Induction Motor

1

• Induction Motor Scalar Control
• By
• Mr.M.Kaliamoorthy
• Department of Electrical Electroncis
  Engineering
• PSNA College of Engineering and Technology

2
Outline

• Introduction
• Speed Control of Induction Motors
• Pole Changing
• Variable-Voltage, Constant Frequency
• Variable Frequency
• Constant Volts/Hz (V/f) Control
• Open-loop Implementation
• Closed-loop Implementation
• Constant Airgap Flux Control
• References

3
INDUCTION MOTOR DRIVES
Three-phase induction motor are commonly used in
adjustable-speed drives (ASD).
Basic part of three-phase induction motor

• Stator
• Rotor
• Air gap

4
The stator winding are supplied with balanced
three-phase AC voltage, which produce induced
voltage in the rotor windings. It is possible to
arrange the distribution of stator winding so
that there is an effect of multiple poles,
producing several cycle of magnetomotive force
(mmf) or field around the air gap. The speed of
rotation of field is called the synchronous speed
ws , which is defined by
?s is syncronous speed rad/sec Ns is
syncronous speed rpm p is numbers of
poles ? is the supply frequency rad/sec f
is the supply frequency Hz Nm is motor
speed
or
5
The motor speed
The rotor speed or motor speed is
Where S is slip, as defined as
Or
6
The motor speed
The rotor speed or motor speed is
Where S is slip, as defined as
Or
7
Equivalent Circuit Of Induction Motor
Where Rs is resistance per-phase of stator
winding Rr is resistance per-phase of rotor
winding Xs is leakage reactance per-phase of the
winding stator Xs is leakage reactance per-phase
of the winding rotor Xm is magnetizing
reactance Rm is Core losses as a reactance
8
Performance Characteristic of Induction Motor
Stator copper loss
Rotor copper loss
Core losses
9
Performance Characteristic of Induction Motor

• Power developed on air gap (Power fropm stator
  to
• rotor through air gap)

• Power developed by motor

or
or

• Torque of motor

or
10
Performance Characteristic of Induction Motor
Input power of motor
Output power of motor
Efficiency
11
Performance Characteristic of Induction Motor
If
and
so, the efficiency can calculated as
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Performance Characteristic of Induction Motor
Generally, value of reactance magnetization Xm gtgt
value Rm (core losses) and also
So, the magnetizing voltage same with the input
voltage
Therefore, the equivalent circuit is
Xm
13
Performance Characteristic of Induction Motor
Xm
The rotor current is
14
Torque speed Characteristic
15
Introduction
Te
Pull out Torque (Tmax)
Trated
What if the load must be operated here?
?r
?s
?rotor
s
Requires speed control of motor
1
0
16
Speed Control of IM

• Given a load T? characteristic, the steady-state
  speed can be changed by altering the T? curve of
  the motor

Varying voltage (amplitude)
2
Varying line frequency
3
Pole Changing
1
17
Speed Control of IM

• Variable-Voltage (amplitude), Constant Frequency
• Controlled using
• Transformer (rarely used)
• Thyristor voltage controller
• thyristors connected in anti-parallel
• motor can be star or delta connected
• voltage control by firing angle control
• (gating signals are synchronized to phase
  voltages and are spaced at 60? intervals)
• Only for operations in Quadrant 1 and Quadrant 3
  (requires reversal of phase sequence)
• also used for soft start of motors

18
Speed Control of IM

• Variable-Voltage (amplitude), Constant Frequency
• Voltage can only be reduced from rated Vs (i.e. 0
  lt Vs Vs,rated)
• From torque equation, Te ? Vs2
• When Vs ?, Te and speed reduces.
• If terminal voltage is reduced to bVs, (i.e. Vs
  bVs,rated)
• Note b ? 1

19
Speed Control of IM

• Variable-Voltage (amplitude), Constant Frequency
• Suitable for applications where torque demand
  reduces with speed (eg fan and pump drives
  where TL ? ?m2)
• Suitable for NEMA Class D (high-slip, high Rr)
  type motors
• High rotor copper loss, low efficiency motors
• get appreciable speed range

Practical speed range
20
Speed Control of IM

• Variable Voltage (amplitude), Constant Frequency
• Disadvantages
• limited speed range ? when applied to Class B
  (low-slip) motors
• Excessive stator currents at low speeds ? high
  copper losses
• Distorted phase current in machine and line
  (harmonics introduced by thyristor switching)
• Poor line power factor (power factor
  proportional to firing angle)
• Hence, only used on low-power, appliance-type
  motors where efficiency is not important
• e.g. small fan or pumps drives

21
Speed Control of IM

• Variable Frequency
• Speed control above rated (base) speed
• Requires the use of PWM inverters to control
  frequency of motor
• Frequency increased (i.e. ?s increased)
• Stator voltage held constant at rated value
• Airgap flux and rotor current decreases
• Developed torque decreases Te ? (1/?s)
• For control below base speed use Constant
  Volts/Hz method

22
Constant Volts/Hz (V/f) Control

• Airgap flux in the motor is related to the
  induced stator voltage E1
• For below base speed operation
• Frequency reduced at rated Vs - airgap flux
  saturates
• (f ? ,?ag ? and enters saturation region oh B-H
  curve)
• - excessive stator currents flow
• - distortion of flux wave
• - increase in core losses and stator copper loss
• Hence, keep ?ag rated flux
• stator voltage Vs must be reduced proportional to
  reduction in f (i.e. maintaining Vs / f ratio)

Assuming small voltage drop across Rs and Lls
23
Constant Volts/Hz (V/f) Control

• Max. torque remains almost constant
• For low speed operation
• cant ignore voltage drop across Rs and Lls (i.e.
  E1 ? Vs)
• poor torque capability(i.e. torque decreased at
  low speeds shown by dotted lines)
• stator voltage must be boosted to compensate
  for voltage drop at Rs and Lls and maintain
  constant ?ag
• For above base speed operation (f gt frated)
• stator voltage maintained at rated value
• Same as Variable Frequency control (refer to
  slide 13)

24
Constant Volts/Hz (V/f) Control
Vs
Vs vs. f relation in Constant Volts/Hz drives
Boost - to compensate for voltage drop at Rs and
Lls

• Linear offset curve
• for high-starting torque loads
• employed for most applications
• Non-linear offset curve
• for low-starting torque loads

f
25
Constant Volts/Hz (V/f) Control

• For operation at frequency K times rated
  frequency
• fs Kfs,rated ? ?s K?s,rated
  (1)
• (Note in (1) , speed is given as mechanical
  speed)
• Stator voltage
  (2)
• Voltage-to-frequency ratio d constant
• (3)

26
Constant Volts/Hz (V/f) Control

• For operation at frequency K times rated
  frequency
• Hence, the torque produced by the motor
• (4)
• where ?s and Vs are calculated from (1) and (2)
  respectively.

27
Constant Volts/Hz (V/f) Control

• For operation at frequency K times rated
  frequency
• The slip for maximum torque is
• (5)
• The maximum torque is then given by
• (6)
• where ?s and Vs are calculated from (1) and (2)
  respectively.

28
Constant Volts/Hz (V/f) Control
Constant Torque Area (below base speed)

• Field Weakening Mode (f gt frated)
• Reduced flux (since Vs is constant)
• Torque reduces
• Constant Power Area
• (above base speed)

Rated (Base) frequency
Note Operation restricted between synchronous
speed and Tmax for motoring and braking regions,
i.e. in the linear region of the torque-speed
curve.
29
Constant Volts/Hz (V/f) Control
Constant Torque Area
Constant Power Area
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Example

• A 4-pole, 3 phase, 400 V, 50 Hz, 1470 rpm
  induction motor has a rated torque of 30 Nm. The
  motor is used to drive a linear load with
  characteristic given by TL K?, such that the
  speed equals rated value at rated torque. If a
  constant Volts/Hz control method is employed,
  calculate
• The constant K in the TL -? characteristic of the
  load.
• Synchronous and motor speeds at 0.6 rated torque.
  
• If a starting torque of 1.2 times rated torque is
  required, what should be the voltage and
  frequency applied at start-up? State any
  assumptions made for this calculation.
• Answers
• K 0.195, synchronous speed 899.47 rpm motor
  speed 881.47 rpm, At start up frequency 1.2
  Hz, Voltage 9.6 V

31
Constant Volts/Hz (V/f) Control Open-loop
Implementation
PWM Voltage-Source Inverter (VSI)
Note ?e ?s synchronous speed
32
Constant Volts/Hz (V/f) Control Open-loop
Implementation

• Most popular speed control method because it is
  easy to implement
• Used in low-performance applications
• where precise speed control unnecessary
• Speed command ?s - primary control variable
• Phase voltage command Vs generated from V/f
  relation(shown as the G in slide 23)
• Boost voltage Vo is added at low speeds
• Constant voltage applied above base speed
• Sinusoidal phase voltages (vabc) is then
  generated from Vs ?s where ?s is obtained
  from the integral of ?s
• vabc employed in PWM inverter connected to motor

33
Constant Volts/Hz (V/f) Control Open-loop
Implementation

• Problems in open-loop drive operation
• Motor speed not controlled precisely
• primary control variable is synchronous speed ?s
• actual motor speed ?r is less than ?s due to ?sl
• ?sl depends on load connected to motor
• ?sl cannot be maintained since ?r not measured
• can lead to operation in unstable region of T-?
  characteristic
• stator currents can exceed rated value
  endangering inverter-converter combination
• Problems (to an extent) can be overcome by
• Open-loop Constant Volts/Hz Drive with Slip
  Compensation
• Closed-loop implementation - having outer speed
  loop with slip regulation

34
Constant Volts/Hz (V/f) Control Open-loop
Implementation
Open-loop Constant Volts/Hz Drive with Slip
Compensation - Slip speed is estimated and added
to the reference speed ?r
Note ?e ?s synchronous speed
35
Constant Volts/Hz (V/f) Control Open-loop
Implementation
Open-loop Constant Volts/Hz Drive with Slip
Compensation

• How is ?sl estimated in the Slip Compensator?
• Using T-? curve, ?sl ? Te
• ?sl can be estimated by estimating torque where
• (8)
• (9)

(7)
Note In the figure, ?slip ?sl slip
speed ?syn ?s synchronous speed
36
Constant Volts/Hz (V/f) Control Closed-loop
Implementation
Open-loop system (as in slide 23)
Slip Controller
Note ?e ?s synchronous speed
37
Constant Volts/Hz (V/f) Control Closed-loop
Implementation

• Reference motor speed ?r is compared to the
  actual speed ?r to obtain the speed loop error
• Speed loop error generates slip command ?sl from
  PI controller and limiter
• Limiter ensures that the ?sl is kept within the
  allowable slip speed of the motor (i.e. ?sl ?
  slip speed for maximum torque)
• ?sl is then added to the actual motor speed ?r
  to generate synchronous speed command ?s (or
  frequency command)
• ?s generates voltage command Vs from V/f
  relation
• Boost voltage is added at low speeds
• Constant voltage applied above base speed
• Scheme can be considered open loop torque control
  (since T ? s) within speed control loop

38
Constant Airgap Flux Control

• Constant V/f control employs the use of variable
  frequency voltage source inverters (VSI)
• Constant Airgap Flux control employs variable
  frequency current source inverters or
  current-controlled VSI
• Provides better performance compared to Constant
  V/f control with Slip Compensation
• airgap flux is maintained at rated value through
  stator current control
• Speed response similar to equivalent
  separately-excited dc motor drive but torque and
  flux channels still coupled
• Fast torque response means
• High-performance drive obtained
• Suitable for demanding applications
• Able to replace separately-excited dc motor
  drives
• Above only true is airgap flux remains constant
  at rated value

39
Constant Airgap Flux Control

• Constant airgap flux in the motor means
• For ?ag to be kept constant at rated value, the
  magnetising current Im must remain constant at
  rated value
• Hence, in this control scheme stator current Is
  is controlled to maintain Im at rated value

Assuming small voltage drop across Rs and Lls
Controlled to maintain Im at rated
maintain at rated
40
Constant Airgap Flux Control

• From torque equation (with ?ag kept constant at
  rated value),
• since s?s ?sl and ignoring Rs and Lls,
• By rearranging the equation

Te ? ?sl ? ?sl can be varied instantly ?
instantaneous (fast) Te
response
41
Constant Airgap Flux Control

• Constant airgap flux requires control of
  magnetising current Im which is not accessible
• From equivalent circuit (on slide 31)
• From equation (10), plot Is against ?sl when Im
  is kept at rated value.
• Drive is operated to maintain Is against ?sl
  relationship when frequency is changed to control
  speed.
• Hence, control is achieved by controlling stator
  current Is and stator frequency
• Is controlled using current-controlled VSI
• Control scheme sensitive to parameter variation
  (due to Tr and ?r)

(10)
42
Constant Airgap Flux Control - Implementation
Current Controlled VSI

• Current controller options
• Hysteresis Controller
• PI controller PWM

ia
ib
ic
Equation (10) (from slide 33)
?r
43
Current-Controlled VSI Implementation

• Hysteresis Controller

44
Current-Controlled VSI Implementation

• PI Controller Sinusoidal PWM

Voltage Source Inverter (VSI)

• Due to interactions between phases
• (assuming balanced conditions)
• ? actually only require 2 controllers

45
Current-Controlled VSI Implementation

• PI Controller Sinusoidal PWM (2 phase)

ia
abc?dq
id
PWM Voltage Source Inverter (VSI)
ib
iq
ic
iq
id
Motor
46
References

• Krishnan, R., Electric Motor Drives Modeling,
  Analysis and Control, Prentice-Hall, New Jersey,
  2001.
• Bose, B. K., Modern Power Electronics and AC
  drives, Prentice-Hall, New Jersey, 2002.
• Trzynadlowski, A. M., Control of Induction
  Motors, Academic Press, San Diego, 2001.
• Rashid, M.H, Power Electronics Circuit, Devices
  and Applictions, 3rd ed., Pearson, New-Jersey,
  2004.
• Nik Idris, N. R., Short Course Notes on
  Electrical Drives, UNITEN/UTM, 2008.
• Ahmad Azli, N., Short Course Notes on Electrical
  Drives, UNITEN/UTM, 2008.