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 1 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 Drivetech, Inc. 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 4 Drivetech, Inc. 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 5 Drivetech, Inc. 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 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 Drivetech, Inc. 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 9 Drivetech, Inc. 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 Drivetech, Inc. 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? 11 Drivetech, Inc. 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 Drivetech, Inc. 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 13 Drivetech, Inc. 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)| Drivetech, Inc. 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 Drivetech, Inc.