Download AC induction - final notes

EET421
Power Electronic Drives
– Induction Motor & drives
Abdul Rahim Abdul Razak
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INDUCTION MOTORS
Per-phase equivalent circuit ….
(Rashid)
In real world:
•Value of Xm is very large (Rm
can be omitted)
•Xm2 >> (Rs2 + Xs2)  Vs ≈Vm
•Thus, for circuit simplification,
the magnetizing reactance Xm
can be moved-out to the stator
winding.
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INDUCTION MOTORS
The input impedance of the motor becomes:
Power factor angle:
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INDUCTION MOTORS
Rotor current (rms):
Previously noted,
PAG or Pg=3Ir2Rr /s
And developed torque τdev or,
Thus ,
τind =Pdev / ωm = PAG / ωsync
--(15-18)
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INDUCTION MOTORS
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Extended Torque-speed characteristic ..
Pullout torque or
breakdown torque =
max possible
handling torque.
The curve is nearly linear
up to full load (Rr >> Xr,
thus Ir,Br and Tind rise
∞ increasing slip).
s
Plugging = reversing of magnetic
field rotation by switching any two
stator phases. The reversed Tind
will stop the motor rapidly and
rotate it to reverse direction.
If speed driven
faster than ns the
Tind reverses and
motor becomes
generator.
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Extended Torque-speed characteristic ..
Pullout torque or
breakdown torque =
max possible
handling torque.
The curve is nearly linear
up to full load (Rr >> Xr,
thus Ir,Br and Tind rise
∞ increasing slip).
If speed driven
faster than ns the
Tind reverses and
motor becomes
generator.
Plugging = reversing of magnetic
field rotation by switching any two
stator phases. The reversed8Tind
will stop the motor rapidly and
ABD
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rotate it to reverse
direction.
INDUCTION MOTORS – DRIVES
Speed & torque control strategies
A) Change nsync
1. Pole changing
2. Line frequency
control
B) Change slip, s
3. Rotor-resistance control
4. Rotor slip-energy
recovery
5. Line/stator voltage
control
C) Parameters alteration
6. Rotor voltage control
7. Stator current control
8. Stator voltage and
frequency control
9. Voltage, current and
frequency control
120f e
ns 
P
n m  ( 1  s )n sync
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INDUCTION MOTORS – DRIVES
With the existence & development of modern
solid-state variable-frequency motor drives.
•
The modern favorable speed and torque controller of
induction motors :
1.
2.
3.
4.
5.
6.
•
Stator voltage control
Rotor voltage control
Frequency control
Stator voltage and frequency control
Stator current control
Voltage, current, and frequency control
To meet the torque-speed duty cycle of a drive, the voltage,
current, and frequency control are normally used.
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INDUCTION MOTORS – Speed Control
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1.
Stator voltage control
 the torque is proportional to the square of the stator supply voltage,
a reduction in stator voltage will produce a reduction in speed.
 If the terminal voltage is reduced to bVs, where b ≤ 1,Typical torquespeed characteristics for various values of b :
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1.
Stator voltage control
In any magnetic circuit, the induced voltage is proportional to flux
and frequency, and the rms air-gap flux can be expressed as
or
(15-31)
Where,
Km is a constant and depends on the number of turns of
the stator winding.
As the stator voltage is reduced, the air-gap flux and the torque
are also reduced.
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
The stator voltage can be varied by three-phase
1.
2.
3.




AC voltage controllers,
voltage-fed variable DC-link inverters, or
pulse-width modulation (PWM) inverters.
However, due to limited speed range requirements, the AC
voltage controllers are normally used to provide the voltage
control.
The ac voltage controllers are very simple. However, the
harmonic contents are high and the input PF of the controllers is
low.
They are used mainly in low-power applications, such as fans,
blowers, and centrifugal pumps, where the starting torque is low.
They are also used for starting high-power induction motors to
limit the in-rush current.
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1) AC voltage controller & soft start
(to apply an adjustable voltage to the motor and increase this voltage
gradually over a user-selectable acceleration period)
Advantages :
•Reduced starting current
•Reduced starting torque
•Less mechanical stress
•Improved control of
acceleration and deceleration
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Example 15.2a:
A 3-phase 460V, 4-pole Y-connected induction motor has below
parameters:
Rs = 1.5ohm, Rr = 0.75ohm, Xs = 1.5ohm, Xr = 2.25ohm and Xm =
65ohm.
The no-load loss, Pno-load is negligible. The load torque is
proportional to the speed squared, is 45Nm at 1740rpm. If the
motor speed is 1550rpm, determine :
a) Load torque, τL
b) The rotor current, IR
c) Stator supply voltage , Va
d) Motor input current, Ii
e) Motor input power , Pi
f) The slip for maximum current ,sa
g) Maximum rotor current , Ir(max)
h) Speed at maximum rotor current, ωa
i) Torque at maximum current, τ a
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2. Rotor voltage control (Rotor resistance control)
Wound rotor induction motor.
Breakdown torque peak is shifted to zero speed by increasing
rotor resistance.
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2. Rotor voltage control (Rotor resistance control)
 applicable for a wound-rotor motor type only.
 an external three-phase resistor may be connected thru slip rings as
shown in figure:
The method may increase starting torque while limiting starting
current.
 However, this is an inefficient method and there would be imbalance
voltages and currents if the resistances in the rotor circuit are not equal.
The increase in resistance does not affect the value of maximum
torque but increases the slip at maximum torque.
 The wound-rotor motors are widely used in application which need
frequent starting and braking with large motor torque (hoist, crane).
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-Applicable for wound-rotor induction
motors only
-The shape of torque-speed curve is
altered by inserting extra
resistances to the rotor circuit
-Inserting extra resistances, will
seriously reduce efficiency of the
motor
-Usually applied for short period only.
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2. Rotor voltage control (Rotor resistance control)
•
•
The three-phase resistor may be replaced by a three-phase diode
rectifier and a dc converter, as shown in Figure, where the gateturn-off thyristor (GTO) or an insulated-gate bipolar transistor (IGBT)
operates as a dc converter switch.
The inductor Ld acts as a current source Id and the dc converter
varies the effective resistance, which can be found from Eq. (14.45):
Re = R (l - k)
•
•
(15.41)
where k is the duty cycle of the dc converter and the motor speed
can be controlled by varying the duty cycle.
The portion of the air-gap power, which is not converted into
mechanical power, is called slip power. The slip power is dissipated
in R.
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2. Rotor voltage control (Rotor resistance control)
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2. Rotor voltage control (Rotor resistance control)
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2. Rotor voltage control (Rotor resistance control)
Example 15.3a:
Rs = 0.05Ω, Rr =0.055Ω, Xs = 0.31Ω, Xr = 0.35Ω and Xm = 7.1Ω
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3.
•
•
•
•
•
Frequency control
The torque and speed of induction motors can be controlled by
changing the supply frequency.
We can notice from Eq. (15.31) that at the rated voltage and rated
frequency, the flux is the rated value.
If the voltage is maintained fixed at its rated value while the frequency
is reduced below its rated value, the flux increases.
This would cause saturation of the air-gap flux, and the motor
parameters would not be valid in determining the torque-speed
characteristics.
At low frequency, the reactances decrease and the motor current may
be too high. This type of frequency control is not normally used.
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3.
•
Frequency control
If the frequency is increased above its rated value, the flux and torque
would decrease. If the synchronous speed corresponding to the rated
frequency is called the base speed ω b, the synchronous speed at
any other frequency becomes,
thus,
and
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3.
Frequency control
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3.
Frequency control
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3.
Frequency control
- Rotation of magnetic fields nsync will change in direct proportion to
change in electrical frequency, fe. so does the no-load point on the
torque-speed characteristic curve.
-The base speed will change with the same pattern.
-Controlling under base speed, terminal voltage level and maximum
operating torque MUST be derated accordingly.
-Beyond based speed (up to 2p.u), voltage should be maintained to
take care on its insulation.
-but the maximum operating torque will automatically decreasing.
-The motor is now operating in so called “field weakening mode”
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INDUCTION MOTORS – Speed Control
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Line frequency Control
The important of derating…
the stator voltage is not decreased nearly with decreasing of stator
frequency, the steel in the core of the induction motor will saturate and
excessive magnetization currents will flow in the machine.
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Line frequency Control
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4.
Voltage and frequency control (volt/hertz control )
-From previous equation , flux is direct proportional to stator voltage and
inverse proportional with frequency :
- thus, if the ratio is kept constant, the flux value will remain constant.
- the maximum torque (τm) can be maintained constant as it only
depends on base speed (ωb) and β.
- Once ωb ↓, β ↓ thus Va
need to be ↓ as well in
order to maintain the
same Tm value.
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4.
Voltage and frequency control (volt/hertz control )
-As the frequency reduced, β decreases, thus the slip at maximum torque
increases.
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4.
Voltage and frequency control (volt/hertz control )
- Volt/hertz control usually applied to maintain torque while speed is varied.
- voltage supply are obtained from 3-phase inverter or cycloconverters
- as for very large power applications (locomotives , cement mills)
cycloconverters are used.
- As the frequency reduced, β decreases, thus the slip at maximum torque
increases.
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3 possible circuit arrangement for obtaining variable voltage and
frequency :
- regeneration is not possible and inverter would generate harmonics into the AC
supply.
- harmonics into the AC supply is reduced.
- input power factor is low
- Regeneration is possible
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5.
Current control
- torque of induction motor can be decreased by reducing rotor
current.
- but only the accessible input
current is varied.
- at fixed input current, the rotor
current depends on the relative
values of magnetizing and rotor
circuit impedance.
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5.
Current control
- maximum torque depends on squared of current.
-starting torque is low due to Xm is large compared to Xs an Xr
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5.
Current control
- as the speed increase, slip decrease, stator voltage rises and the torque
increase.
-Torque is increased due to increasing flux. Magnetizing current also increased
and saturation will happen if the input current is not limited.
- the constant current can be supplied by 3-phases CSI.
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2 possible CSI circuit for Current control :
-Inductor act as current source
- controlled rectifier controlling
the current source
- input power factor is very low.
- the chopper controlling the
current source
- input power factor is higher.
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6.
Voltage, current and frequency control
- In some application or requirement, all the 3 parameters might have to
be adjusted.
-
- 1st region: speed is varied by voltage (or current) control at constant
torque.
- 2nd region: motor operated at constant current and slip is varied.
- 3rd region: speed is controlled by frequency at reduced stator current.42
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6.
Voltage, current and frequency control
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electric drives control :
closed loop : means that some sort of feedback device
attached to the motor feed speed information back to the drive
for use in correcting any speed discrepancies.
Open loop: like most AC Drives, means no such feedback
exists, and the drive assumes that what it told the motor to do
is actually being done.
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Modern electric drives (With closed-loop control )
- closed-loop control is required to satisfy the steady-state and
transient performance specs of AC drives
- control strategies:
1) scalar control: control variable are DC quantities and only
magnitude are controlled.
2) Vector control : both magnitude and phase of control variables.
3) Adaptive control: parameters are continuously varied to adapt
the output variables variations.
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Modern electric drives (With closed-loop control )
Input
Output
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Modern electric drives (With closed-loop control )
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Choice of Electrical Drives
Choice of an electrical drive depends on a number of factors. Some of the
important factors are :
1.
Steady state operation requirement : nature of speed-torque
characteristics, speed regulation, speed range, efficiency, duty cycle,
quadrants of operation, speed fluctuations if any, ratings.
2.
Transient operation requirement: values of acceleration and
deceleration, starting, braking and reversing performance.
3.
Requirement related to the source : type of source, and its capacity,
magnitude of voltage, voltage fluctuations, power factor, harmonics and
their effect on other load, ability to accept regenerated power.
4.
Capital and running cost, maintenance needs, lifespan expectation.
5.
Space and weight restrictions if any.
6.
Environment and location. 7. Reliability.
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