Download DC Motor Construction:

DC Motor Construction:
 A DC generator can be used as a DC motor without any constructional changes
and vice versa is also possible.
A DC machine consists two basic parts; stator and rotor.
Stator – The static part that houses the field windings and receives the supply,
Rotor – The rotating part that brings about the mechanical rotations.
Basic constructional parts of a DC machine are described below,
1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast
iron or steel. It not only provides mechanical strength to the whole
assembly but also carries the magnetic flux produced by the field winding.
2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or
welding. They carry field winding and pole shoes are fastened to them. Pole
shoes serve two purposes; (i) they support field coils and (ii) spread out the
flux in air gap uniformly.
3. Field winding: They are usually made of copper. Field coils are former
wound and placed on each pole and are connected in series. They are wound
in such a way that, when energized, they form alternate North and South
poles.
4. Armature core: Armature core is the rotor of the machine. It is cylindrical in
shape with slots to carry armature winding. The armature is built up of thin
laminated circular steel disks for reducing eddy current losses. It may be
provided with air ducts for the axial air flow for cooling purposes. Armature
is keyed to the shaft.
Cross Section View/Construction of Dc Motor
5. Armature winding: It is usually a former wound copper coil which rests in
armature slots. The armature conductors are insulated from each other and
also from the armature core. Armature winding can be wound by one of the
two methods; lap winding or wave winding. Double layer lap or wave
windings are generally used. A double layer winding means that each
armature slot will carry two different coils.
Lap Winding
In this case the number of parallel paths between conductors A is equal to the
number of poles P. i.e A = P
hint: An easy way of remembering it is by remembering the word LAP-----→
L A=P
Wave Winding
Here in this case, the number of parallel paths between conductors A is always
equal to 2 irrespective of the number of poles. Hence the machine designs are
made accordingly.
6. Commutator and brushes: Physical connection to the armature winding is
made through a commutator-brush arrangement. The function of
a commutator, in a dc generator, is to collect the current generated in
armature conductors.
Whereas, in case of a dc motor, commutator helps in providing current to the
armature conductors. A commutator consists of a set of copper segments
which are insulated from each other. The number of segments is equal to the
number of armature coils. Each segment is connected to an armature coil and
the commutator is keyed to the shaft. Brushes are usually made from carbon
or graphite. They rest on commutator segments and slide on the segments
when the commutator rotates keeping the physical contact to collect or supply
the current.
Working of DC Motor:
 The principle of working of a DC motor is that "whenever a current carrying
conductor is placed in a magnetic field, it experiences a mechanical force".
 The direction of this force is given by Fleming's left hand rule & it's
magnitude is given by ,
F = B*I*L
Where,
B = magnetic flux density, I = current and L = length of the conductor within
the magnetic field.
 When armature windings are connected to a DC supply, current sets up in the
winding. Magnetic field may be provided by field winding (electromagnetism)
or by using permanent magnets. In this case, current carrying armature
conductors experience force due to the magnetic field, according to the
Flemings Left hand rule.
 Commutator is made segmented to achieve unidirectional torque. Otherwise,
the direction of force would have reversed every time when the direction of
movement of conductor is reversed the magnetic field.
Working of DC Motor:
 According to fundamental laws of nature, no energy conversion is possible until
there is something to oppose the conversion. In case of generators this opposition
is provided by magnetic drag, but in case of dc motors there is back emf.
 When the armature of the motor is rotating, the conductors are also cutting the
magnetic flux lines and hence according to the Faraday's law of electromagnetic
induction, an emf induces in the armature conductors. The direction of this
induced emf is such that it opposes the armature current (Ia) .
 Magnitude of Back emf can be given by the emf equation/ Voltage equation of
DC Motor,
Magnitude of back emf is directly proportional to speed of the motor. Consider the
load on a dc motor is suddenly reduced. In this case, required torque will be small
as compared to the current torque. Speed of the motor will start increasing due to
the excess torque. Hence, being proportional to the speed, magnitude of the back
emf will also increase. With increasing back emf armature current will start
decreasing. Torque being proportional to the armature current, it will also decrease
until it becomes sufficient for the load. Thus, speed of the motor will regulate.
Working of DC Motor:
 On the other hand, if a dc motor is suddenly loaded, the load will cause
decrease in the speed. Due to decrease in speed, back emf will also decrease
allowing more armature current. Increased armature current will increase the
torque to satisfy the load requirement. Hence, presence of the back emf
makes a dc motor ‘self-regulating’.
 To see the Video Demonstration of DC Motor Working Please click on the
given link ,DC Motor, How it works_ - YouTube (360p).mp4
Derivation of Back EMF:
 As the armature rotates, a voltage is generated in its coils. In case of a
generator, the emf of rotation is called the Generated emf or Armature
emf and is denoted as Eg. In case of a motor, the emf of rotation is known
as Back emf or Counter emf and represented as Eb.
 Let, assume
P – Number of poles of the machine
ϕ – Flux per pole in Weber.
Z – Total number of armature conductors.
N – Speed of armature in revolution per minute (r.p.m).
A – Number of parallel paths in the armature winding.
 In one revolution of the armature, the flux cut by one conductor is given as,
 Time taken to complete one revolution is given as
Derivation of Back EMF
 Therefore, the average induced e.m.f in one conductor will be
 Putting the value of (t) from Equation (2) in the equation (3) we will get
 The number of conductors connected in series in each parallel path = Z/A.
Therefore, the average induced e.m.f across each parallel path or across the
armature terminals is given by the equation shown below.
Derivation of Back EMF
Where, n is the speed in revolution per second (r.p.s) and given as
 For a given machine, the number of poles and the number of conductors per
parallel path (Z/A) are constant. Hence, the equation (5) can be written as
Where, K is a constant and given as
 Therefore, the average induced emf equation can also be written as
Derivation of Back EMF
Where K1 is another constant and hence induced emf equation can be written as
Where ω is the angular velocity in radians/second is represented as
 Thus, it is clear that the induced emf is directly proportional to the speed and
flux per pole. The polarity of induced emf depends upon the direction of the
magnetic field and the direction of rotation. If either of the two is reverse the
polarity changes, but if two are reversed the polarity remains unchanged.
 This induced emf is a fundamental phenomenon for all the DC Machines
whether they are working as a generator or motor.
Derivation of Back EMF
 If the machine DC Machine is working as a Generator, the induced emf is
given by the equation shown below.
 If the machine DC Machine is working as a Motor, the induced emf is given
by the equation shown below.
 In a motor, the induced emf is called Back Emf (Eb) because it acts opposite
to the supply voltage.
Significance / Importance of Back EMF:
 Magnitude of back emf is directly proportional to speed of the motor. Consider
the load on a dc motor is suddenly reduced. In this case, required torque will
be small as compared to the current torque. Speed of the motor will start
increasing due to the excess torque. Hence, being proportional to the speed,
magnitude of the back emf will also increase. With increasing back emf
armature current will start decreasing. Torque being proportional to the
armature current, it will also decrease until it becomes sufficient for the load.
Thus, speed of the motor will regulate.
 On the other hand, if a dc motor is suddenly loaded, the load will cause
decrease in the speed. Due to decrease in speed, back emf will also decrease
allowing more armature current. Increased armature current will increase the
torque to satisfy the load requirement.
 Hence, presence of the back emf makes a dc motor ‘self-regulating’.
Derivation of Torque:
 When a DC machine is loaded either as a motor or as a generator, the rotor
conductors carry current. These conductors lie in the magnetic field of the air
gap. Thus, each conductor experiences a force. The conductors lie near the
surface of the rotor at a common radius from its center. Hence, a torque is
produced around the circumference of the rotor, and the rotor starts rotating.
 When the machine operates as a generator at a constant speed, this torque is
equal and opposite to that provided by the prime mover. When the machine is
operating as a motor, the torque is transferred to the shaft of the rotor and
drives the mechanical load. The expression is same for the generator and
motor.
 When the current carrying current is placed in the magnetic field, a force is
exerted or it which exerts turning moment or torque F x r. This torque is
produced due to the electromagnetic effect, hence is called Electromagnetic
torque.
Derivation of Torque:
 As we known that the voltage of the Motor is,
Multiplying the equation (1) by Ia we get
Where,
VIa is the electrical power input to the armature & I2aRa is the copper loss in
the armature.
As We know that,
Total electrical power supplied to the armature = Mechanical power
developed by the armature + losses due to armature resistance
Derivation of Torque:
 Now, the mechanical power developed by the armature is Pm.
 Also, the mechanical power rotating armature can be given regarding torque T
and speed n.
Where, n is in revolution per seconds (rps) and T is in Newton-Meter.
Hence,
Derivation of Torque:
But,
Where N is the speed in revolution per minute (rpm) and
Where, n is the speed in (rps).
Therefore,
So, the torque equation is given as
Derivation of Torque:
 For a particular DC Motor, the number of poles (P) and the number of
conductors per parallel path (Z/A) are constant.
Where,
Thus, from the above equation (5) it is clear that the torque produced in the
armature is directly proportional to the flux per pole and the armature current.
Derivation of Speed of DC Motor:
 We know, back emf Eb of a DC motor is the induced emf in the armature
conductors due to the rotation of armature in magnetic field. Thus, magnitude
of the Eb can be given by the EMF equation of a DC generator,
Eb = PØNZ/60A
Where, P = no. of poles, Ø = flux/pole, N = speed in rpm, Z = no. of armature
conductors and A = parallel paths.
 Eb can also be given as,
Eb = V- IaRa
 Thus, from the above equations,
N = Eb 60A/PØZ
 But, for a DC motor A, P and Z are constants
Therefore,
N ∝ K Eb/Ø
(where, K=constant)
 This shows the speed of a dc motor is directly proportional to the back emf
and inversely proportional to the flux per pole.
Characteristics of DC Series Motor:
 Motor is machine which convert electrical energy into mechanical energy
 The performance of a motor can be judged from its characteristic curves known as
motor characteristics.
 from the working of a DC motor there are three characteristic curves ,those are as
follows,
1. Torque and armature current (Ta/Ia characteristic)
 It is also known as electrical characteristic
 We have seen that in series motor Ta ∝ Φia
 In a series motor, as field windings also carry the armature current, Φ ∝ Ia up to
the point of magnetic saturation.
 Since Ta ∝ ΦIa and theirfore Ta ∝ Ia2
 As Ia increases, Ta increases as the square of the current. Hence, Ta/Ia curve is a
parabolic in nature.
 After saturation, Φ is almost independent of Ia hence Ta ∝ Ia only. So the
characteristic becomes a straight line. The shaft torque Tsh is less than armature
torque due to stray losses
 So we conclude that (prior to magnetic saturation) on heavy loads, a series motor
exerts a torque proportional to the square of armature current. Hence, in cases
where huge starting torque is required for accelerating heavy masses quickly as in
hoists and electric trains etc., series motors are used.
Characteristics of DC Series Motor:
Characteristics of DC Series Motor:
2. Speed and armature current (N/Ia characteristic)

N α Eb/Φ
Change in Eb, for various load currents is small and hence may be neglected
for the time being. With increased Ia, Φ also increases. Hence, speed varies
inversely as armature current

Eb is constant
Φ α Ia
So, N α 1/ Ia
3. Speed and torque (N/Ta characteristic)
 It is also known as mechanical characteristic.
 It is found from above that when speed is high, torque is low and viceversa
 It is because an increase in torque requires an increase in armature current,
which is also the field current. The result is that flux is strengthened and
hence the speed drops.

N α Eb/Φ
Eb is constant
T α Φ Ia, so,
N α 1/T
Characteristics of DC Shunt Motor:
1. Torque and armature current (Ta/Ia characteristic)
 It is also known as electrical characteristic
 Since Ta ∝ ΦIa
In this characteristic torque is directaly proportional to the armature
current. T α Φ Ia , where, Φ is constant, so, T α Ia , which gives straight line
in practically.
Characteristics of DC Shunt Motor:
2. Speed and armature current (N/Ia characteristic)

N α Eb/Φ=(V-IaRa)/Φ
 Φ is constant, but practically, there is a slope because of some parameters.
This equation shows that if armature current ( Ia ↑ ) , ( Eb ↓ ) so
speed ( N ↓ ) slightlly as shown in figure.
3. Speed and torque (N/Ta characteristic)
It is also known as Mechanical characteristic
N α Eb/Φ =(V-IaRa)/Φ
Eb is constant
T α Φ Ia, so,
N α 1/T
 In dc Motor Ta α Ia (Φ constant ) So if ( Ia ↑ ) then the ( Ta ↑ )
hence ( Eb ↓ ) so speed N will slightly decrease.
Characteristics of DC Shunt Motor:
Characteristics of DC Compound Motor:
 A compound motor has both shunt and series field winding .The shunt
field winding always stronger then series field winding.
 There are mainly two types of compound motor
1. Cumulative compound motor:
 current direction is same in series and shunt.
2. Differentially compound motor:
 current is in opposite direction in series and shunt winding.
Characteristics of DC Compound Motor:
 Characteristics of Cumulative and differentially compound motor are as below,
DC Motor Stators:
1.
3 Point Stator:
Drawbacks of 3 Point Stator:
 The 3 point starter suffers from a serious drawback for motors with a large
variation of speed by adjustment of the field rheostat.
 To increase the speed of the motor, the field resistance should be increased.
Therefore, the current through the shunt field is reduced.
 The field current may become very low because of the addition of high
resistance to obtain a high speed.
 A very low field current will make the holding electromagnet too weak to
overcome the force exerted by the spring.
 The holding magnet may release the arm of the starter during the normal
operation of the motor and thus, disconnect the motor from the line. This is not
a desirable action.
DC Motor Stators:
1.
4 Point Stator:
Electrical Braking of DC Motor:
 A running motor may be brought to rest quickly by either mechanical
braking or electrical braking.
 Electrical braking is used in applications where frequent, quick, accurate or
emergency stops are required.
 Smooth braking of a motor can be achieved by electric braking
 When a loaded hoist is lowered, electric braking keeps the speed within safe
limits. Otherwise, the machine or drive speed will reach the dangerous values.
When a train goes down a steep gradient, electric braking is employed to hold
the train speed within the prescribed safe limits. Electrical Braking is more
commonly used where active loads are applicable.
 The mechanical braking is applied by means of mechanical break shoes.
Hence the smoothness of mechanical braking is dependent on the surface and
physical condition of brakes.
Disadvantages of Mechanical Braking:
The main disadvantages of the Mechanical Braking are as follows:• It requires frequent maintenance and replacement of brake shoes.
• Braking power is wasted in the form of heat.
Electrical Braking of DC Motor:
In spite of having some disadvantages of mechanical braking, it is also used
along with the electric braking to ensure reliable operation of the drive. It is
also used to hold the drive at the standstill because many braking methods do
not produce torque at standstill condition
Types of Electrical Braking:
There are three types of Electric Braking in a DC motor.
1.
2.
3.
Dynamic or Rheostatic Braking,
Plugging or Reverse Current Braking and
Regenerative Braking
Electrical Braking of DC Motor….
1. Dynamic or Rheostatic Braking:
 In case of DC shunt motors, armature is disconnected from the supply and a
rheostat (variable resistor) is connected across it. The field winding is left
connected across the supply.
Thus the machine will now feed the current to the connected rheostat and
heat will dissipate at the rate of I2R. Braking effect is controlled by varying
the resistance connected across the armature.
 In case of DC series motor, motor is disconnected from the supply and field
connections are reversed and a rheostat is connected in series.
Electrical Braking of DC Motor….
2. Plugging or Reverse Current Braking:
In this method, armature connections are reversed and hence motor tends to
run in opposite direction.
Due to reversal of the armature terminals, applied voltage V and back emf Eb
starts acting in the same direction and hence the total armature current
exceeds.
Electrical Braking of DC Motor….
To limit this armature current a variable resistor is connected across the
armature. This is similar for both series and shunt wound methods.
Plugging gives greater braking torque as compared to rheostatic braking.
Applications: in controlling elevators, Rolling Mills, machine tools, printing
presses etc.
3.
Regenerative Braking:
Regenerative braking is used where, load on the motor has very high inertia
(e.g in electric trains).
When applied voltage to the motor is reduced to less than back emf Eb,
obviously armature current Ia will get reversed, and hence armature torque is
reversed. Thus speed falls.
As generated emf is greater than applied voltage (machine is acting as a DC
generator), power will be returned to the line, this action is called as
regeneration. Speed keeps falling, back emf Eb also falls until it becomes
lower than applied voltage and direction of armature current again becomes
opposite to Eb.
Electrical Braking of DC Motor….
•
•
•
•
Applications of Regenerative Braking:
Regenerative braking is used especially where frequent braking and slowing
of drives is required.
It is most useful in holding a descending load of high potential energy at a
constant speed.
Regenerative braking is used to control the speed of motors driving loads such
as in electric locomotives, elevators, cranes and hoists.
Regenerative braking cannot be used for stopping the motor. It is used for
controlling the speed above the no-load speed of the motor driving.
Brushless DC (BLDC) Motors:
 Brushless DC Motors are a type of synchronous motor
– magnetic fields generated by the stator and rotor rotate at the same
frequency
– no slip
 Available in single-phase, 2-phase, and 3-phase configurations
 BLDC Motor Stator
BLDC Motor Rotors
Hall-Effect:
 If a current-carrying conductor is kept in a magnetic field, the magnetic
field exerts a force on the moving charge carriers, tending to push them to
one side of the conductor, producing a measurable voltage difference
between the two sides of the conductor.
 Hall-Effect Sensors:
• Need 3 sensors to determine the position of the rotor
• When a rotor pole passes a Hall-Effect sensor, get a high or low signal,
indicating that a North or South pole
Transverse Sectional View of Rotor
Commutation Sequence:
 Each sequence has
• one winding energized positive (current into the winding)
• one winding energized negative (current out of the winding)
• one winding non-energized
Torque-Speed Characteristic