make torque measurements.
The torque versus speed curve shown in Figure 2.17 is typical of the simplest of control
systems. More complex control systems sometimes introduce electronic resonances that act to increase
the available torque above the motor's low-speed torque. A common result of this is a peak in the
available torque near the cutoff speed.
Characteristic Parameters
 Holding torque: the maximum torque which can be applied to an energized stationary motor
without causing spindle rotation
 Pull-out torque: the maximum torque which can be applied to a motor, running at a given
stepping rate, without losing synchronism
 Pull-in torque: the maximum torque against which a motor will start, at a given pulse rate, and
reach synchronism without losing a step.
 Pull-out rate: the maximum switching rate at which a motor will remain in synchronism while the
switching rate is gradually increased.
 Pull-in rate: the maximum switching rate at which a loaded motor can start without losing steps
 Slew range: the range of switching rates between pull-in and pull-out in which a motor will run in
synchronism but cannot start or reverse.
8. Explain in detail the linear and non – linear analysis of stepper motor?
The torque produced by a specific rotary stepper motor is a function of:
• The step rate
• The current through the windings
• The type of drive used
(The force generated by a linear motor is also dependent upon these factors.)
Torque is the sum of the friction torque (Tf) and inertial torque (Ti).
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The frictional torque (ounce-inches or gram-cm) is the force (F), in ounces or grams, required
to move a load multiplied by the length, in inches or cm, of the lever arm used to drive the load (r) as
shown in figure 2.14
Figure2.14
The inertial torque (Ti) is the torque required to accelerate the load (gram-cm2).
Where,
I = the inertial load in g-cm2
= step rate in steps/second
t = time in seconds
= the step angle in degrees
K = a constant
It should be noted that as the step rate of a motor is increased, the back electro-motive force
(EMF) (i.e. the generated voltage) of the motor also increases. This restricts current flow and results in
a decrease in useable output torque.
Using basic principles of electromechanical energy conversion, a simplified analysis of one
stack of the stepper motor is presented here. It is assumed that the magnetic circuit is linear
(unsaturated). Even then the resulting model is highly nonlinear so that no generalized conclusions can
be drawn. Let
e(t) = voltage applied per stack
R = winding resistance per stack
L(θ) = winding inductance per stack (a function of rotor position only and
independent of coil current because of linear magnetic circuit assumption)
i(t) = current per stack
θ(t) = angular position of rotor
Kirchoff’s mesh equation for stator winding is
e(t )  Ri (t ) 
where λ = flux linkages of stator winding = iL(θ).
Therefore,
 Ri (t )  L( )
d
dt
di
dL( ) d
i
…………....(1)
dt
d dt
Transformer
emf
Energy stored in air gap is
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speed
emf
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2
W = ½ L(θ) i (t)……………………………..(2)
Mechanical torque developed is given by

T
W (i, )

1
dL( )
 i 2 (t )
…………………………....(3)
2
d
Rotor dynamics is governed by
d 2
d
……………………………….(4)
TJ 2  f
dt
dt
In a toothed structure, reluctance and therefore winding inductance varies continuously (even
function) as function of θ over and above an average value, i.e.,
L( )  L1  L2 cos T ……………………………….(5)
Substituting in equation 3,
1
T   L2Ti 2 (t ) sin T
2
2
  Ki (t )sin T ……………………………….(6)
This indeed is the reluctance torque and has sinusoidal form compared to the torque-angle
curve. Notice the similarity of the overall shape of the curve to a sinusoidal wave.
Equations 1, 6 and 4 govern the dynamic behavior of one stack of a stepper motor under
application of e(t), a pulse wave shape. Assuming that mechanical and electrical transients are over in
the time intervening between pulses, single-stack analysis would suffice. Equations being highly
nonlinear, approximate linear model is not feasible. Solution must be obtained on an analog or digital
computer.
Intervening time between the applications of two pulses must be long enough to allow for the motor
rotor to lock in position otherwise the motor may skip a step which is undesirable. Step skipping may
also occur if rotor oscillation amplitude about the locking position is too large.
9. Explain in detail the drive system of a stepping motor?
In the above circuits, the details of the necessary switches have been deliberately ignored. Any
switching technology, from toggle switches to power MOSFETS will work! Figure contains some
suggestions for implementing each switch, with a motor winding and protection diode included for
orientation purposes:
Each of the switches shown in Figure is compatible with a TTL input. The 5 volt supply used
for the logic, including the 7407 open-collector driver used in the figure, should be well regulated. The
motor power, typically between 5 and 24 volts, needs only minimal regulation. It is worth noting that
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these power switching circuits are appropriate for driving solenoids, DC motors and other inductive
loads as well as for driving stepping motors.
`
The SK3180 transistor shown in Figure is a power darlington with a current gain over 1000;
thus, the 10 milliamps flowing through the 470 ohm bias resistor is more than enough to allow the
transistor to switch a few amps current through the motor winding. The 7407 buffer used to drive the
Darlington may be replaced with any high-voltage open collector chip that can sink at least 10
milliamps. In the event that the transistor fails, the high-voltage open collector driver serves to protect
the rest of the logic circuitry from the motor power supply.
The IRC IRL540 shown in Figure is a power field effect transistor. This can handle currents of
up to about 20 amps, and it breaks down nondestructively at 100 volts; as a result, this chip can absorb
inductive spikes without protection diodes if it is attached to a large enough heat sink. This transistor
has a very fast switching time, so the protection diodes must be comparably fast or bypassed by small
capacitors. This is particularly essential with the diodes used to protect the transistor against reverse
bias! In the event that the transistor fails, the zener diode and 100 ohm resistor protect the TTL
circuitry. The 100 ohm resistor also acts to somewhat slow the switching times on the transistor.
For applications where each motor winding draws under 500 milliamps, the ULN200x family
of darlington arrays from Allegro Microsystems, also available as the DS200x from National
Semiconductor and as the Motorola MC1413 darlington array will drive multiple motor windings or
other inductive loads directly from logic inputs. Figure 3.8 shows the pin out of the widely available
ULN2003 chip, an array of 7 darlington transistors with TTL compatible inputs:
The base resistor on each darlington transistor is matched to standard bipolar TTL outputs.
Each NPN darlington is wired with its emitter connected to pin 8, intended as a ground pin, Each
transistor in this package is protected by two diodes, one shorting the emitter to the collector,
protecting against reverse voltages across the transistor, and one connecting the collector to pin 9; if
pin 9 is wired to the positive motor supply, this diode will protect the transistor against inductive
spikes.
The ULN2803 chip is essentially the same as the ULN2003 chip described above, except that it
is in an 18-pin package, and contains 8 darlingtons, allowing one chip to be used to drive a pair of
common unipolar permanent-magnet or variable-reluctance motors.
For motors drawing under 600 milliamps per winding, the UDN2547B quad power driver
made by Allegro Microsystems will handle all 4 windings of common unipolar stepping motors. For
motors drawing under 300 milliamps per winding, Texas Instruments SN7541, 7542 and 7543 dual
power drivers are a good choice; both of these alternatives include some logic with the power drivers.
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10. Explain in detail the multi stack Variable Reluctance Stepping motor?
• The VR stepper motors mentioned up to this point are all single-stack motors. That is, all the
phases are arranged in a single stack, or plane.
• The disadvantage of this design for a stepper motor is that the steps are generally quite large
(above 15°).
• Multi-stack stepper motors can produce smaller step sizes because the motor is divided along its
axial length into magnetically isolated sections, or stacks.
• Each of these sections is excited by a separate winding, or phase.
• In this type of motor, each stack corresponds to a phase, and the stator and rotor have the same
tooth pitch.
11. What is the motor torque Tm required to accelerate an initial load of 3x10-4 kg – m2 from f1 = 1000Hz to f2
= 2000Hz during 100msec. The frictional torque Tf is 0.05 Nm and the step angle is 1.8°.
1
T   L2Ti 2 (t ) sin T
2
12. What is the maximum acceleration of an initial load of 4x10-4 kg – m2 driven by a motor torque of 0.3 Nm.
Frictional loads are negligible and the step angle is 3°.
1
T   L2Ti 2 (t ) sin T
2
13. With a neat block diagram, explain the closed loop operation of stepping motor.
14. Explain the concept of torque production in variable reluctance stepping motor.
. Let
e(t) = voltage applied per stack
R = winding resistance per stack
L(θ) = winding inductance per stack (a function of rotor position only and
independent of coil current because of linear magnetic circuit assumption)
i(t) = current per stack
θ(t) = angular position of rotor
Kirchoff’s mesh equation for stator winding is
e(t )  Ri (t ) 
where λ = flux linkages of stator winding = iL(θ).
Therefore,
 Ri (t )  L( )
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d
dt
di
dL( ) d
i
…………....(1)
dt
d dt
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Transformer
emf
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speed
emf
Energy stored in air gap is
W = ½ L(θ) i2(t)……………………………..(2)
Mechanical torque developed is given by

T
W (i, )

1
dL( )
 i 2 (t )
…………………………....(3)
2
d
Rotor dynamics is governed by
d 2
d
……………………………….(4)
TJ 2  f
dt
dt
In a toothed structure, reluctance and therefore winding inductance varies continuously (even
function) as function of θ over and above an average value, i.e.,
L( )  L1  L2 cos T ……………………………….(5)
Substituting in equation 3,
1
T   L2Ti 2 (t ) sin T
2
UNIT – III SWITCHED RELUCTANCE MOTOR
PART - A
1. What is switched reluctance motor?
The switched reluctance motor is a doubly salient, singly excited motor. This means that it has salient
poles on both the rotor and the stator, but only one member (usually the stator) carries windings. The rotor has
no windings; magnet is or cage windings but is built up from stacks of salient pole laminations.
2. What is meant by self control?
In the open loop system, if a load is suddenly applied, the rotor momentarily slow down, making the
torque angle delta increase beyond 90 and leading to loss of synchronism. If the rotor speed adjusts the stator
frequency the drive system is called as self controlled drive.
3. What are the advantages of Switched Reluctance motor?
Rotor is simple and it tends to have a low inertia, The stator is simple to wind, In most applications the
bulk of the losses appear on the stator, which is relatively easy to cool, Due to the absence of magnet the
maximum permissible rotor temperature may be higher than in PM motors, Under fault conditions the open
circuit voltage and short circuit current are zero or varying small, Extreme by high speeds are possible
4. What is the difference between Switched Reluctance motor and variable reluctance stepper motor?
Switched Reluctance motor
Variable reluctance stepper motor
Conduction angle for phase current is controlled and Stepper motor is usually fed with a square wave of
synchronized with the rotor position, usually by means phase current without rotor position feedback.
of a shaft position sensor
The SRM is designed for efficient power conversion at It is usually designed as a torque motor with a
high speed
limited speed.
5. What are the disadvantages of a Switched Reluctance motor?
The absence of free PM excitation imposes the burden of excitation on the stator windings and the controllers
and increases the per unit copper losses,  is limited, torque / ampere is limited, Non uniform nature of the
torque production which leads to torque ripple and may contribute to acoustic noise.
6. Distinguish between co-energy and field energy
Co-energy
Field energy
i

Co-energy is defined as w  di
1
0


Field energy is defined as w f   id
0
7. Determine the step angle of a 3 phase Switched Reluctance motor having 12 stator poles and 8 rotor
poles. What is the commutation frequency in each phase at a speed of 6000 rpm.
f1  n N r
16
rpm
Nr
60
6000

8
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Given: q = 3; NS = 12; Nr = 8 ; N = 6000 rpm
To find Step angle (), Frequency Solution:
Step angle () =
2
rad
qN r
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
2
3 8
8. What are the applications of Switched Reluctance motor?
Precision position control system for Robotics, Low power servo motor
9. Give basic features of Switched Reluctance motor.
The switched reluctance motor is a doubly-salient, singly-excited motor. This means that it has salient
poles on both the rotor and the stator but only one member (usually the stator) carries windings. The rotor has
no windings, magnets, or cage windings, but is built up from a stack of salient-pole laminations.
Low inertia and simple manufacturing, Losses appear only on the stator and easy to cool, No magnets and so
permissible rotor temperature is higher than in PM motors, Torque is independent of the polarity of phase
current. Reduction in no of semiconductor devices in controller Open circuit voltage and Short circuit current
are zero or very small under faulty condition, Immune from shoot through failure, High starting torque,
extremely high speeds possible.
10. Write the relations between the speed and fundamental switching frequency.
f = n Nr = (r.p.m./60) Nr Hz Nr = No. of rotor poles, If there are q phases there are q Nr steps per
revolution and the step angle or stroke is given by
ε = 2 π /(q Nr) rad. The no of stator poles usually exceeds
the no of rotor poles.
11. What is co-energy?
In the ψ –i curve of a motor, the area between the curve and horizontal i axis is the co-energy W’ and
the other part is the stored field energy Wf.
12. Give the expression for torque of a Switched Reluctance motor.
The torque is given by T = [∂ W’ / ∂θ] i=const Or by T = [∂ Wf / ∂θ] ψ =const With magnetic saturation
negligible and with ψ –i curve straight line , ψ = L i ,W’ = Wf = (½) L i2 ,T = (½) i2 dL /d θ Nm
13. Explain torque speed characteristics of Switched Reluctance motor.
For speed below ωb the torque is limited by the motor current. Up to base speed it is possible to
maintain the torque constant by means of the regulators. In the speed range below ω b the firing angles can be
chosen to optimize efficiency of minimum torque ripple. The corner point or base speed wb is the highest speed
at which maximum current can be supplied at rated voltage with fixed firing angle. If these angles are still kept
fixed, then the maximum torque at rated voltage decreased with speed squared. However if the conduction
angle is increased there is a considerable speed range over which maximum current can still be forced into the
motor and thus sustain the torque at a level high enough to maintain constant power change. This is shown
between points B and P. The angle D is dwell or conduction angle of the main switching device in each phase.
It should generate can be possible to maintain constant power up to 2.3 times base speed.
14. What are the types of power controllers used for Switched Reluctance motor?
Using two power semiconductors and two diodes per phase, (n + 1) power switching devices and (n + 1)
diodes per phase, Phase windings using Bifilar wires , Dump – C – converter, Split power supply converter
15. Why rotor position sensor is essential for the operation of Switched Reluctance motor?
It is normally necessary to use a rotor position sensor for communication and speed feedback. The
turning ON and OFF operation of the various devices of power semiconductor switching circuit are influenced
by signals obtained from rotor position sensor.
16. What are the two types of current control techniques?
Hysteresis type control, PWM type control
17. What is meant by energy ratio?
Energy ratio = (Wm / Wm + R) ≈ 0.45 Wm = Mechanical energy transformed
This energy ratio cannot be called as efficiency. As the stored energy R is not wasted as a loss but it is feedback
to the source through feedback diodes.
18. What is phase winding of SRM?
Stator poles carrying field coils. The field coils of opposite poles are connected in series such that
mmf’s are additive and they are called “Phase winding” of Switched Reluctance motor.
19. What is Hysteresis current control?
This type of current controller maintains a more or less constant throughout the conduction period in
each phase. This controller is called hysteresis type controller.
20. Define: Chopping mode of operation of Switched Reluctance motor?
In this mode, also called low – speed mode, each phase winding gets excited for a period which is
sufficiently long.
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21. Define: Single pulse mode of operation of Switched Reluctance motor?
In this mode, also called high – speed mode, the current rise is within limits during the small time interval of
each phase excitation.
22. What is the step angle of an 5 phase Switched Reluctance motor and commutation frequency in each
phase for the speed of 6000 rpm. SRM having 10 stator poles and 4 rotor poles.
Solution: Step angle () = ( 2π / qNr ) = ( 360° / 5*4) = 18°
Commutation frequency at each phase = ( Nr * ω ) / 2π = ( 4*6000) / 60 = 400 Hz. [ω = 2πN]
23. What are the merits of Dump C – Converter?
This topology uses lower number of switching devices and has only one switch voltage drop, the converter has
full regenerative capability, and there is faster demagnetization of phases during commutation
24. What are the merits of split power supply Converter?
It requires lower number of switching devices, there is faster demagnetization of phases during commutation
25. What are the merits of classic converter?
Control of each phase is completely independent of the other phases; the energy from the off going phase is
feedback to the source, which results in useful utilization of the energy.
26. What are the different power converters used for the control of switched reluctance motor?
Classic converter, (n+1) power semiconductor switch, phase windings using bifilar wires, split power
supply converter, dump c converter.
27. Draw the speed-torque characteristics of switched reluctance motor.
PART B
1. Explain the construction and working of Switched Reluctance motor?
2. With a block diagram explain the importance of closed loop control of Switched Reluctance motor.
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3. Describe the Hysteresis type and PWM type current regulator for one phase of Switched Reluctance motor.
4. Explain in detail about microprocessor based control of Switched Reluctance motor.
BLOCK DIAGRAM
DC SUPPLY
Power semiconductor
Switching circuit
SRM
REF SPEED
CONTROLLER
ROTOR POSITION SENSOR
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5. Describe the various power controller circuits to Switched Reluctance motor and explain the operation of any
one scheme with suitable circuit diagram.
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6. Describe the various operating modes of Switched Reluctance motor.
7. Derive the torque equation of Switched Reluctance motor.
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8. Draw and explain torque – speed characteristics of Switched Reluctance motor.
9. Discuss the advantages and disadvantages of Switched Reluctance motor.
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10. Discuss in detail the frequency of variation of the inductance of each phase winding of Switched Reluctance
motor.
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UNIT – IV PERMANENT MAGNET BRUSHLESS D.C. MOTORS
PART - A
1. Why adjustable speed drives are preferred over a fixed speed motor?
The common reasons for preferring an adjustable speed drives over a fixed speed motor are:
Energy saving e.g. Fan or pump flow process, Velocity and position control e.g. Electric train, portable tools,
washing machine, Amelioration of transients: Starting and stopping of motors produce sudden transients. It
can be smoothened using adjustable speed drives.
2. What is the structure of an adjustable speed drive system?
The general structure of a motion control system or drive consists of the following elements:
The load, the motor, the power electronic converter; and the control.
3. Write briefly about the construction and types of a Brushless DC machines.
Brushless PM machines are constructed with the electric winding on the stator and PMs on the rotor.
There are several conventional PM machine configurations and other more novel concepts conceived in recent
years to improve performance. The configuration of a PM machine and the relationship of the rotor to the stator
determine the geometry and the shape of the rotating magnetic field. PM machines in which the magnetic flux
travels in the radial direction are classified as radial-flux machines. They are cylindrical in shape, and the rotor
is usually located inside the stator but can also be placed outside the stator. PM machines in which the magnetic
flux travels in the axial direction are classified as axial-gap machines. They can have multiple disk or pancakeshaped rotors and stators. The stator-rotor-stator configuration is typical.
4. What are the advantages of PM machine?
In general, PM machines have a higher efficiency as a result of the passive, PM-based field excitation.
PM machines have the highest power density compared with other types of electric machines, which implies
that they are lighter and occupy less space for a given power rating. The amount of magnet material that is
required for a given power rating is a key cost consideration. The cost of magnet material is high compared with
the cost of the other materials used in electric motors, and design attributes that minimize the required amount
of magnet material are important considerations in motor selection. The stators of PM machines are generally
fabricated in the same manner as induction machine stators; however, modifications are sometimes necessary,
such as the design of a stator lamination to accommodate high flux density.
5. What are the types of PM machines?
1. Interior PM Machine and 2. Surface mounted PM machine.
6. Sketch different types of IPM machines
7. Sketch different types of Surface PM machines.
8. Discuss briefly about the types of Permanent Magnets used in electrical machines.
PM strength and other key properties the various types of PMs include the following:
Alnico—a family of magnets made from aluminum, nickel, and cobalt characterized by excellent temperature
stability, high residual induction, and enough energy for a number of industrial and commercial applications.
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Ceramic—a hard, low-cost ferrite made of barium and strontium ferrite with excellent stability. Ceramic
magnets tend to be brittle, hard, and resistant to corrosion.
Ferrites—a softer (non-sintered) version of the ceramic magnet. Lowest-cost magnets.
Samarium cobalt (Sm2Co17)—a rare-earth magnet with outstanding magnetic properties including high
BHmax, excellent thermal stability, and excellent corrosion resistance. Samarium cobalt is formulated as
SmCo5 (1:5 material) and Sm2Co17 (2:17 material). This magnet is well suited for applications demanding
high magnetic strength in a high temperature environment.
9. What are the methods for containing the magnets in PMBLDC motor?
As the base speed increases, magnet containment becomes a critical problem. For a machine with
surface-mounted magnets operating as a brushless direct current motor (BDCM) with trapezoidal back-emf,
there are four options: Wrap the magnets with a suitable B-stage, Encase the rotor in a non-magnetic can, Use
an “inside-outside configuration” in which the stator is inside the structure containing the magnets and rotor
laminations. The centrifugal forces on the magnets are then restrained by this structure, Use an axial gap
configuration. In this arrangement, the stator and rotor are in the shape of a toroid. The armature current is in
the radial direction and the magnets are charged in the axial direction. The air gap is in the axial direction and
the centrifugal force on the magnets is in the radial direction. Thus a restraining ring can be used that does not
impact the air gap volume.
10. Compare brushless DC motor with P.M. commutator motor.
Brushless DC motor
P.M. Commutator motor
1. No Brushes. Maintenance problems (RFI, sparking, 1. Commutator based DC machines need carbon
ignition and fire accidents) eliminated.
brushes, so sparking and wear and tear is un
avoidable.
2. More cross sectional available for armature windings. 2. Armature winding is inside and the magnet is
Conduction of heat through the frame is improved.
on the stator outside.
3. Increase in electric loading is possible, providing a 3. Efficiency less.
greater specific torque. Higher efficiency.
4. Space saving, higher speed possible, with reduced 4. Commutator restricts speed.
inertia.
5. Maximum speed limited by retention of magnet 5. Magnet is on the stator. No problem.
against centrifugal force.
6. Shaft position sensor is a must.
6. Not mandatory.
7. Complex electronics for controller.
7. Simple
11. What are the modes of operation of the square wave BLDC motor?
There are two modes of operation: 120 degree mode and 180 degree mode. The mode corresponds to
the conduction period of the switches in the voltage source inverter. Each switch conducts for the period 120 or
180 degree as per the mode. Commutation from one conducting device to another takes place at every 60 degree
intervals.
12. Why the shape of the emf waveform of a BLDC is trapezoidal?
The ideal shape of the emf waveform is rectangular. The effect of slotting and fringing causes its
corner to be rounded and hence the shape is trapezoidal with flat top portion with 120 degree
13. Give the emf and torque equations of the square wave BLDC motor.
The emf equation is given by E = kφω and the torque equation is given by T = kφI. where k is the
armature constant depending on the number of turns in series per phase in the armature winding, ω is rotor
speed in rad / sec and φ is the flux ( mainly contributed by the Permanent Magnet on the rotor). I is the load
current.
14. Give the expression for torque speed characteristics of BLDC motor.
The emf equation is given by E = kφω and the torque equation is given by T = kφI. E = V - IR
At no load, I = 0, so the no load speed ω0 is given by ω0 =V/ (kφ) (Since at no load I=0, E= V= kφ ω0 )
The stall torque( motor at zero speed) is given by T 0 = kφI0, where I0 is the stall current. At zero speed E = 0 ,
V= I0R, R = V/ I0, E = V- RI, Kφω = V – (V/ I0) I , = V(1-I/ I0) ω = (V/Kφ)( 1-I/ I0) , = ω0 (1-T/ T0)
since T is proportional to I. This is the equation describing the torque speed characteristics of BLDC motor.
15. Compare 120 degree and 180 degree operation of BLDC motor.
The 180 degree magnetic arc motor uses 120 degree mode of inverter operation. The motor with 120
degree magnetic arc uses 180 degree mode of inverter operation.
In 180 degree mode of inverter has 1.5 times copper losses but produce same torque with only 2/3 of magnetic
material. Motor operation is less efficient.
16. Give the expression for self and mutual inductances of a BLDC motor.
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Self inductance is given by Lg = (ψ/i) = (πμ0 N2lr1 )/(2g”) where g” = g’ + lm/ μrec , g’ = Kcg, N =
Number of conductors in the slot, I = current, lm = magnet length in radial direction , g’ = air gap, g” = air gap
including radial thickness of the magnet, μrec = relative recoil permeability, Mutual inductance is given by Mg =
- (1/3) Lg
17. What are the types of sensors used with PMBLDC motors?
Hall effect sensors are most commonly used for speed, position sensing with PMBLDC motors.
Optical Disc based sensors are also used. Presently rotor position sensors are avoided by using alternative
methods called as Sensorless control methods, which uses terminal emf measurement, third harmonic voltage
measurement, flux estimation and neuro – fuzzy techniques etc.
18. Why MOSFET or IGBTs are used in inverters for PMBLDC motors?
These devices operate at very high switching frequencies for PWM method of operation. The duty
cycle of the PWM decides the average voltage applied to the motor and hence the speed is adjusted. These
devices are easy to commutate by using microprocessor or microcontroller based software. (Base drive)
19. Write the dynamic equations of the PMBLDC motor.
The dynamic model equations of PMBLDC motor is given by di /dt =( v – R i – e (θ) ) / L
a
an
a
a
di /dt =( v – R i – e (θ) ) / L , di /dt =( v – R i – e (θ) ) / L , dω/dt = [ T – T – B ω ] / J , dθ/dt = P ω / 2
b
bn
b
b
c
cn
c
c
e
l
where the Torque developed is given by , T = (e (θ)i +e (θ)i + e (θ) i ) / ω , T = Load torque applied, B is the
e
a
a
a
b
a
c
L
coefficient of friction and J is the moment of inertia.
20. What are the relative merits and demerits of brush less DC motor drives?
Merits: Commutator less motor , Specified electrical loading is better, Heat can be easily dissipated, No
sparking takes place due to brush, Source of EMI is avoided
Demerits: Above 10 kW, the cost of magnet is increase, Due to centrifugal force the magnet may come out.
21. How the direction of rotation is reversed in case of PMBLDC motor
The direction of rotation can be reversed by reversing the logic sequences in PMBLDC motor
22. What are the difference between conventional DC motor and PMBLDC motor?
DC
PMBLDC
Brushes are present
Brushes are not present
Sparking may occur due to brush
Sparking will not occur as brush is not present
Brush tend to produce RF1
RF1 problem does not occur
There is a need for brush maintenance
No need of brush maintenance
23. Write the torque and emf equation of square wave brushless motor
e  2 N pn B g lr1 ,
Te  4 N ph B g lr1 I
24. What are the various kinds of permanent magnets?
There are basically three different types of permanent magnets which are used in small DC motors
Alnico magnets, Ferrite or ceramic magnet, and Rare - earth magnet (samarium – cobalt magnet )
25. Bring out the differences between conventional and brushless DC motors?
Description
Conventional motors
Brushless DC motors
Mechanical structure Field magnets on the stator
Field magnets on the rotor.
Similar to AC synchronous motor.
Distinctive features
Quick response and excellent controllability Long – lasting Easy maintenance
Commutation method Mechanical contact between brushes and
Electronic switching using transistors
commutator
26. Write the difference between square wave PMBL and sine wave PMBL motor.
Square wave PMBL
Sine wave PMBL motor
It has rectangular distribution of It has sinusoidal or quasi – sinusoidal distribution of magnetic flux in
magnetic flux in the air gap.
the air gap.
It has rectangular current waveforms.
It has sinusoidal or quasi – sinusoidal current waveforms.
It has concentrated stator winding
It has quasi – sinusoidal distribution of stator winding (conductors)
which is short pitch and distributed or sometimes distributed in some
cases.
27. Write the expression of torque equation of a sine wave PMBL motor.
For an ideal case T 
3
2 2
I Bm  r1 l N s sin  Nm and for the practical case T 
3p

E ph I sin  Nm
Where Bm = Maximum flux density in the air gap Wb / m2, l = axial length of the magnetic path, m
Ns = stator poles,  = torque angle , p = Number of pole pairs
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28. Write the expression for induced emf / phase of the sine wave PMBL motor.
For
an
ideal
case
E ph 
1
2p 2
Bm  r1 l  N s and
for
the
practical
case
E ph  2 m1 K w1 N ph  f Where Bm = Maximum flux density in the air gap Wb / m2, l = axial length of the
magnetic path, m, Ns = stator poles,  = torque angle, p = Number of pole pairs
28. Write down the torque equation of permanents magnet synchronous motor.
T = π BArl sinβ
29. State the objective of vector control of permanent magnet synchronous motor.
I = Id + Iq controlling the PMSM taking into consideration above mentioned aspects is known as vector
control of PMSM.
PART B
1. Derive the torque and EMF equations of the permanent magnet brushless DC motor.
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2. What are the differences between mechanical and electronic commutator?
Less spark, easy to control, less maintenance, more efficient, small in size (compact)
3. What are the advantages and disadvantages of Brushless DC motor drives?
High power density, size, absence of brushes, no sparking and less maintenance, more efficient than
conventional Dc motor, high speed
Drawbacks
Presence of magnets
Temperature limit
Ageing
4. Draw the diagram of mechanical commutator. Explain the operation of the same.
5. Sketch the structure of controller for permanent magnet brushless DC motor and explain the functions of
various blocks.
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6. Explain in detail the various rotor position sensors used in permanent magnet brushless DC motor.
7. Sketch torque – speed characteristics of a permanent magnet brushless DC motor.
8. Draw the diagram of electronic commutator. Explain the operation of the same.
9. Explain in detail the magnetic circuit analysis of brushless DC motor on open circuit.
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10. Discuss in detail the various driving circuits used in permanent magnet brushless DC motor.
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UNIT – V PERMANENT MAGNET SYNCHRONOUS MOTORS
PART - A
1. Compare PMSM and PMBLDC motors.
PM Brushless DC motor
PMSM
1. Rectangular distribution of magnetic flux in 1. Sinusoidal or quasi–sinusoidal distribution of magnetic flux
the air gap.
in the air gap.
2. Rectangular current waveforms.
2. Sinusoidal or quasi-sinusoidal current waveforms.
3. concentrated stator windings.
3. Quasi-sinusoidal distribution of stator conductors. (short
pitched and distributed or concentric stator windings)
2. Give the expression for torque and emf of a PMSM motor.
T = (3/2) I √2 [ π r1 l B Ns sin(β)]/2, where β is the torque angle +ve for motoring,
Eph = [2 π (kw1Nph) ΦM1 f ] / √2
3. Compare the performance of PMSM with BLDC motor.
With equal r.m.s. phase currents the torque of the square wave motor exceeds that of sine wave motor
by a factor 1.47. With equal peak currents the factor is 1.27. For the same flux-density flux per pole of a square
wave motor exceeds that of a sine wave motor by a factor π/2. Square wave motor has a slightly better
utilization of the peak current capability of the converter switches. In PMSM three devices conduct at a time
(180 degree mode of inverter), where as in BLDC only two devices conduct at a time in 120 degree mode.
4. What is meant by field oriented control of PMSM?
In general for field oriented control the stator currents are transformed into a frame of reference
moving with the rotor flux. In the PMSM the rotor flux is stationary relative to the rotor. The rotor flux is
therefore defined by the mechanical angle of rotation α, this is obtained from a rotor position sensor. Thus, the
control is much easier to implement than in the case of induction motor.
5. What are the inherent advantages of variable reluctance motor?
Simple in construction due to absence of rotor windings and or magnets and the use of small number of
concentrated stator coils similar to the field coils of a DC motor, Efficient motor cooling, Suitable for
sustained high speed operations, Low motor inertia and high torque / weight rotor
6. Write the expression for the self and synchronous reactance of PMSM
The sum of the magnetizing and leakage reactance define synchronous reactance.
X S  X M  X l , Xs = Synchronous reactance, XM = Magnetizing reactance, Xl = Leakage
reactance
7. Bring out the differences between synchronous and Induction motor?
Synchronous motor
Induction motor
It is not a self starting one and some external means are required
It has got self starting torque
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Requires DC excitation or PM
Does not required DC excitation
Speed control not possible
Speed can be controlled but to small range
It can be operated under wide range of power factor both lag and It operates at only lagging power factor
lead
which becomes very simple for light load
More complicated and costly
More simple and less cost
8. What are the applications of synchronous motor?
Used in power houses and sub station in parallel to the bus bar to improve the power factor, Used in textile
mills, rubber mills, mining and the big industry, Fans, Blowers centrifugal pumps
9. What are the applications of PMBLDC and PMSM motors?
PMBLDC: (Low rating application)turn table drives for record players, Hard disc drives, Low cost instruments,
Small fans for cooling electronic equipment,
(High rating application)Air craft, Satellite system, Traction
system (in future)
10. What are advantages and disadvantages of PMSM?
Advantages: Elimination of field copper loss, High power density, Lower rotor inertia, Robust construction of
rotor, High efficiency Disadvantages: Loss of flexibilities of field flux control, Demagnetization effect, High
cost
14. What are the features of permanent magnet synchronous motor?
Robust, compact and less weight, No field current or rotor current in PMSM, unlike in induction motor,
Copper loss due to current flow which is largest loss in motors is about half that of induction motor,
High efficiency.
12. What are the applications of PMSM?
PMSM are used in low to medium power (up to several hundred HP) Applications, Fiber spinning mills
Rolling mills, Cement mills, Ship propeller, Electric Vehicles, Servo and robotic drives, Starters / generator for
air craft engine
13. Explain the difference between synchronous motor and PMSM
Synchronous Motor
PMSM
3 phase AC or six step voltage or current source 3 phase sine wave ac or PWM ac is used as supply
inverter is used as supply
This type of motor is used in very large compressor and Here it is used in low integral HP industries drives,
fan drives
fiber spinning mills.
14. What are disadvantages of PMSM relative to the commutator motor?
Need for shaft position sensing, Increased complexion in the electronic controller
15. What are the assumptions made in derivation of emf equation for PMSM?
Flux density distribution in the air gap is sinusoidal, Rotor rotates with an uniform angular velocity of ωm (
rad/sec), Armature winding consists of full pitched, concentrated similarly located coils of equal number of
turns
16. What are the advantages of load commutation?
It does not require commutation circuits, Frequency of operation can be higher, It can operate power levels
beyond the capability of forced commutation.
17. Write down the expressions for power input of a PMSM?
Power input = 3EqIa + 3Ia2Ra
18. Write down the expressions for torque of a PMSM?
T = 3EIsin β / ωm N-m
19. What are the features of closed- loop speed control of load commutated inverter fed synchronous
motor drive?
Higher efficiency, Four quadrant operation with regeneration braking is possible, Higher power ratings and run
at high speeds ( 6000 rpm)
20. Write down the emf expressions of PMSM?
Eph = 4.44 f Фm Kw1 Tph volts, This is the rms value of induced emf per phase, where f = Frequency in
Hertz, Фm = flux per pole, Kw1 = Winding factor, Tph = Turns per phase
21. What is load commutation?
Commutation of thyristors by induced voltages of load is known as “load commutation”. Here, frequency of
operation is higher and it does not require commutation circuits.
22. What is meant by slotless motor?
In slotless motor, the stator teeth are removed and resulting space is partially filled with addition copper.
23. Distinguish between self control and vector control of PMSM?
S.No.
Self control
Vector control
1.
Dynamic performance is poor
Dynamic performance is better
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2.
Control circuit is simple
Control circuit is complex
24. State the two classifications of PMSM?
Sinusoidal PMSM, Trapezoidal PMSM
25. Write down the expressions for the self and synchronous reactance of PMSM?
Xs = (3 π μ0 Ns2 l r1 ω) / 8 p2 g2 Ω
26. Differentiate between mechanical and electronic commutators of permanent magnet brushless DC
motors.
Mechanical commutator
Electronic commutator
Commutator arrangement is located in the rotor
Commutator arrangement is located in the stator
Shaft position sensing is inherent in the arrangement It requires a separate rotor position sensor
No. of commutator segments are very high
No. of switching devices is limited to 6
Sparking takes place
There is no sparking
27. Name the two commutators used in the power controllers of permanent magnet brushless DC motors.
Mechanical and electronic commutators
PART B
1. Derive the EMF and torque equation of BLPM Sine wave motor.
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2. Explain the microprocessor based control of permanent magnet synchronous motors with a neat block
diagram.
BLOCK DIAGRAM
DC SUPPLY
Power semiconductor
Switching circuit
SRM
REF SPEED
CONTROLLER
ROTOR POSITION SENSOR
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3. Explain in detail the vector control of permanent magnet synchronous motor.
4. With neat sketch, discuss the torque – speed characteristics of PMSM.
5. What are the differences in the constructional features of PMBLDC and PMSM?
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6. Draw and explain the phasor diagram of permanent magnet synchronous motor.
7. With a neat diagram, explain the operation of power of controller for permanent magnet synchronous motor.
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8. Explain circle diagram of permanent magnet synchronous motor.
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