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 V DC Switch Actuators – gate drivers Algorithm converting angle and voltage to switch times Algorithm convert desired speed to angle and voltage ( ) Speed sensor Error detection and controller to set driving speed to sufficient slip to get the correct motor voltages

Switch Bridge

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:FZ50R65KE3IXBF32N300FI40-06DIRFP22N50AEPC2025 C2M D Device TypeIGBT IGBT Half-bridgeN-MOSFET GaN HEMT enhancementSiC FET VendorInfineonIXYS Vishay Eff. Power. ConversionCree VDS or VCE Max6500 V3200 V (1500 V typ.) V300 V1200 V IDS or IC max.750 A40 A (22 A practical)30 A (15 A practical)22 A (14 A practical) 3A 10 A Saturation voltage A A A 125 C A A1.2 6A Gate voltage20 V25 V15 V 5 V20 V Gate turn-on voltage: VTH equiv.6 V4 V 3 V2.2 V2.5 V Reverse transfer capacitance (Miller effect)3.2 nF 27 pf0.1 pf3 pf Gate series R0.75 ohm2 ohm 4.3 ohm Q G11.5 uC130 nc??100 nC120 nC1.8 nC20 nC T turn on 800 ns delay; 400 ns rise time800 ns80 ns 26 ns delay; 94 ns rise time 6 ns delay; 16 ns rise time T turn off 7.6 us delay;.5 us fall time600 ns 300 ns delay; 40 ns fall 47 ns delay; 47 ns rise Limited by gate drive16 ns delay/fall Thermal Resistance junction to case17.5 deg. C/kW0.8 C/W1.0 C/W0.45die package1.8 C/W Unit cost$3,026$43$11$3$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 (C GSS & C RSS ) with R G 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 – Maximum V/ns – Maximum junction temperature: 150 C (for reliability limit to 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 Motor winding peak volts volts and VA peak < 2/3 Vbus Bus voltage VBUS = 300 VDC 5 A is sensitive to TJ as C, C and 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 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 + ( )*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* = 12 volts

VGS Level Shift Problem Source of M HI goes from 0 to V bus – a range of several hundred volts Gate drive of M HI is referenced to that source voltage Electrical isolation needed between the controller and M HI

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

Table of Winding Voltages for Switch Settings and Possible PWM Vector Bases ABCVAVBVC

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 T S over the output cycle Figure shows or deg.

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