Motor & Drive Systems 2015
Voltage and Current Measurement
Techniques for
Motor Drive & Power Converter Design
Dal Y. Ohm & Levent U. Gokdere
Drivetech, Inc., Chantilly, Virginia
(703) 489-2474 www.drivetechinc.com
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Why measure V&I?
• Simple applications
– Circuit breakers or Ovecurrent relays/contactors
• Measurement Needs
(1) Detection of abnormal operation & protection
(2) High bandwidth servo control (T ∝ It)
(3) Model-based control including Sensorless
(4) Power & efficiency calculation
PV Inverter, EV, Pump & other steady-state load
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Measuring Locations
• Vin, Vbus, Vab, Va• Iac_in, Idc_in, Iph, Ileg, Idc_inv
+
Q1
Q5
Q3
A
AC IN
3
B
Cbus
Q2
Q4
Q6 C
Drivetech, Inc.
Current Sensing Technologies
• Closed-loop Hall-effect sensors
– Accurate, High bw, Expensive & Limited temp
• Open-loop Hall-effect sensors
• Mageto-resistive sensors – similar performance
– Internal (ASIC) Temp compensation
• CT and Rogowski coil – AC only
• Shunt Resistor (4-terminal vs 2-terminal)
– Isolation and amplification necessary
– with signal processing IC
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Current Sensing in 6-step drive (1R)
•
•
•
•
Popular: 2Q voltage drives with overcurrent detection
Sampling time – Mid point of pwm ON
Current control (magnitude only) possible
Simple 4Q drive – Vrs can be negative (regen)
– Torque reversal by pwm logic
+
Q1
A
B
Cbus
Q2
Q5
Q3
Q6
Q4
C
Rs
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Various 6-step Switching Schemes
Sch 0 (2Q)
Sch 1
Sch 2
Sch 3
Sch 4
Sync or Split swtg
Difference in
Current ripple
Switching loss
Sch 1-4: 4Q
Sch 2&4 - Iph sample at (1/4)Ts after pwm start
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Drivetech, Inc.
Sinusoidal FOC Drive & Sensorless Observer
Id* +
Iq*
Regulator
PI
+
-
Regulator
PI
Vα
Vd
Vector
Saturation
Algorithm
Inverse
Park
Vq
Inverse
Clarke
Vβ
θe
PWM
(SVM)
Vabc
θe
Iα
Id
Iabc
Iq
Park
Iβ
Clarke
(FOC - Outer Vel/Pos loop Omitted)
θe
ω
7
Vabc
Angle&
Vel Est
Observer
(Angle estimator)
Motor
Model
Iabc
Sine Drives with 3R Sensing
• Sense during “000” state (Ia ∝ -Vra)
• Bidirectional current (amplification with bias)
• Limitation in max PWM duty
• Can detect shoot-through
Q1
A
+
Cbus
B
Q2
Ra
8
Q5
Q3
Q4
Q6
Rb
Rc
C
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Current Sensing at High SVPWM duty
• Max voltage: Inscribed circle
• 6 small circle region (2 shown
Near 100% pwm. A-ph shown)
Ia = -(Ib + Ic)
011
b,c
• 3 diamond region
Two phases near 100% pwm
Covers full SVM if
Tmeas < 6.7% of Tpwm
• 2R Sensing?
010
B
110
a,b
0
C
001
A
100
c,a
101
Space Vector Hexagon
– Must sacrifice max pwm duty!!
– In-line 2/3R sensing possible
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Locations of Shunt Resistors
• DC High-side sensing?
– Load can be directly grounded
– Can detect high-side load current from short
– High common mode voltage
– Level-shifting necessary
• In-line sensing
– Wide-input common mode voltage
– May sacrifice high frequency accuracy
– Dedicated “difference amplifiers” available
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Sine Drives with 1R
• Sample during two active pwm
states
• Min duty for sampling at each
pwm state (modfy pwm duty)
•(A) •0 •1 •1 •1•1 •1 •1 •0
• Requires hardware-triggered
sampling
•(B)
• More computational burden
•(C) •0 •0 •0 •1•1 •0 •0 •0
– Ix,Iy,pwm -> assign Iabc.
• Assumed currents within pwm
period are constant
•0 •0 •1 •1•1 •1 •0 •0
•To •Tx •Ty•Tu•Tu•Ty •Tx •To
•Ts
• Cost vs performance?
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Offset and Temperature variation
• Change of resistance wrt temperature
– Sizing (accuracy, power loss, and temperature)
– Temperature compensation possible
– In feedback control system, effects are minimal
• (velocity, position, pressure, etc…)
• Balanced 3-phase system
– FOC algorithm assumes balanced system
– Unbalanced current still generated balanced voltage!
– Minimize offset.
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Voltage measurements
• Input AC voltage (R or transformer)
– PFC (to extract phase angle)
• Bus voltage (R)
• Motor voltage (pwm)
– Model calculation, Sensorless, power calc.
– Filtering causes phase delay
– Low frequency harmonics are difficult to remove
– Mag: Vph = Vbus*Vph# - Vdrop
#
= Per Unit value)
Vdrop: Dead-time and Semiconductor drop, etc.
– Angle: Use Commanded angle
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Extraction of Vph(1)
Vph(1): Fundamental motor phase voltage
from direct measurement
ω1
Use of synchronous transform
Usage
Vdrop = Vcmd – Vph(1)
ω1
Fundamental power calculation
Vabc
Analog
LPF
Fwd
Transform
θ
14
Vdq
Digital
LPF
Rev
Transform
Vector
Magnitude
V(1)abc
θ
|V(1)|
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Concluding Remarks
• Phase current sense is critical for high performance
control
• Simple 3R method can be used for low power
applications.
Sampling and Limitation should be considered
Simple algorithm allows acquisition of all currents up to
maximum SVPWM operation.
• Output Voltage measurement
– Commanded mag. & angle with Vdrop compensation
– Sync. Transform can be used to extract fundamental
magnitude.
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Drivetech, Inc.