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 ( )• Speed sensor• Error detection and controller to set driving speed to

sufficient slip to get the correct motor voltages

V f

Switch Bridge

0 0.00333 0.00666 0.00999 0.01332 0.01665 0.01998 0.02331-200

0

200

400

600

800

1000

-1.5

0

1.5

3

4.5

6

7.5Three-Phase Switch Bridge Output With No PWM

Va

Va as sinusoid

Vb (offset + 400)

Vb as sinusoid (offset + 400)

Vc (offset + 400)

Vc as sinusoid (offset + 800)

Ia with 1 H inductive load

Time (seconds) - Excitation Frequency is 50 Hz.

Out

put (

volt

s) fo

r a,b

,c A

xes

wit

h O

ffset

to S

epar

ate

Curv

es

Win

ding

Cur

rent

(am

pere

s) fo

r Ind

uctiv

e Lo

ad o

f 1 H

.

Switch Pattern: a,b,c - 1 implies direct connection to VDC101 100 110 010 011 001 101

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: FZ50R65KE3 IXBF32N300 FI40-06D IRFP22N50A EPC2025 C2M0280120D

Device Type IGBT IGBT IGBT Half-bridge N-MOSFETGaN HEMT enhancement SiC FET

Vendor Infineon IXYS IXYS VishayEff. Power. Conversion Cree

VDS or VCE Max 6500 V 3200 V (1500 V typ.) 600 500 V 300 V 1200 VIDS or IC max. 750 A 40 A (22 A practical) 30 A (15 A practical) 22 A (14 A practical) 3A 10 ASaturation 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 @ 6AGate 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 pf 0.1 pf 3 pfGate series R 0.75 ohm 2 ohm 4.3 ohm Q G 11.5 uC 130 nc?? 100 nC 120 nC 1.8 nC 20 nC

T turn on800 ns delay; 400 ns rise time 800 ns 80 ns

26 ns delay; 94 ns rise time

6 ns delay; 16 ns rise time

T turn off7.6 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/WUnit 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 (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– Maximum 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)

5DSdV

dt

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• 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

• PWM sampling 25 KHz – 40 usec period• Switching loss is 2.1 W

21 12 2/SWT BUS LOAD RISE PWM OSS BUS PWMP V I C V f

120 2 170

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

0 0.00333 0.00666 0.00999 0.01332 0.01665 0.01998 0.02331-200

0

200

400

600

800

1000

-1.5

0

1.5

3

4.5

6

7.5Three-Phase Switch Bridge Output With No PWM

Va

Va as sinusoid

Vb (offset + 400)

Vb as sinusoid (offset + 400)

Vc (offset + 400)

Vc as sinusoid (offset + 800)

Ia with 1 H inductive load

Time (seconds) - Excitation Frequency is 50 Hz.

Out

put (

volt

s) fo

r a,b

,c A

xes

wit

h O

ffset

to S

epar

ate

Curv

es

Win

ding

Cur

rent

(am

pere

s) fo

r Ind

uctiv

e Lo

ad o

f 1 H

.

Switch Pattern: a,b,c - 1 implies direct connection to VDC101 100 110 010 011 001 101

0 30 60 90 120 150 180 210 240 270 300 330 360-200

-150

-100

-50

0

50

100

150

200

Simulated SVPWM 3-Phase Voltage to a Wye-Wound Motor Using 300 VDC Bus

VA Wye-WoundVB Wye-WoundVC Wye-Wound

Cycle Angle (seg.)

Win

ding

Vol

tage

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• Figure shows or deg. 2

1 3

sin( )0.75

sin( )

t

t

mod2 25.4t 1 2

0 1 2

20.4

3DD

S DD

V t tV V

t t t

0 60 120 180 240 300 360-300

-250

-200

-150

-100

-50

0

50

100

150

200

250

300Comparison of PWM Outputs Relative to Ground with Wye and Delta Winding

Voltages

Drive Output ADrive Output BDrive Output CVA Wye-WoundVAB Delta Wound

Cycle Angle (deg.)

Volt

age

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