University of Nottingham
School of Electrical and Electronic Engineering
Sensorless Control of AC Motor Drives at low and zero frequency
Professor Greg Asher
Power Electronics, Machines and Control Group
OverviewOverview
General principles
Problems of sensorless control at low speed
How to get best performance from standard approaches
Principle of sensorless control by signal injection
Practical problems
Illustrative results for SM Permanent Magnet Machine
Illustrative results for standard cage Induction Machine
Review and Conclusions
General Principles Vector Control
•• Flux angle tracking Flux angle tracking –– SM Permanent Magnet MachineSM Permanent Magnet Machine
isd = 0
isq = stator current controlling motor torqueisq ΨR
λr
Knowledge of λ r at any time means:
Can control stator currents (mag and phase) so that stator current distribution gives correct iS for required isq
Now λ r = rotor position θr
General Principles Vector Control
•• Flux angle tracking Flux angle tracking –– Induction MotorInduction Motor
Knowledge of λ r at any time means:
Can control stator currents (mag and phase) so that stator current distribution gives correct iS for required isq and isd
But must know λ r at all times as it rotates in space
ΨR
iS
λr
isq
isd
isd = stator current controlling motor field
isq = stator current controlling motor torque
Need for sensorless controlNeed for sensorless control
isd* , isq*
Is
PWM
Coordinate
Trans
Coordinate
TransPI
θr (PM)
isd , isq
λr
Speed
Position
control
ωr (IM or PM)
Flux angle
calculator
Is
VsMachine
model
• Expensive on small drives• Fragile• Mounting problems
• Power supply required• Isolation• Noise and glitches
Flux angle
calculator
ModelModel--based Sensorless Drivebased Sensorless Drive
PWM
Machine
model
Coordinate
Trans
Coordinate
TransPI
θr (PM)
Isisd , isq
isd* , isq*
Is
Vs
λr
• λr incorrect if model parameters are wrong– lose flux and torque control
• Fails asymptotically if ωe→ 0
Sensorless performance of 4-pole 5kW IM under full load(Adaptive Luenburger Observer)
- 2 0 0
- 1 5 0
- 1 0 0
- 5 0
0
5 0
0 5 0 1 0 0L o a d to r q u e [ % ]
Nrre
f [rpm
]
U n s t a b l e r e g io n
• ωe = 0 (theoretical failure) occurs in re-generative region - red line
• ωe→ 0 region of instability (grey region) for a given scheme
• ωe→ 0 region of instability widens if parameters in model are in error
Sensorless performance of 5kW IM under full load(Adaptive Luenburger Observer)
-300
0
300
600
900
1200
1500
0 4 8 12 16 20 24 28 32 36 40Time [s]
Nrre
f , Nr [
rpm
]
Holds full load at zero speed for 30s followed by accelaration to 1200rpm
-1500
-1200
-900
-600
-300
0
300
600
900
1200
1500
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Time [s]
Nrre
f , Nr [
rpm
]1200 to –1200 rpm Quickly through zero
-1500
-1200
-900
-600
-300
0
300
600
900
1200
1500
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Time [s]
Nrre
f , Nr [
rpm
]1200 to –1200 rpm Slowly through zero
Going to zero in 30rpm stepsRs
est = 0.8Rs and 50 % load torque
-60
-30
0
30
60
90
120
150
180
0 1 2 3 4 5 6 7 8 9 10Time [s]
Nrre
f , Nr
[rpm
]
How to get best performance from model-based PWM drive
1. When integrating to find flux e.g.
- Do NOT use DC blocking filter to eliminate integrator drift
- Use non-linear feedback integrator exploiting fact that flux is constant
( )∫ −= ssss iRVΨ
2. Use linear (!) power amplifier to get sinusoidal V, I as ωe→ 0
- best possible dead-time compensation (ie. hardware sgn(i) detection)
- must compensate for IGBT & diode voltage drops
- avoid high switching frequencies (VTA distortion)
3. Exploit best Rs identifier you can find
4. Avoid heavily saturated machines and under flux slightly
**** BUT IT WILL STILL FAIL AS ωe→ 0 ****
How to get best performance frommodel-based PWM drive - 2 0 0
- 1 5 0
- 1 0 0
- 5 0
0
5 0
0 5 0 1 0 0L o a d t o r q u e [ % ]
Nrre
f [rpm
]
U n s t a b l e r e g i o n
Step reduction of torque to zero speed – motoring quadrant
From J. Holtz, “Drift and parameter compensated flux estimator for persistent zero frequency operation of sensorless induction motors”, IAS 2002
University of Nottingham
School of Electrical and Electronic Engineering
Sensorless Control of AC Motors Drives at low and zero frequency
Using Signal Injection
Principle of high frequency injectionPrinciple of high frequency injection
Saturation
No saturation
• Apply high frequency signal e.g. 20V at 1kHz• Impedance seen by hf varies around machine due to
saturation effects• Measured hf currents will be amplitude modulated and will
contain information of d-axis (ψr) position
A axis
Fit hall probe in air gap aligned to Phase A coil
Principle of high frequency injectionPrinciple of high frequency injection
A or α axis
pα
• Demodulate to get pα
β axis
pβ• Apply quadrature hf to β coilDemodulate to get pβ
• Have two “resolver” signals
ResolverResolver signals psignals pαβαβ in practice in practice -- SM PM machineSM PM machine
Harmonics exist on resolver signals
0 4 8 12 16 20 24 28 32 36 40 44 480
0.2
0.4
0.6
0.8
1Before Compensation
Frequency [Hz]
Amplit
ude [
% of
Salien
cy Po
sition
Harm
onic]
0 40
0.2
0.4
0.6
0.8
1
Amplit
ude [
% of
Salien
cy Po
sition
Harm
onic]
desired undesired
• Non-sinusoidal distribution of saturation
• Inverter effects – dead time & device voltage drop
• Dirty resolver signal must be cleaned up
Harmonic Compensation using machine signatureHarmonic Compensation using machine signatureof PM Machine of PM Machine
0 4 8 12 16 20 24 28 32 36 40 44 480
0.2
0.4
0.6
0.8
1Before Compensation
Frequency [Hz]
Amplit
ude [%
of Sal
iency
Positio
n Harm
onic]
0 4 8 12 16 20 24 28 32 36 40 44 480
0.2
0.4
0.6
0.8
1After Compensation
Frequency [Hz]
Amplit
ude [%
of Sal
iency
Positio
n Harm
onic]
Sensorless rotor position control structure for SMSensorless rotor position control structure for SM--PM machine using PM machine using αβ injection
Is
Is
isd , isq
isd* , isq*
PWM
Coordinate
Trans
Coordinate
TransPI
λr
Speed
Position
control
BPFDemodulationSignal
Cleaningatan
λr*
Encoder for monitoring
vαβ_hf
5kW Surface mount PM machine5kW Surface mount PM machineSensorless Position Control Sensorless Position Control –– 0% load0% load
• Response to 180° position demand
5kW Surface mount PM machine5kW Surface mount PM machineSensorless Position Control Sensorless Position Control –– 100% load100% load
• Response to 180° position demand- no integrator in control loop (incremental position only)- isq (torque current) limited to 1.3 x rated
PM Machine PM Machine Hybrid model / hf for wide speed range operationHybrid model / hf for wide speed range operation
injru Ψ= ˆmodˆ ru Ψ=
i αβv *
αβ
Rotor Flux model
modˆ rΨ
injrΨ̂
injrλ̂
rλ̂tan-1
12% 20%
)ˆsin(
)ˆcos(ˆ
injr
injrm
j λ
λ
+
Ψ
rω̂ to controller processingfrom hf
processing
• Under 12% speed use angle from hf processing
• Over 20% speed use angle from flux model
• Fuzzy interpolation between these these values
• Injection turned off e.g. 22% speed
PM machine Sensorless Hybrid control Hybrid control 1500 to 1500 to ––1500 rpm under 100% load1500 rpm under 100% load
0.6 0.8 1 1.2 1.4 1.6 1.8-2000
-1000
0
1000
2000
Rot
or sp
eed
[rpm
]
0.6 0.8 1 1.2 1.4 1.6 1.8-2000
-1000
0
1000
2000
Time [s]
Estim
ated
spee
d [r
pm]
Time [s]0.9 1 1.1 1.2 1.3 1.40
90
180
270
360
Inje
ctio
n θ r
[oel
ec]
0.9 1 1.1 1.2 1.3 1.40
90
180
270
360
θ rand
hyb
rid θ
r[o
elec
]^
^
Top: real speed
Bottom: estimated speed
Top: final rotor position
Bottom: hf estimate injrλ̂
rλ̂
PM machine Sensorless Hybrid controlHybrid controlStep position through 16 revolutions under 100% loadStep position through 16 revolutions under 100% load
0 1 2 3 4 5 6 7 8 9 10
0
1080
2160
3240
4320
5400
Rot
or p
ositi
on [
oel
ec]
0 1 2 3 4 5 6 7 8 9 10
0
1080
2160
3240
4320
5400
Time [s]
Est.
roto
r pos
ition
[o
elec
]
Real and estimated speed
0 1 2 3 4 5 6 7 8 9 10-2000
-1000-600
6001000
2000
Rot
or sp
eed
[rpm
]
0 1 2 3 4 5 6 7 8 9 10-2000
-1000-600
6001000
2000
Estim
ated
spee
d [r
pm]
Time [s]
Real and estimated position
• Below 400rpm – position estimated exclusively from hf injection
• Above 600rpm - position estimated exclusively from machine model
5kW Surface mount PM machine5kW Surface mount PM machineSensorless Position Control Sensorless Position Control –– 100% load100% load
1°
1°
1°
Position holding ((θθr r (ref)(ref) -- θθr r ))
at three different demand positions 45° apart
(illustrates cogging effect)
Actual Actual ResolverResolver signals psignals pαβCage Induction Cage Induction MachineMachine αβ
• Clean up to leave slotting harmonic to give rotor position tracking
• Better to have un-skewed, open slot, exhibiting good slot harmonic
Case 1 – closed slot, skewed machine with low rotor slotting
• Same effects as PM machine but higher undesired harmonics
• Clean up to get fundamental to give flux tracking
0 4 8 12 16 20 24 28 32 36 40 44 480
0.2
0.4
0.6
0.8
1Before Compensation
Frequency [Hz]
Amplit
ude [
% of
Salien
cy Po
sition
Harm
onic]
0 40
0.2
0.4
0.6
0.8
1
Amplit
ude [
% of
Salien
cy Po
sition
Harm
onic]
desired undesired
2fe
0 4 8 12 16 20 24 28 32 36 40 44 480
0.2
0.4
0.6
0.8
1Before Compensation
Frequency [Hz]
Amplit
ude [
% of
Salien
cy Po
sition
Harm
onic]
0 40
0.2
0.4
0.6
0.8
1
Amplit
ude [
% of
Salien
cy Po
sition
Harm
onic]
undesired desired
Case 2 – closed skewed machine with high rotor slotting
nfr2fe
Test signal injection for IM machinesTest signal injection for IM machines
Case 1 – closed slot, skewed machine with low rotor slotting
• Only flux position obtained
• Good for torque and flux control
• Need auxiliary structure for speed control e.g. model
Case 2 – open slot, exhibiting rotor slotting effects
• Rotor position obtained
• Good for torque, flux, speed and position control
Control mode & performance is machine dependent
Inverter and (selected machine) sold together
Technology most suitable for INTEGRATED DRIVES
Real Integrated Induction Motor DriveReal Integrated Induction Motor Drive
+ =
Integrated Motor Drive(Power Electronics housed in a redesigned End Plate)
Matrix ConverterInduction Motor
Power Electronics housed in the motor end plate
IGBTs, diodes and filter capacitors
Redesigned end plate with extra fins to cool the devices
Using the zeroUsing the zero--sequence current in an sequence current in an Integrated IM Drive Integrated IM Drive
Given an integrated drive
• Machine terminal connections at inverter
• Have access to machine phase currents in ∆ machine so can measure the zero sequence current, Iz
• Gives best method of tracking rotor or rotor flux position
• Applying test vector combinations to PWM, can show that λr = f(dIz/dt)
• dIz/dt measured using single non-integrating Rogowski coil
• Diagram shows torque control (rotor flux tracking) since have closed-slot machine
Machine terminals at inverter of integrated drive
Induction Machine Zero sequence currentsInduction Machine Zero sequence currentsSensorless Torque Control Sensorless Torque Control ωωrr= = ±±30 rpm at 30 rpm at 100% torque100% torque
Sensorless IM dynamometerDrive machine
ω = -30rpm
ω = 30rpm T = 100%
IA
Drive speed
IM flux angle
IA ∝ Driving torque
Induction Machine Zero sequence currentsInduction Machine Zero sequence currentsSensorless Torque Control Sensorless Torque Control 100% torque at 100% torque at ωωrr=0=0
Drive speed
IM flux angle
IM torque demand
IA ∝ Driving torque
IA
Sensorless IM dynamometerDrive machine
T = -100%ω = zero
T = 100%
Induction Machine Zero sequence currentsInduction Machine Zero sequence currentsSensorless Torque Control Sensorless Torque Control 100% torque at zero frequency100% torque at zero frequency
ω = -40rpm
Drive speed
IM flux angle
IA ∝ Driving torque
IA
T = 100%
Sensorless IM dynamometerDrive machine
Observer based Sensorless Observer based Sensorless DrivesConclusions DrivesConclusions
• Asymptotic failure as fe→ 0
• For best performance should focus on:
- obtaining linear behaviour for switching converter
- good integrator and resistance estimators
• Estimator/observer type not so important
• Expect failure as fe→ 0 under regeneration
• Performance will depend on machine to a certain extent
- sensorless drives assume validity of 2-axis dq theory
Signal injection based Signal injection based Sensorless PM DrivesConclusions Sensorless PM DrivesConclusions
• Position tracking effective down to and at fe→ 0 under all loads• No dependence on machine parameters
• Works for Surface mount PM machines (saturation saliency)
- very effective for buried magnet machines (salient machine)
• Hybrid technique appropriate for wide-speed range drive
• Harmonic compensation (signal cleaning) advisable- position accuracy down to 0.1° mechanical- sensorless closed loop position bandwidth 3 - 5Hz
• Improvements in performance if:- machine designed to minimise cogging- obtain linear behaviour for switching converter
Signal injection based Signal injection based Sensorless IM DrivesConclusions Sensorless IM DrivesConclusions
• Rotor Flux/ Rotor Position tracking effective down to and at fe→ 0 under all loads
• No dependence on machine parameters
• Control mode heavily machine dependent
- skewed, closed slot machines yield torque and flux control
- open-slot machines of given slot combination for rotor position control
• Commercial penetration likely to be as integrated drive
• Hybrid technique appropriate for wide-speed range drive
• Harmonic compensation (signal cleaning) essential
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