Download Variable Frequency Induction Motor Drives

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
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