Variable Frequency Induction Motor Drives Simplest Control set frequency for steady state operation only Use digital control Block Diagram Vf Variable Frequency Motor Drive Nothing Fancy ID: 768778
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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 V DC Switch Actuators – gate drivers Algorithm converting angle and voltage to switch timesAlgorithm convert desired speed to angle and voltage ( )Speed sensorError 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 MOSFETAll 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: FZ50R65KE3 IXBF32N300 FI40-06D IRFP22N50A EPC2025 C2M0280120D Device Type IGBT IGBT IGBT Half-bridge N-MOSFET GaN HEMT enhancement SiC FET Vendor Infineon IXYS IXYS Vishay Eff. Power. Conversion Cree VDS or VCE Max 6500 V 3200 V (1500 V typ.) 600 500 V 300 V 1200 V IDS or IC max. 750 A 40 A (22 A practical) 30 A (15 A practical) 22 A (14 A practical) 3A 10 A Saturation voltage 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 1.2 V @ 6A Gate voltage 20 V 25 V 15 V 15 V 5 V 20 V Gate turn-on voltage: VTH equiv. 6 V 4 V 3 V 2.2 V 2.5 V Reverse transfer capacitance (Miller effect) 3.2 nF 27 pf0.1 pf 3 pfGate series R 0.75 ohm2 ohm 4.3 ohm Q G 11.5 uC130 nc?? 100 nC 120 nC 1.8 nC20 nC T turn on 800 ns delay; 400 ns rise time800 ns 80 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 time 600 ns 300 ns delay; 40 ns fall 47 ns delay; 47 ns rise Limited by gate drive 16 ns delay/fall Thermal Resistance junction to case 17.5 deg. C/kW 0.8 C/W 1.0 C/W 0.45 die package 1.8 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 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 Maximum V/nsMaximum junction temperature: 150 C (for reliability limit to 100 - 125 C)Maximum gate voltage: +/- 30 VOther Properties:Minimum recommended RG = 5 ohms Case type: TO-247Thermal 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 VbusBus voltage VBUS = 300 VDCVDS @ 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 PWM sampling 25 KHz – 40 usec periodSwitching 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 CGate charge for 12 Volt VGS and 300 Volt VDS is CQ = 120 nC For rise/fall times = 100 ns this requires 1.2A gate driveTo limit dVDs/dt, the vendor recommends 5 ohm series gate resistorVGS 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 M HI goes from 0 to V bus – a range of several hundred voltsGate drive of MHI is referenced to that source voltageElectrical 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
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 T S over the output cycleFigure shows or deg .
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