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Download Variable Frequency Induction Motor Drives
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Variable Frequency Induction Motor Drives • Simplest Control – set frequency for steady state operation only • Use digital control Block Diagram: V/f Variable Frequency Motor Drive – Nothing Fancy! The Grocery List: Building Blocks for Induction Motor Control • DC Power Supply (Batteries or Rectifier – rectifier needs complex design to minimize mains harmonics) • Switch Bridge – connects motor to VDC • Switch Actuators – gate drivers • Algorithm converting angle and voltage to switch times • Algorithm convert desired speed to angle and voltage (V f ) • Speed sensor • Error detection and controller to set driving speed to sufficient slip to get the correct motor voltages Switch Bridge Three-Phase Switch Bridge Output With No PWM 7.5 Switch Pattern: a,b,c - 1 implies direct connection to VDC 101 100 110 010 011 800 6 001 101 Va Va as sinusoid 600 4.5 Vb (offset + 400) Vb as sinusoid (offset + 400) Vc (offset + 400) Vc as sinusoid (offset + 800) 400 Ia with 1 H inductive load 200 3 1.5 0 0 -200 0 0.00333 0.00666 0.00999 0.01332 0.01665 Time (seconds) - Excitation Frequency is 50 Hz. 0.01998 Winding Current (amperes) for Inductive Load of 1 H. Output (volts) for a,b,c Axes with Offset to Separate Curves 1000 -1.5 0.02331 What Are the Switches? • Three types: IGBT, MOSFET, HEMT • Rapid development: SiC, GaN, HV Si MOSFET • All controlled by gate-source voltage IGBT MOSFET/HEMT Switch On/Off IGBT (IXBF32N300) MOSFET/HEMT (IRFP22N50A) Random Comparison of IGBT and MOSFET Capabilities Part Number: Device Type FZ50R65KE3 IXBF32N300 4V Reverse transfer capacitance (Miller effect) Gate series R QG 2 ohm 130 nc?? T turn on T turn off Thermal Resistance junction to case Unit cost C2M0280120D EPC2025 GaN HEMT IGBT IGBT IGBT Half-bridge N-MOSFET enhancement Eff. Power. Infineon IXYS IXYS Vishay Conversion 6500 V 3200 V (1500 V typ.) 600 500 V 300 V 750 A 40 A (22 A practical) 30 A (15 A practical) 22 A (14 A practical) 3A 3.0 V @ 500 A 125 C 3.25V @ 30 A 125 C 1.6V @ 15 A 125 C 2.5 V @ 14 A 0.6 V @ 3 A 20 V 25 V 15 V 15 V 5V Vendor VDS or VCE Max IDS or IC max. Saturation voltage Gate voltage Gate turn-on voltage: VTH equiv. 6V 3.2 nF 0.75 ohm 11.5 uC 800 ns delay; 400 ns rise time 7.6 us delay; .5 us fall time FI40-06D 3V 27 pf 4.3 ohm 100 nC 120 nC 26 ns delay; 94 ns 80 ns rise time 300 ns delay; 40 ns 47 ns delay; 47 ns fall rise 800 ns 600 ns 17.5 deg. C/kW 0.8 C/W $3,026 IRFP22N50A 1.0 C/W $43 $11 SiC FET Cree 1200 V 10 A 1.2 V @ 6A 20 V 2.2 V 2.5 V 0.1 pf 3 pf 1.8 nC 20 nC 6 ns delay; 16 ns rise time Limited by gate drive 0.45 die package $3 16 ns delay/fall 1.8 C/W $8 $5 Gate Drive Functionality FET Model with Capacitances and Gate Current Limiting Resistor • Constant current load represents slow change of inductive load current with voltage – PWM much faster than average current can change • Capacitance (CGSS & CRSS) with RG sets rise and fall times Maximum Ratings: The things you have to worry about building a switch bridge Example Device: IRFP22N50A: – – – – – – Peak drain current: 22 A Continuous drain current: 14 A Maximum drain voltage: 500 V dVDS Maximum dt 5 V/ns Maximum junction temperature: 150 C (for reliability limit to 100 - 125 C) Maximum gate voltage: +/- 30 V Other Properties: – Minimum recommended RG = 5 ohms – Case type: TO-247 – Thermal resistance: 0.75 deg. C/watt junction to heat sink (no thermal washer) Design Example: 2 HP 208V 3-phase Wye-wound Motor (85 % eff.) • • • • Motor power = 1770 W and current 5 A RMS 120 2 170 Motor winding peak volts volts and VA peak < 2/3 Vbus Bus voltage VBUS = 300 VDC VDS @ 5 A is sensitive to TJ as 1.2 V @ 25 C, 2.3 V @ 125 C and 2.6 V @ 150 C. Choose 2.3 V for design needing to check that TJ will not get to 125 C. • PD from ID RMS = 11.5 W 2 PSWT 12 VBUS I LOAD RISE / PWM 12 COSSVBUS f PWM • PWM sampling 25 KHz – 40 usec period • Switching loss is 2.1 W Design Example: 2 HP 208V 3-phase Motor (Continued) • Total power dissipation: 13.5 W • Thermal resistances: Junction to case = .25 deg./W; case to heat sink = .45 deg./W and heat sink to ambient 2.8 deg./W. • Ambient temperature max = 40 C. (Probably unrealistically low!) • Maximum junction temperature = 40 + (.25+.45+2.8)*13.5 = 87 C • Gate charge for 12 Volt VGS and 300 Volt VDS is CQ = 120 nC • For rise/fall times = 100 ns this requires 1.2A gate drive • To limit dVDs/dt, the vendor recommends 5 ohm series gate resistor • VGS for turn-on is about 6 volts • Required VGS for final clamping is turn on plus peak drop in the 5 ohm resistor so VGDRV > 5*1.2 + 6 = 12 volts VGS Level Shift Problem • Source of MHI goes from 0 to Vbus – a range of several hundred volts • Gate drive of MHI is referenced to that source voltage • Electrical isolation needed between the controller and MHI Gate Drive with Low Power (< 10 KW) • • • Multiple vendors Coupling techniques include open-drain HV drivers, transformers, giant magnetoresistance coupling, and capacitors. Limited to 600 V, 30 A (very roughly – set by required gate current) How International Rectifier Does It Three-Phase Switch Bridge Output With No PWM 7.5 Switch Pattern: a,b,c - 1 implies direct connection to VDC 101 100 110 010 011 800 6 001 101 Va Va as sinusoid 600 4.5 Vb (offset + 400) Vb as sinusoid (offset + 400) Vc (offset + 400) Vc as sinusoid (offset + 800) 400 Ia with 1 H inductive load 200 3 1.5 0 0 -200 0 0.00333 0.00666 0.00999 0.01332 0.01665 Time (seconds) - Excitation Frequency is 50 Hz. 0.01998 Winding Current (amperes) for Inductive Load of 1 H. Output (volts) for a,b,c Axes with Offset to Separate Curves 1000 -1.5 0.02331 Simulated SVPWM 3-Phase Voltage to a Wye-Wound Motor Using 300 VDC Bus 200 VA WyeWound VB Wye-Wound 150 Winding Voltage 100 50 0 -50 -100 -150 -200 0 30 60 90 120 150 180 210 Cycle Angle (seg.) 240 270 300 330 360 Table of Winding Voltages for Switch Settings and Possible PWM Vector Bases ABC VA VB VC 100 0.667 -0.333 -0.333 110 0.333 0.333 -0.667 010 -0.333 0.667 -0.333 011 -0.667 0.333 0.333 001 -0.333 -0.333 0.667 101 0.333 -0.667 0.333 How to Interpolate: • • • • Three switch changes per PWM sample interval Single switch change in each subinterval Uses both zero output values One of several ways that SVPWM can be done depending on supporting hardware • Current harmonic optimization implemented by varying TS over the output cycle t 2V t t sin( ) • Figure shows t sin( ) 0.75 or t mod 2 25.4 deg. V 3 t t t 0.4V 2 DD 1 2 S 1 3 DD 0 1 2 Comparison of PWM Outputs Relative to Ground with Wye and Delta Winding Voltages 300 250 200 150 100 Voltage 50 0 -50 -100 Drive Output A -150 Drive Output B Drive Output C -200 VA Wye-Wound VAB Delta Wound -250 -300 0 60 120 180 Cycle Angle (deg.) 240 300 360 How to EDM a Ball Bearing Race The Idea of Space Vector PWM