1
Electrical Machine PWMLoss Evaluation
BasicsTutorial Demo
EDPE2005 - International Conference on Electrical
Drives and Power Electronics26 28 September
2005, Dubrovnik, Croatia

• Dr. Alex Ruderman
• The School of Engineering
• Bar-Ilan University
• Ramat-Gan 52900, Israel
• ruderma_at_eng.biu.ac.il

2
Electrical Machine PWM Loss in General - I

• Pulse Width Modulation (PWM) voltage control
  moves energy loss from power electronics to
  electrical machine and chokes
• In-depth understanding of PWM loss mechanisms is
  important for predicting losses and improving
  efficiency of electrical machines
• There is no accepted electrical machine PWM loss
  theory applicable in engineering practice, myths
  and misconceptions instead
• The first misunderstanding is that PWM loss is
  mainly copper PWM current ripple resistive loss.
  It is wrong for iron machines that are in the
  scope of this Tutorial for them PWM core eddy
  current loss is a dominating PWM loss mechanism
• Another misconception is about PWM loss
  dependence on switching frequency. Some people
  erroneously assume PWM loss increasing with PWM
  frequency extrapolating main flux induced PWM
  loss frequency behavior. Others expect PWM loss
  reduction following current ripple decrease
• It may look somewhat surprising but a reasonable
  approximation is PWM loss independent of
  switching frequency

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Electrical Machine PWM Loss in General - II

• For AC motor inverter operation, suggested rule
  of thumb is 20 motor power derating due to PWM
  loss - T. Haring, Design of motors for inverter
  operation, Energy Efficiency Improvements in
  Electronic Motors and Drives, P. Bertoldi, A. T.
  de Almeida, and H. Falkner, Eds., Springer,
  Berlin, 2000
• Experimental induction motor PWM loss
  investigation
• M. Sokola, V. Vuckovic, and E. Levi,
  "Measurement of Iron Losses in PWM Inverter Fed
  Induction Machines", Proc. UPEC'95, London, UK,
  September 1995
• showes 3 times iron loss increase compared
  with pure sinusoidal supply
• Another important result is that twice switching
  frequency increase practically causes no change
  in PWM loss

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Suggested PWM Loss Approach

• Our practical PWM loss engineering approach
  comprises
• - theoretical evaluation of normalized PWM
  loss this includes time averaging (on PWM and
  fundamental period) and space averaging
  (integration)
• - PWM loss characterization (separation)
  experiment to quantify PWM loss
• We recommend measuring PWM loss for maximal PWM
  loss (current ripple) conditions
• Once maximal total PWM loss is obtained, it is
  then scaled for an arbitrary operating point
  using simple formula of PWM core eddy current
  envelope to provide PWM loss upper bound
• Considered are different types of electrical
  machines / converters and different DC / AC
  modulation techniques / switching patterns
• The same ideas are applicable to numeric PWM loss
  calculation by coupled circuit and
  electromagnetic field analysis

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PWM Core Loss in a Solenoid

• Solenoid controlled by an H-bridge converter
• Average normalized voltage equal to PWM duty
  cycle D defines average current
• Voltage pulsation (b) causes current ripple and
  PWM loss

• Elementary voltage induced in local core eddy
  current path is proportional to voltage pulsation

• As voltage pulsation is defined by an average
  output voltage, we shall formulate our results
  normalized average PWM loss in terms of
  converter normalized output voltage (duty cycle D)

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PWM Core Loss Frequency Dependence

• If PWM frequency (a) is increased twice, current
  ripple is reduced in the same proportion (b)
• However, local eddy current shaping is the same
  meaning the same PWM core eddy current loss
• This will hold with frequency increase until
  local eddy current finite rise / fall times due
  to eddy current path finite time constant can not
  any more be neglected compared with PWM period
• It is expected to take place at switching
  frequencies of some hundred KHz while typical
  switching frequencies of modern servo amplifiers
  and inverters are in the range of 20-60 KHz
• PWM core eddy current loss is a dominating PWM
  loss mechanism providing that total PWM loss is
  practically switching frequency independent

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PWM Loss for Solenoid DC Excitation

• Normalized PWM eddy current loss is obtained by
  averaging on PWM period
• It can be further normalized for maximum unity
• Normalized
• copper loss
• Normalized core
• hysteresis loss

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PWM Loss for Permanent Magnet Machine

• For permanent magnet machines, PWM eddy current
  loss takes place mostly in PWM excited armature
  yoke because of relatively large equivalent air
  gap
• PWM loss is also expected in permanent magnet
  alloy materials that, opposed to ferrites, have
  relatively high conductivity
• For DC motor, as there is only one excitation
  winding actual rotor core flux distribution is of
  no importance for normalized PWM loss
  consideration and previous results for solenoid
  are applicable
• For multi-phase AC machine, total PWM local eddy
  current is obtained by superposition of local
  currents induced by different windings.
  Normalized PWM eddy current loss is obtained by
  averaging squared eddy current distribution
  time averaging on PWM and fundamental frequency
  period and space averaging
• Calculations show that two- and three-phase AC
  permanent magnet machines are equivalent from
  normalized PWM eddy current loss perspective for
  two-dimensional core field consideration

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PWM Loss for AC Permanent Magnet Machine

• Normalized PWM eddy current loss
  , where
• - normalized fundamental phase voltage
  amplitude for two-phase and three-phase
  delta-connected motor
  - for Y-connected motor
• PWM loss is maximal for
• For AC induction motor, despite relatively small
  air gap there is no major PWM loss in rotor yoke
  because it is effectively shielded by rotor
  winding that may be considered superconducting
  for PWM frequencies

• PWM copper loss in induction motor is higher
  because of increased stator current ripple and
  PWM copper loss in rotor winding. However, no
  major increase of PWM stator core eddy current
  and hysteresis loss is expected compared with a
  permanent magnet motor because stator current
  ripple increase is exactly compensated by a
  current ripple induced in rotor winding that has
  an opposite phase (transformer short-circuit
  effect)

10
PWM Loss Characterization Experiment

• For permanent magnet motors, we suggest
  electromagnetic no-load experiment at maximum PWM
  loss point
• The motor is back driven by a prime mover at such
  a speed that back EMF equals motor fundamental
  voltage at maximum PWM loss point. Servo
  amplifier / inverter is run with a zero current
  command. The power into the motor from servo
  amplifier / inverter side is a maximal total PWM
  loss (commutator loss neglected for DC
  brush motor)
• PWM eddy current loss envelope gives an upper
  bound of total PWM loss for an arbitrary point
  for AC PM motor
• The same equation is applicable to AC induction
  motor if PWM loss is characterized for maximal
  PWM loss point
• The known induction motor PWM loss experiment is
  not that simple and accurate as it requires back
  driving the motor, carrying out different
  experiments and calculations and further
  separating PWM loss from mechanical and main
  electromagnetic process losses. We are developing
  AC motor PWM loss experiment that will allow for
  effective PWM loss measurement at stall condition
  without a prime mover

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Electrical Machine PWM Loss Reduction

• Dont try to reduce machine PWM loss by PWM
  frequency increase, this will not work!
• Thinner laminations reduce PWM core loss just as
  that induced by main electromagnetic flux
• DC link voltage regulation while keeping maximal
  duty cycle without overmodulation was suggested
  by Prof. F. Profumo for low dynamic applications
  like pumps

• Multi-level PWM reduces PWM loss at the expense
  of power electronics loss. Three-level PWM
  reduces PWM loss about 4 times on average
• Other PWM loss reduction opportunities
  multi-winding multi-converter for core PWM flux
  compensation