ECE 8830 - Electric Drives

1
ECE 8830 - Electric Drives
Topic 11 Slip-Recovery Drives for
Wound-Field Induction Motors
Spring 2004
2
Introduction

• In a wound-field induction motor the slip
  rings allow easy recovery of the slip power which
  can be electronically controlled to control the
  speed of the motor.
• The oldest and simplest technique to invoke
  this slip-power recovery induction motor speed
  control is to mechanically vary the rotor
  resistance.

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Introduction (contd)

• Slip-power recovery drives are used in the
  following applications
• Large-capacity pumps and fan drives
• Variable-speed wind energy systems
• Shipboard VSCF (variable-speed/constant
  frequency) systems
• Variable speed hydro-pumps/generators
• Utility system flywheel energy storage systems

4
Speed Control by Rotor Rheostat

• Recall that the torque-slip equation for an
  induction motor is given by
• From this equation it is clear that the
  torque-slip curves are dependent on the rotor
  resistance Rr. The curves for different rotor
  resistances are shown on the next slide for four
  different rotor resistances (R1-R4) with
  R4gtR3gtR2gtR1.

5
Speed Control by Rotor Rheostat (contd)

6
Speed Control by Rotor Rheostat (contd)

• With R10, i.e. slip rings shorted, speed is
  determined by rated load torque (pt. A). As Rr
  increases, curve becomes flatter leading to
  lower speed until speed becomes zero for Rr gtR4.
• Although this approach is very simple, it is
  also very inefficient because the slip energy is
  wasted in the rotor resistance.

7
Speed Control by Rotor Rheostat (contd)

• An electronic chopper implementation is also
  possible as shown below but is equally
  inefficient.

8
Static Kramer Drive

• Instead of wasting the slip power in the rotor
  circuit resistance, a better approach is to
  convert it to ac line power and return it back to
  the line. Two types of converter provide this
  approach
• 1) Static Kramer Drive - only allows
• operation at sub-synchronous speed.
• 2) Static Scherbius Drive - allows
• operation above and below
• synchronous speed.

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Static Kramer Drive (contd)

• A schematic of the static Kramer drive is
  shown below

10
Static Kramer Drive (contd)

• The machine air gap flux is created by the
  stator supply and is essentially constant. The
  rotor current is ideally a 6-step wave in phase
  with the rotor voltage.
• The motor fundamental phasor diagram referred
  to the stator is as shown below

Vs stator phase voltage, Isstator current,
Irf fundamental rotor current referred to
the stator, ?g air gap flux, Immagnetizing
current, and ?PF angle.
11
Static Kramer Drive (contd)

• The voltage Vd is proportional to slip, s and
  the current Id is proportional to torque. At a
  particular speed, the inverters firing angle can
  be decreased to decrease the voltage VI. This
  will increase Id and thus the torque. A
  simplified torque-speed expression for this
  implementation is developed next.

12
Static Kramer Drive (contd)

• Voltage Vd (neglecting stator and rotor
  voltage drops) is given by
• where sper unit slip, VL stator line voltage
  and n1stator-to-rotor turns ratio. The inverter
  dc voltage VI is given by
• where n2transformer turns ratio (line side to
  inverter side) and ?inverter firing angle.

13
Static Kramer Drive (contd)

• For inverter operation, ?/2lt?lt?. In steady
  state VdVI (neglecting ESR loss in inductor)
• gt
• The rotor speed ?r is given by
• 
  if n1n2
• Thus rotor speed can be controlled by
  controlling inverter firing angle, ?.
  At ??, ?r0 and at ??/2 , ?r?e.

14
Static Kramer Drive (contd)

• It can be shown (see text) that the torque may
  be expressed as
• The below figure shows the torque-speed curves
  at different inverter angles.

15
Static Kramer Drive (contd)

• The fundamental component of the rotor current
  lags the rotor phase voltage by ?r because of a
  commutation overlap angle ? (see figure below).
  At near zero slip when rotor voltage is small,
  this overlap angle can exceed ?/3 resulting in
  shorting of the upper and lower diodes.

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Static Kramer Drive (contd)

• The phasor diagram for a static Kramer drive at
  rated voltage is shown below
• Note All phasors are referred to stator.

IL
17
Static Kramer Drive (contd)

• On the inverter side, reactive power is drawn
  by the line -gt reduction in power factor (?Lgt
  ?s). The inverter line current phasor is IT. The
  figure shows IT at s0.5 for n1n2. The real
  component ITcos? opposes the real component of
  the stator current but the reactive component
  ITsin? adds to the stator magnetizing current.
  The total line current IL is the phasor sum of IT
  and IS. With constant torque, the magnitude of IT
  is constant but as slip varies, the phasor IT
  rotates from ?90? at s0 to ?160? at s1.

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Static Kramer Drive (contd)

• At zero speed (s1) the motor acts as a
  transformer and all the real power is transferred
  back to the line (neglecting losses). The motor
  and inverter only consume reactive power.
• At synchronous speed (s0) the power factor is
  the lowest and increases as slip increases. The
  PF can be improved close to synchronous speed by
  using a step-down transformer. The inverter line
  current is reduced by the transformer turns ratio
  -gt reduced PF.

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Static Kramer Drive (contd)

• A further advantage of the step-down
  transformer is that since it reduces the inverter
  voltage by the turns ratio, the device power
  ratings for the switching devices in the inverter
  may also be reduced.
• A starting method for a static Kramer drive is
  shown on the next slide.

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Static Kramer Drive (contd)

• The motor is started with switch 1 closed and
  switches 2 and 3 open. As the motor builds up
  speed, switches 2 and 3 are sequentially closed
  until desired smax value is reached after which
  switch 1 is opened and the drive controller takes
  over.

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AC Equivalent Circuit of Static Kramer Drive

• Use an ac equivalent circuit to analyze the
  performance of the static Kramer drive. The
  slip-power is partly lost in the dc link
  resistance and partly transferred back to the
  line. The two components are
• PlId2Rd and
• Thus the rotor power per phase is given by

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AC Equivalent Circuit of Static Kramer Drive
(contd)

• Therefore, the motor air gap power per phase is
  given by
• where Irrms rotor current per phase,
• Rr rotor resistance, and
• Pm mech. output power per phase.

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AC Equivalent Circuit of Static Kramer Drive
(contd)

• Only the fundamental component of rotor
  current, Irf needs to be considered. For a 6-step
  waveform,
• Thus, the rotor copper loss per phase is given
  by

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AC Equivalent Circuit of Static Kramer
Drive(contd)

• The mechanical output power per phase is then
  given by
• Pm (fund. slip power) (1-s)/s

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AC Equivalent Circuit of Static Kramer
Drive(contd)

• The resulting air gap power is given by
• where
• and

26
AC Equivalent Circuit of Static Kramer
Drive(contd)

• The per-phase equivalent circuit derived from
  these equations (referred to the rotor) is shown
  below

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Static Kramer Drive Example

• Example 6.3 Krishnan

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Torque Expression

• The average torque developed by the motor
  total fundamental air gap power
• synchronous speed of motor
• ?
• where Pgf fundamental frequency per-phase
  air gap power.

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Torque Expression (contd)

• A torque expression in terms of inverter
  firing angle may be derived (see text pg. 320)
  resulting in

30
Torque Expression (contd)

• The torque-speed curves at different firing
  angles of the inverter are shown below

31
Harmonics in a Static Kramer
Drive

• The rectification of slip-power causes
  harmonic currents in the rotor which are
  reflected back into the stator. This results in
  increased machine losses. The harmonic torque is
  small compared to average torque and can
  generally be neglected in practice.

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Speed Control of a Static Kramer Drive

• A speed control system for a static Kramer
  drive is shown below

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Speed Control of a Static Kramer Drive
(contd)

• The air gap flux is constant and the torque is
  controlled by the dc link current Id (controlled
  in the inner control loop). The speed is
  controlled via the outer control loop (see
  performance curves below).

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Power Factor Improvement

• As indicated earlier, the static Kramer drive
  is characterized by poor line PF because of phase
  controlled inverter.
• One scheme to improve PF is the
  commutator-less Kramer drive - see Bose text pp.
  322-324 for description.

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Static Scherbius Drive

• The static Scherbius drive overcomes the
  forward motoring only limitation of the static
  Kramer drive.
• Regenerative mode operation requires the slip
  power in the rotor to flow in the reverse
  direction. This can be achieved by replacing the
  diode bridge rectifier with a thyristor bridge.
  This is the basic topology change for the static
  Scherbius drive from the static Kramer drive.

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Static Scherbius Drive (contd)

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Static Scherbius Drive (contd)

• One of the limitations of the previous
  topology is that line commutation of the
  machine-side converter becomes difficult near
  synchronous speed because of excessive
  commutation angle overlap. A line commutated
  cycloconverter can overcome this limitation but
  adds substantial cost and complexity to the
  drive.

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Static Scherbius Drive (contd)

• Another approach is to use a double-sided PWM
  voltage-fed converter system as shown below

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Modified Scherbius Drive for Shipboard VSCF Power
Generation

• Another approach that has been used for
  stand-alone shipboard power generation is shown
  below

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Modified Scherbius Drive for Ship-board VSCF
Power Generation (contd)

• In this approach an induction generator
  provides real stator power Pm to a 3? 60Hz
  constant voltage bus which is equal to the
  turbine shaft power and the slip power fed to
  the rotor by a cycloconverter. The stator
  reactive power QL is reflected to the rotor as
  sQL which adds to the machine magnetizing power
  requirement to give the total reactive power QL
  of the cycloconverter. This power is further
  increased to QL at the cycloconverter input by
  the shaft-mounted synchronous exciter.

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Modified Scherbius Drive for Ship-board VSCF
Power Generation (contd)

• The slip frequency and its phase sequence are
  adjusted for varying shaft speed so that the
  resultant air gap flux rotates at synchronous
  speed.
• At subsynchronous speeds the slip power sPm
  is supplied to the rotor by the exciter and so
  the remaining ouptut power (1-s)Pm is supplied
  to the shaft. At supersynchronous speeds, the
  rotor output power flows in the opposite
  direction so that the total shaft power increases
  to (1s)Pm.

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Modified Scherbius Drive for Ship-board VSCF
Power Generation (contd)

• Rotor voltage and frequency vary linearly with
  deviation from synchronous speed. For example, if
  the shaft speed varies in the range of 800-1600
  rpm with 1200 rpm as the synchronous speed
  (s?0.33) the range of slip frequency will be
  0-gt20Hz for a 60Hz supply frequency.