Sensorless Control of AC Machines

1
Sensorless ControlofAC Machines

• Marie Curie ECON2
• Nottingham Summer School 08

Cedric Caruana
2
Objectives

• To review the sensorless control of ac machines
  at low and zero speed
• To present two techniques
• Zero Vector Current Derivative Technique for PMSM
• Use of PWM Harmonics for IM Rotor Position
  Detection

3

• 1. Sensorless Control of AC Machines at Low
  and Zero Speed

4
Topics

• Background
• Injection and Demodulation Techniques
• Multiple Saliencies
• Saturation Saliency Shift with Load

5
Why go Sensorless

• Objective is to enable vector control without the
  need of the encoder on the machine shaft
• Gains
• remove drive dependency on sensors that are
  external to itself
• cost, robustness, reliability
• Which market
• precision drives, possibly integrated solutions
• lower cost, general purpose drives

6
Methods for Sensorless Control

• Fundamental model based Methods
• simple realization, however
• parameter dependent
• generally fail at low and zero frequency
• Signal Injection Methods
• exploit saliencies that are not seen by
  fundamental signals
• excite machine at much higher frequency than
  fundamental
• injection setting can ensure that high frequency
  effects are superimposed to fundamental machine
  operation
• rotor- or flux- position obtained indirectly
  through response of machine (reflects hf
  impedance)
• parameter independent

7
AC Machine Saliency

• Salient Pole machine geometric saliency

• Symmetric machines
• geometric saliencygenerally negligible
• saturation saliency (main fieldand local
  saturation)
• rotor slotting saliency (IMs)

ls
8
Signal Injection Methods

• High Frequency Carrier Injection
• Rotating Carrier Injection
• Pulsating Carrier Injection
• Transient Injection
• Test Voltage Vector injection superimposed on
  fundamental PWM
• Standard PWM Switching
• exploit the switching of the fundamental PWM
  waveforms

9
Signal Injection MethodsHigh Frequency Rotating
Carrier Injection

• derives two orthogonal position signals
• can detect instantaneous rotor / flux position
• different demodulation schemes heterodyning,
  synchronous filters
• easy to implement
• requires no additional sensors

10
Signal Injection MethodsHigh Frequency
Pulsating Carrier Injection

• Injection in estimated dqe frame

11
Signal Injection MethodsTransient Injection

• test voltage vector superimposed on fundamental
  PWM

• can detect instantaneous rotor / flux position
• needs to measure current derivative
• requires synchronous sampling of current
  derivative
• simple combination of readings to obtain position
• can be realized WITHOUT test vector injection,
  using fundamental PWM switching

12
Comparison of Methods

• Different levels of complexity in setting up the
  injection
• hf carrier injection obtained easily through same
  hardware but observing demodulation scheme
  complex. Tracking scheme is easy.
• transient and PWM switching schemes require
  synchronization of sampling but algorithm is very
  easy
• Latter techniques require extra sensor. However
  these can be integrated in the drive.
• industrial current transducers have a current
  derivative signal available internally (Kennel)

13
Corrupting Harmonics

• Ideally machine will only exhibit one saliency
• Practical machines will exhibit multiple
  saliencies the saliency that is not tracked acts
  as a disturbance corrupting the position signal/s
• Similar effect if the saliency distribution is
  not sinusoidal
• Need to couple the unwanted saliencies to improve
  the estimation accuracy

14
Corrupting HarmonicsSaturation and Rotor
Slotting Effects (IM)

• both rotor geometry and saturation saliency
  present
• saturation saliency acts as a disturbance

15
Corrupting HarmonicsHigher Order Saturation
Harmonics

• Ideal and experimental position loci over the
  stator current angle

• Harmonic spectrum of position signal pa for
  closed slot IM under motoring conditions (a)
  20 and (b) 100 rated torque at 60r/min

16
Corrupting HarmonicsNonlinearity of the PWM
Inverter

• Effect caused by the inverter, not the machine
• Generates additional harmonics that coincide with
  the harmonics of the saturation saliency in high
  frequency signal injection drives
• standard, simple dead-time compensation
  strategies not effective
• complex compensation schemes published in the
  literature
• Less effect on transient excitation schemes

17
Decoupling of Corrupting Harmonics

• Various engineering solutions proposed
• Harmonic compensation table (frequency approach)
• table stores frequency, amplitude and phase of
  different harmonics
• not effective against inverter nonlinearity
  effects
• Space Modulation Profiling (SMP) (time approach)
• table stores corrupting magnetic signature of the
  machine (obtained during commissioning stage)
• effective against inverter nonlinearity effects
• tedious to commission
• Neural Networks
• ease the commissioning of the table
• Synchronous filters with memory

18
Decoupling of Corrupting HarmonicsSpace
Modulation Profiling (SMP)

• Open slot IM under rotating hf carrier injection
• shows clear signs of inverter nonlinearity effects

• Closed slot IM under transient excitation
• not influenced by inverter nonlinearity effects

• Both profiles quite complex

19
Decoupling of Corrupting HarmonicsReal Time
Implementation using SMP Table

• SMP table referenced through measurable variables
  like stator current angle
• Can compensate saturation saliency, higher order
  saturation harmonics and inverter nonlinearity
  effects

20
Decoupling of Corrupting HarmonicsConsiderations

• How complex is this compensation
• more drive memory required not considered a
  problem
• Commissioning required. What tests can be done?
• How often do we need to commission
• might not be portable to different machines
• Do we go for a high quality inverter with less
  nonlinearity effects ?
• will be more costly
• Do we go for off-the-shelf machine or custom
  machine?
• will depend on the application
• Can we define criteria for sensorless friendly
  machines?
• lcd ? lcq not sufficient

21
Load Dependent Saturation Saliency EffectsPhase
Displacement

• Load dependent phase displacement between the
  identified and real flux position
• Displacement will depend on
• machine type, construction
• injection scheme used

• Closed slot IM
• relationship is nonlinear
• Upper hf injection (max of around 30)
• Lower transient injection (max of around 18)

22
Load Dependent Saturation Saliency
EffectsCompensation of Phase Displacement

• Linear approximation possible based on armature
  reaction effect
• parameter dependent
• not applicable to closed slot IMs
• Lookup table referenced through isq
• Might be less on PMSM due to design (Kennel)

23
Load Dependent Saturation Saliency
EffectsConsiderations

• More memory required
• Not portable to different machines, hence need of
  commissioning
• Do we go for purposely designed machine where
  this phenomenon is insignificant or predictable

24

2. Zero Vector Current Derivative Technique for
PMSMs
25
Objectives

• Sensorless Operation of a PMSM Drive without
  Additional Test Signal Injection
• To define a position error signal utilizing both
  back emf and PMSM magnetic saliency
• Analysis and Decoupling of Inverter Nonlinearity
  Effects
• Sensorless operation

26
Mathematical Model

• Consider the voltage equations for a PMSM, in a
  dq frame orientated to the rotor flux

• On the application of a zero voltage vector

27
Definition of Position Error Signal

• Under sensorless operation, drive operates in
  estimated dqe frame
• current control forces current in estimated de
  axis to zero (i.e. ide 0)
• In the dqe frame, assuming ide0

28
Examining the Position Error Signal
29
Proposed Tracking Controller
30
Test Rig
31
Acquisition of Current Derivative

• TPWM 266.7?s (fPWM 3.75kHz)
• TADC 66.7?s

32
Position Error Signal ComponentsPlotting perr
against dqe frame position error
33
Inverter Nonlinearity Effects
34
Inverter Non-linearity Effects
35
Inverter Non-linearity Effects

• Theoretical

• Experimental

36
Sensorless Torque Control at higher speeds

• Torque control in quadrants II and III, -12rpm ?
  55 rated

• Torque control in quadrants I and IV, 12rpm ? 55
  rated

37
Sensorless Torque Control at Low Speed

• Speed change from 0 to 6 rpm (0.4Hz ele) at ide
  0 A

38
Sensorless Position Initialization

• Start from zero speed, rated current
• Initial position intentionally set wrong

39
Sensorless Speed Control

• Position error signal polarity correction function

40
Sensorless Speed Control

• ? 30rpm speed transients

41
Conclusions

• The Principle of the Zero Vector Current
  Derivative Technique was shown
• Back EMF signal is strong enough for sensorless
  control providing the possibility of implementing
  this algorithm in any PMSM, even if no saliency
  is available
• Saliency component allows operation of the drive
  even down to zero speed
• Limitation of the method in the very low speed
  region in two of the four quadrants of operation
• dide/dt error signal polarity needs to be
  adjusted for four operation quadrant operation
• Torque and speed control of the sensorless PMSM
  were shown

42

• 3. Use of PWM Harmonics for IM Rotor Position
  Detection

43
Objectives

• Rotor position detection of an IM using the PWM
  Harmonics, without Additional Hf Signal Injection
• To define suitable position signals
• Analysis and Decoupling of Corrupting harmonics
• Rotor position reconstruction
• Sensorless operation

44
PWM Carrier Harmonics

• Typical PWM waveforms

• Frequency spectrum of 1 PWM cycle

• 2nd PWM harmonic shows highest amplitude
• Can be regarded as injection signal

• Floating FFT spectrum

45
2nd PWM Voltage Harmonic (fPWM2)

• fPWM2 harmonic pulsates at 2fPWM and rotates at
  we
• Amplitude and direction cannot be controlled
  without compromising the fundamental PWM scheme
• Amplitude is variable, reflecting fundamental
  conditions

46
Definition of Position Signals

• To overcome the variable hf excitation, define an
  equivalent impedance tensor zPWM2 as follows

• In IMs, i does not drop to zero due to
  magnetizing current

• Demodulation scheme

47
Test Rig

• Off-the-shelf MEZ induction machine
• Skewed rotor with semi open slots
• Parameters

Nominal Power 5.5 kW No. of pole pairs 2
Nominal voltage 415V (D) No. of rotor slots 32
Nominal current 10.3A (D) Nominal speed 1450
48
Examining the position signals

• Rotor geometry modulation quite visible
• Additional modulation, apart from saturation
  saliency effect, depending on position of iPMW2
  in ab frame. Assumed to be inverter nonlinearity
  effect.

49
Decoupling Saturation Saliency Modulation

• Saturation saliency modulation depends on
• imposed stator currents is
• relative position of iPWM2 to saturation saliency
  axis
• Latter dependency makes compensation more
  challenging as relative position is speed
  dependent

• SMP referenced by isq and

50
Decoupling Saturation Saliency Modulation

• Additional dimension required to decouple
  inverter nonlinearity effects

51
Rotor Position Reconstruction
52
Sensorless Torque Control

• Zero to rated torque transients at -52 rpm

53
Sensorless Speed Control

• ?60 rpm speed transients, rated torque

54
Conclusion

• A technique for extracting IM rotor position
  information using only PWM voltage harmonics has
  been proposed
• The 2nd PWM harmonic shows the strongest signal,
  forming a pulsating HF vector rotating with the
  fundamental frequency.
• An equivalent impedance tensor zPWM2 is defined
  as position signals
• The saturation and inverter nonlinearity effects
  are decoupled by using a simple look up table.
  Appropriate table references were defined.
• Fully sensorless operation in torque and speed
  control have been achieved with a standard
  of-the-shelf 5.5 kW induction machine.

55

• Thank you for your attention