ECE 8830 Electric Drives

1
ECE 8830 - Electric Drives
Topic 17 Wound-Field Synchronous
Machine Drives Spring 2004
2
Introduction

• For high power (multi-MW) applications, the
  high efficiency of synchronous motors makes them
  more appealing than induction motors. Indeed,
  most of todays electrical power generators are
  3? synchronous generators.

3
Brushless dc Excitation

• Wound-field synchronous motors require dc
  current excitation in the rotor winding. This
  excitation is traditionally done through the use
  of slip rings and brushes. However, these have
  several disadvantages such as requiring
  maintenance, arcing (which means they cannot be
  used in hazardous environments), etc. An
  alternative approach is to use brushless
  excitation which is illustrated on the next
  slide.

4
Brushless dc Excitation (contd)

5
Brushless dc Excitation (contd)

• A wound-rotor induction motor (WRIM) is
  mounted on the same shaft as the wound-field
  synchronous motor. This is acting as a rotating
  transformer with the rotor as the primary and the
  stator as the secondary. The stator of the WRIM
  is fed by a 60Hz supply and the rotor of the WRIM
  rotates at a speed set by the supply frequency.
  The slip voltage in the rotor winding of the WRIM
  is rectified to provide the current feed to the
  rotor windings of the synchronous motor.

6
Load Commutated Current-Fed Inverters

• Thyristor current-fed, load commutated
  inverters (LCIs) are very popular for high power
  (multi-MW) wound-field synchronous motor drives.
• We will briefly review current-fed thyristor
  inverters and then discuss load commutated
  inverters in some detail. We will then see how to
  apply them to wound-field synchronous motor
  drives.

7
Review of Current-Fed Thyristor Inverter

• Let us first briefly review the operation of
  the current-fed thyristor inverter.

8
Review of Current-Fed Thyristor Inverter (contd)

• Initially, ignore commutation considerations.
• Induction motor load is modeled by back emf
  generator and leakage inductance in each phase of
  the winding.
• The constant dc current Id is switched through
  the thyristors to create a 3? 6-step symmetrical
  line current waves as shown on the next slide.

9
Review of Current-Fed Thyristor Inverter (contd)

10
Review of Current-Fed Thyristor Inverter (contd)

• The load or line current may be expressed by a
  Fourier series as
• where the peak value of the fundamental
  component is given . Each thyristor
  conducts for radians. At any instant one
  upper thyristor and one lower thyristor conduct.

11
Review of Current-Fed Thyristor Inverter (contd)

• The dc link is considered harmonic-free and
  the commutation effect between thyristors is
  ignored.
• At steady state the voltage output from the
  rectifier block input voltage of inverter.
• For a variable speed drive the inverter can be
  operated at variable frequency and variable dc
  current Id.

12
Review of Current-Fed Thyristor Inverter (contd)

• If thyristor firing angle ? gt 0, inverter
  behavior.
• If thyristor firing angle ?0, rectifier
  behavior.
• Max. power transfer occurs when ??.

13
Inverter Operation Modes

• Two inverter operation modes are established
  depending on the thyristor firing angle
• 1) Load-commutated inverter
• Applies when ?/2lt?lt?.
• 2) Force-commutated inverter
• Applies when ?lt?lt3?/2.

14
Load-Commutated Inverter Mode

• Consider ?3?/4. In this case vca lt 0 gt
  thyristor Q5 is turned off by the load. This
  requires load to operate at leading power factor
  gt motoring mode of a synchronous machine
  operating in over-excitation.
• Vd-Vd0cos?

15
Load Commutated Inverters

• Let us initially consider a single-phase
  inverter before discussing the 3? case. A
  single-phase, current-fed, parallel resonant
  inverter with load commutation is shown below

16
Load Commutated Inverters (contd)

• A phase-controlled rectifier provides the dc
  input and a large capacitor C provides the load
  commutation of the thyristors. Assuming perfect
  filtering of harmonics by the capacitor and the
  dc link inductor, the inverter load voltage and
  current waves are shown below

17
Load Commutated Inverters (contd)

• The thyristor pairs Q1Q2 and Q3Q4 are switched
  alternately for ? angle to produce the square
  wave output. The fundamental of the current wave
  leads the sinusoidal voltage wave by ??. Thus,
  when Q1Q2 turn on, the Q3Q4 pair has a negative
  voltage for duration ?? which provides the load
  commutation. Since ??tq, the time tq must be
  sufficiently long for the thyristors to turn off.

18
Load Commutated Inverters (contd)

• The equations for the inverter circuit are
• where Rd is the resistance of the inductor Ld.

19
Load Commutated Inverters (contd)

• These equations can be expressed in
  state-variable form and solved to model the
  steady state and dynamic performance of the
  inverter.
• We will now consider an approximate steady
  state analysis assuming that Ld is of infinite
  size and is lossless. We will also assume that
  the load is highly inductive, i.e. ?LgtgtR.

20
Load Commutated Inverters (contd)

• Consider the series R-L load to be resolved
  into parallel R1 and L1 components in which real
  current IP flows through R1 and reactive current
  IQ flows through L1. The load impedance ZL can be
  written as

21
Load Commutated Inverters (contd)

• If the load is highly inductive (as we had
  assumed) R1gtgt?L1 and
• and L ? L1.
• The fundamental component of the current is
  given by

22
Load Commutated Inverters (contd)

• The real and reactive components of the load
  current are given by
• and
• where . Through some
  algebraic manipulation we get
• and

23
Load Commutated Inverters (contd)

• From the above equations we can calculate the
  load voltage, currents, and commutation angle ?.
• Example
• Single-phase synchronous motor Vd200V,
  f60Hz, R0.2?, L1.2mH, Id240A, C150?F. Find ?.

24
Load Commutated Inverters (contd)

• There are basically two control variables for
  the load commutated inverter - the dc link
  current and the frequency. For a variable load, a
  variable capacitance can be used to provide
  desired margin of commutation angle ?. However, a
  better way to operate is to use a PLL to control
  the inverter frequency to just above the load
  resonance frequency.

25
Load Commutated Inverters (contd)

• The single-phase inverter concepts can be
  extended to 3? LCIs. The figure below shows a
  three-phase LCI with lagging power factor load.
  Here load commutation is achieved by using a
  leading VAR load connected at the load terminal.

26
Load Commutated Inverters (contd)

• In the case of a variable load, a fixed
  capacitor bank is connected at the terminals and
  the inverter frequency adjusted so that the
  effective inverter load has a leading PF so that
  commutation occurs at a fixed angle ?.

27
Load Commutated Inverters (contd)

• As mentioned earlier, thyristor current-fed,
  load commutated inverters (LCIs) are very
  popular for high power (multi-MW) wound-field
  synchronous motor drives where it is easy to
  maintain the required leading PF angle by
  adjusting the field excitation.

28
Load Commutated Inverters (contd)

• The fundamental frequency phasor diagram for a
  salient pole synchronous machine under motoring
  condition is shown below
• Note The winding resistance and the
• commutation overlap effect have been neglected.

29
Load Commutated Inverters (contd)

• A flux linkage has been included in the phasor
  diagram where ?f field flux linkage, ?aarmature
  reaction flux linkage and ?Sresultant stator
  flux. We can write the de and qe components of
  ?a as follows
• For a salient pole machine, Ld?Lq the phasors
  ?a and Is are not in phase.

30
Load Commutated Inverters (contd)

• The motor phase voltage and current waves are
  shown below including the commutation overlap
  effect

31
Load Commutated Inverters (contd)

• The load commutated inverter with an
  over-excited synchronous machine load depends on
  sufficient back emf which is not available at low
  speeds. The critical speed required for load
  commutation to work is about 5 of base speed. A
  forced commutation approach is required below
  these speeds and to start the motor. (see Bose
  text pp. 284-285 for details).

32
Load-Commutated Inverter Drive

• Having seen how a current-fed, thyristor
  inverter can be load commutated with a
  wound-field synchronous motor by operating the
  machine at a leading power factor, we can now
  consider how to design a self-controlled drive
  system for a wound-field synchronous motor based
  on a load-commutated inverter drive. As
  mentioned earlier, this type of drive is popular
  for high power (multi-MW) drives for compressors,
  pumps, ship propulsion, etc.

33
Load-Commutated Inverter Drive (contd)

• A block diagram of a self-controlled
  load-commutated, current-fed inverter drive for a
  wound-field synchronous motor is shown below

34
Load-Commutated Inverter Drive (contd)

• The phasor diagram for the LCI in motoring
  mode driving a synchronous motor is shown below
• Note The saliency and stator resistance have
  been neglected.

35
Load-Commutated Inverter Drive (contd)

• The field flux ?f is established by the field
  current If and depends on the rotor position. The
  armature flux ?a IsLs is determined by the
  stator current and stator winding inductance. The
  delay angle command ?d sets the position of ?a
  relative to ?f since ?a leads ?f by ? given by
• where ? torque angle.

36
Load-Commutated Inverter Drive (contd)

• Thyristors require a minimum turn-off time
  toff for successful commutation. This corresponds
  to a turn-off angle ??toff. For reliable
  operation of a LCI drive and minimum reactive
  current loading to the synchronous motor, turning
  off the thyristors at a fixed time every cycle is
  a good approach. A complete speed control system
  for a LCI synchronous motor drive incorporating
  constant turn-off angle control is shown on the
  next slide.

37
Load-Commutated Inverter Drive (contd)

38
Load-Commutated Inverter Drive (contd)

• This drive operates in the constant torque
  region in motoring mode with stator flux ?s
  maintained constant (open loop). There are four
  control elements
• Speed and dc link current control
• Field flux/field current control
• Generation of ?f, ?d, ? and ? command signals
  (where ? is the commutation overlap angle)
• Delay angle control.

39
Load-Commutated Inverter Drive (contd)

• Speed and dc link current control
• ?r compared to ?r and error goes through P-I
  controller and absolute value circuit -gt Id. Id
  and Id compared and controls thyristor firing
  angle ? in rectifier to control dc link current.
• The generated motor torque ? Id (see Bose text
  pg. 499 for derivation).

40
Load-Commutated Inverter Drive (contd)

• Field flux/field current control
• The command field flux ?f is given by
• where ?s constant, ?aLsIsKaId and
• ? ?k?. To obtain ? we need ? which can
  either be measured or calculated using the
  expression

41
Load-Commutated Inverter Drive (contd)

• The command flux current If is then generated
  from the command flux ?f by through a function
  generator that corrects for saturation effects. A
  phase-controlled rectifier can then be used to
  control the flux current as shown in the system
  block diagram.

42
Load-Commutated Inverter Drive (contd)

• Generation of ?f, ?d, ? and ? command
  signals
• We have discussed how all of the command
  signals can be obtained with the exception of the
  ? angle. This is obtained from the equation

43
Load-Commutated Inverter Drive (contd)

• Delay Angle Control
• For a six-step inverter we need six discrete
  firing pulses at ?/3 intervals apart within a
  cycle. A block diagram showing how this can be
  achieved is shown on the next slide.

44
Load-Commutated Inverter Drive (contd)

45
Load-Commutated Inverter Drive (contd)
46
Load-Commutated Inverter Drive (contd)

• The corresponding alignment of reference
  signal S1 and the waveforms for phase a in
  motoring mode are shown below. These diagrams can
  be used to determine the inverter firing angles.

47
Load-Commutated Inverter Drive (contd)

• The absolute position sensor can be eliminated
  and the machine terminal voltage signals can be
  used instead to estimate the rotor position for
  inverter firing angle determination. Details are
  presented in the Bose textbook pp. 504-507.

48
Cycloconverter Drive

• High power, wound-field synchronous motors can
  be operated at unity power factor when excited by
  phase-controlled, line-commutated, thyristor
  cycloconverters. Drive control for such drives
  can be both scalar and vector control, similar to
  that of the voltage-fed inverter drive.
• The next slide shows a simple scalar control
  method for a cycloconverter drive for a
  wound-field synchronous motor.

49
Cycloconverter Drive (contd)

50
Cycloconverter Drive (contd)

• There are three control variables in this
  control system
• The stator current amplitude,
• Phase angle, ? (see phasor diagram below)
• The field current, If.

51
Cycloconverter Drive (contd)

• The torque generated by the motor is
  proportional to the in-phase stator current. The
  command stator current Is is generated from the
  error in the speed control loop.
• The angle ? and the field current If can be
  determined from Is as
• shown in the figure. Thus,
• Is is used to generate ?
• and If using function
• generators.

52
Cycloconverter Drive (contd)

• The position sensor and encoder generate the
  cos?e and sin?e signals and the speed signal, ?r.
  The 2-phase unit signals are converted to 3-phase
  unit signals using the following transformations

53
Cycloconverter Drive (contd)

• Each of the 3? unit signals is then multiplied
  by Is and phase shifted by angle ? to produce
  the three phase current command signals as
  follows

54
Cycloconverter Drive (contd)

• The performance of the cycloconverter drive
  can be enhanced if vector control is used rather
  than scalar control. In the constant torque
  region, the field current must be increased if we
  want to increase the developed torque at the
  constant rated stator flux. However, the field
  current response is slow and this leads to
  sluggish motor response. In vector control we
  inject a transient magnetizing current in the
  direction of the stator flux to obtain a much
  faster response than with scalar control. This
  current is set to zero in steady state to
  maintain unity PF.

55
Cycloconverter Drive (contd)

• A vector control implementation is shown

56
Cycloconverter Drive (contd)

• A phasor diagram for the vector control
  approach is shown below

57
Cycloconverter Drive (contd)

• Notable points from phasor diagram
• The torque component of the stator current IT is
  in phase with Vs and forms a triangle with Is and
  the injected magnetizing current IM. IM0 at
  steady state and ITIs.
• The magnetizing current, Im, the field current
  If, and the torque component of the stator
  current IT form a right-angled triangle (which is
  a scaled version of the flux triangle).

58
Cycloconverter Drive (contd)

• There are three sets of d-q axes
• - de-qe in reference frame of rotor
• - ds-qs in reference frame of stator
• - de-qe with qe aligned with Vs and de
  aligned
• with ?s.
• At steady state, ?s and ?a are at quadrature.
  Also, Is is in phase with Vs which leads ?s by
  90 gt unity PF.

59
Cycloconverter Drive (contd)

• From the phasor diagram, at steady state, we
  can write
• This equation gives the control equation for
  If. Under transient conditions, the command
  injected transient magnetizing current, IM, is
  given by
• Under steady state conditions, IM0 and the
  above steady state equation is re-established.

60
Cycloconverter Drive (contd)

• Control features of the vector control of a
  wound-field synchronous motor drive
• Speed control error generates the torque
  component of current through P-I controller.
• Command currents IT and IM are compared to
  feedback currents, IT and IM to generate command
  voltages vT and vM through P-I compensators.
• A vector rotator uses unit control signals cos?
  and sin? to transform the vT and vM signals to
  phase command voltages va, vb, and vc.

61
Cycloconverter Drive (contd)

• A transient change in the required torque causes
  IM to be injected because of the sluggish
  response of If, thereby maintaining a constant
  flux ?s. As If builds up, IM drops down to reach
  zero when If has reached its new steady state
  value.
• The complete vector control feedback signal
  processing is shown on the next slide.

62
Cycloconverter Drive (contd)