Voltage and Current Measurement Techniques for Motor Drive & Power Converter Design

Transcription

1 Motor & Drive Systems 2015 Voltage and Current Measurement Techniques for Motor Drive & Power Converter Design Dal Y. Ohm & Levent U. Gokdere, Chantilly, Virginia (703)

2 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 2

3 Vin, Vbus, Vab, Va- Measuring Locations Iac_in, Idc_in, Iph, Ileg, Idc_inv AC IN + Cbus Q1 Q2 A Q3 Q4 B Q5 Q6 C 3

4 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 4

5 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 + Cbus Q1 Q2 A Q3 Q4 B Q5 Q6 C Rs 5

6 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 6

7 Sinusoidal FOC Drive & Sensorless Observer Id* + - Iq* + - θe Regulator PI Regulator PI Vector Saturation Algorithm Vd Vq Id Iq Inverse Park Park θe Vα Vβ Iα Iβ Inverse Clarke Clarke PWM (SVM) Iabc Vabc (FOC - Outer Vel/Pos loop Omitted) θe ω Angle& Vel Est Observer Motor Model Vabc Iabc 7 (Angle estimator)

8 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 Q3 Q5 + Cbus Q2 A Q4 B Q6 C Ra Rb Rc 8

9 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) 3 diamond region 011 b,c Two phases near 100% pwm Covers full SVM if Tmeas < 6.7% of Tpwm 2R Sensing? Must sacrifice max pwm duty!! In-line 2/3R sensing possible 010 B C c,a 101 a,b Space Vector Hexagon A 100 9

10 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 10

11 Sine Drives with 1R Sample during two active pwm states Min duty for sampling at each pwm state (modfy pwm duty) Requires hardware-triggered sampling More computational burden Ix,Iy,pwm -> assign Iabc. Assumed currents within pwm period are constant Cost vs performance? (A) (B) (C) To Tx Ty Tu Tu Ty Tx To Ts 11

12 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. 12

13 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) 13 Vdrop: Dead-time and Semiconductor drop, etc. Angle: Use Commanded angle

14 Extraction of Vph(1) Vph(1): Fundamental motor phase voltage from direct measurement Use of synchronous transform Usage Vdrop = Vcmd Vph(1) Fundamental power calculation ω1 ω1 Vabc Analog LPF Fwd Transform Vdq Digital LPF Rev Transform V(1)abc θ Vector Magnitude θ V(1) 14

15 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. 15