ECE 8830 Electric Drives

1
ECE 8830 - Electric Drives
Topic 12 Scalar Control of AC Induction
Motor Drives Spring
2004
2
Introduction

• Scalar control of an ac motor drive is only
  due to variation in the magnitude of the control
  variables. By contrast vector control involves
  the variation of both the magnitude and phase of
  the control variables.
• Voltage can be used to control the air gap
  flux and frequency or slip can be used to control
  the torque. However, flux and torque are
  functions of frequency and voltage, respectively
  but this coupling is disregarded in scalar
  control.

3
Introduction (contd)

• Scalar control produces inferior dynamic
  performance of an ac motor compared to vector
  control but is simpler to implement. In
  variable-speed applications in which a small
  variation of motor speed with loading is
  tolerable, a scalar control system can produce
  adequate performance. However, if precision
  control is required, then a vector control system
  must be used.

4
Speed Control

• Three simple means of limited speed control
  for an induction motor are
• 1) Reduced applied voltage magnitude
• 2) Adjusting rotor circuit resistance
• (suitable for a wound rotor machine
• and discussed earlier)
• 3) Adjusting stator voltage and frequency
• These are discussed in section 9.2 Ong text and
  are not presented further here.

5
Constant Air Gap Flux

• Generally, an induction motor requires a
  nearly constant amplitude of air gap flux for
  satisfactory working of the motor. Since the air
  gap flux is the integral of the voltage impressed
  across the magnetizing inductance, and assuming
  that the air gap voltage is sinusoidal,
• Thus a constant volts/Hz ratio results in a
  constant air gap flux.

6
Constant Air Gap Flux (contd)

• The torque-speed curves with a constant air
  gap flux at different excitation frequencies are
  shown below

7
Constant Air Gap Flux (contd)

• From the curves on the previous slide, it can
  be seen that we will obtain the same torque at
  the same value of slip speed if we operate at a
  constant air gap flux. This is the basis for
  constant volts/Hz control of an induction motor.
  This type of control may be implemented either in
  open loop or in closed loop.

8
Constant Air Gap Flux (contd)

• A set of 6-step voltage waveforms illustrating
  constant volts/Hz is shown below

Ref D.W. Novotny and T.A. Lipo, Vector Control
and Dynamics of AC Drives
9
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter

• Three regions of operation for the induction
• motor are possible
• 1) Holds slip speed constant and regulates stator
  current to obtain constant torque.
• 2) Holds stator voltages at its rated value and
  regulates stator current to obtain constant
  power.
• 3) Holds stator voltage at its rated value and
  regulates slip speed just below its pull-out
  torque value.

10
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)

11
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)
12
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)

• The open loop volts/Hz control of an induction
  motor is very popular because of its simplicity.
  A block diagram of such a control system is shown
  below

13
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter

• The power circuit comprises
• 1) A diode rectifier supplied by either a
• single-phase or three-phase supply
• 2) An LC filter
• 3) A PWM voltage-fed inverter.
• The primary control variable is the frequency
  ?e??r. The commanded phase voltage Vs is
  generated by a gain stage based on the speed ?e
  to maintain a constant air gap flux.

14
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)

• As the frequency becomes small at low speed,
  the voltage drop across the stator resistance can
  no longer be neglected and so a boost voltage V0
  needs to be supplied allowing the rated flux (and
  thus the full torque) to be available down to
  zero speed. The effect of the boost voltage is
  negligible at higher frequencies.

15
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)

• The drives steady state performance for a fan
  or pump-type load (TLK?r2) is shown below

16
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)

• As the frequency is increased the speed
  increases almost proportionally and we move along
  the load torque curve from points 1-gt2-gt3 etc.
  moving smoothly through the different operating
  modes of the induction motor.

17
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)

• Let us now look at the effects of dynamic
  variations in load torque and line voltage.
• Suppose the load torque is changed from TL to
  TL for the same frequency command, the speed
  will drop slightly from ?r to ?r. This type of
  speed variation can easily be tolerated by a fan
  or pump.
• Now suppose the operating point is a and the
  line voltage drops so that the operating point
  moves to b. Again the speed is tolerable for some
  applications.

18
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)

• The safe acceleration/deceleration
  characteristics are shown below

19
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)

• Assume a pure inertia type load and the motor
  initially operating at point 1. A small step
  increase in command frequency will initially move
  the operating point to point 2 (the rated torque)
  and then steadily increase to point 3. The
  frequency can then be decreased slightly to
  achieve the steady state operating point 4. All
  of these transitions are done in a gradual manner
  to prevent the machine from becoming unstable.
  Decrementing the frequency command in a step will
  shift the operating point from 1 to 5 due to a
  negatively developed torque.

20
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)

• The motor torque and speed are related by
• where J moment of inertia,
• Te torque developed by motor,
• and TL load torque
• With the rated Te the slope of the
  acceleration curve d?r/dt is determined by J. The
  higher J, the smaller the slope.

21
Open Loop Volts/Hz Control of a Voltage-Fed
Inverter (contd)

• Typical Volts/Hz drive performance is shown
  below

22
Energy Savings with Variable Frequency Drives

• Considerable energy savings can be achieved
  with variable frequency drives compared to
  constant frequency drives (see figure below and
  text).

23
Closed Loop Volts/Hz Control with Slip Regulation

• An improvement over open loop Volts/Hz control
  is closed loop Volts/Hz control with slip
  regulation (see block diagram below).

24
Closed Loop Volts/Hz Control with Slip Regulation
(contd)

• Here the speed loop error generates a slip
  command ?sl via a proportional-integral
  controller and limiter. This slip command is
  added to the feedback speed signal ?r to get the
  frequency command ?e which, in turn, generates
  the voltage command through a volts/Hz function
  generator. Since slip is proportional to torque
  at constant flux, this approach may be considered
  as open loop torque control within a speed
  control loop.

25
Closed Loop Volts/Hz Control with Slip Regulation
(contd)

• If a step-up speed command is provided, the
  motor accelerates freely until a slip limit
  (corresponding to the motors torque limit) is
  achieved and then settles down to the steady
  state load-limited torque.
• If ?r is stepped down, the drive behaves as a
  generator and decelerates with constant negative
  slip - ?sl. However, the value of - ?sl must
  be limited to a safe margin below the slip speed
  corresponding to the pull-out torque point.

26
Closed Loop Volts/Hz Control with Slip Regulation
(contd)

• Since the slip speed is relatively small
  compared to the rotor speed, this mode of
  operation requires precise measurement of the
  rotor speed. Also, in negative slip mode of
  operation, the regenerated power must either be
  dissipated in a braking resistor or fed back to
  the ac mains.
• One disadvantage of this approach is that the
  flux may drift due to load torque or supply
  voltage variations.

27
Closed Loop Volts/Hz Control with Slip Regulation
(contd)

• A speed control system with closed loop torque
  and flux control is shown below. However,
  additional feedback control loops increases
  system complexity and potential stability
  problems.

28
Current-Regulated Voltage-Fed Inverter Drive

• Instead of controlling inverter voltage by the
  flux loop, the stator current can be controlled
  which has the benefit of providing inherent
  overcurrent protection to the switching devices
  as well as achieving direct control of the motor
  torque and air gap flux.
• A current-regulated VSI drive, with torque and
  flux control in an outer loop and hysteresis-band
  current control in the inner loop, is shown on
  the next slide.

29
Current-Regulated Voltage-Fed Inverter Drive
(contd)

• Flux control loop -gt stator current amplitude
• Torque control loop -gt frequency command
• Only need 2 current sensors since iaibic 0
• (for an isolated motor neutral).

30
Current-Regulated Voltage-Fed Inverter Drive
(contd)

• The performance of the drive for subway
  traction application is shown below

31
Traction Drives with Parallel Machines

• Multiple voltage-fed inverters can be operated
  in parallel. An example of such a system for a
  locomotive drive is discussed in the Bose text,
  pp. 348-349.

32
Current-Fed Inverter Control

• Some of the same principles for control of
  voltage-fed inverters can be applied to
  current-fed inverters. However, the current-fed
  inverter cannot be operated open loop.
• The simplest implementation of a closed loop
  control system for a current-fed inverter,
  allowing independent control of dc link current
  Id and slip ?sl, is shown on the next slide.

33
Current-Fed Inverter Control (contd)

34
Current-Fed Inverter Control (contd)

• In this implementation, the fed back rotor
  speed ?r and command slip ?sl are added to give
  the command frequency ?e. The dc link current Id
  is controlled by a feedback loop that controls
  the output voltage of the rectifier, Vd.
• With ve slip, acceleration occurs with -ve
  slip, Vd and VI both become -ve and power is fed
  back to the source.
• The torque can be controlled either by Id or
  ?sl. However, no flux control is possible with
  this control scheme.

35
Current-Fed Inverter Control (contd)

• Speed and flux control can be achieved in a
  current-fed inverter using the below control
  system.

36
Current-Fed Inverter Control (contd)

• In this case the speed control loop controls
  the torque by slip control (as before) but also
  controls the current Id by a pre-computed
  function generator to maintain a constant flux.
  This open loop approach is satisfactory but the
  machine flux may still vary with parameter
  variations. An independent flux control loop (as
  shown earlier for the voltage-fed inverter) can
  be implemented for tighter flux control if
  desired.

37
Current-Fed Inverter Control (contd)

• A volts/Hz implementation for a current-fed
  inverter is shown below
• A particular advantage of this approach is
  that the motor flux is unaffected by line voltage
  variation.

38
Efficiency Optimization Control by Flux Program

• Normally a motor is operated at its rated flux
  because the developed torque is high and the
  transient response is fast. Under light loads,
  this can lead to poor efficiency of the drive.
  The rotor flux can be lowered at light loads so
  that the motor losses are reduced and the
  conversion efficiency of the drive optimized. See
  text pp. 352-254 (Bose) to see how this may be
  achieved.