Using GaN Devices to Improve the Power
Efficiency in a Motor-Inverter Drive System
BY
Jim Honea – Transphorm, Inc. – Goleta CA
Richard Welch Jr. – Welch Enterprise – Oakdale MN
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Motor Efficiency: a hot topic worldwide
US DOE push to optimize motor efficiency
Information from DOE , Motor Challenge fact sheet
European efficiency classification scheme for low voltage AC motors
Information from ABB
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Motor Efficiency: a variable, not a constant
Efficiency of the motor itself, and of the motor system, depends on numerous factors:
Temperature, Load, Speed
Brushless DC motor efficiency curve
Why the Exlar T-LAM™ Servo Motors have Become the New
Standard of Comparison for Maximum Torque Density and Power
Efficiency
By Richard Welch Jr. - Consulting Engineer, November 3, 2008
AC Induction Motor system efficiency curve
Induction motors fed by PWM frequency inverters
WEG www.weg.net
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Steps for system efficiency
1. Use a Premium Efficiency Motor. ( but… right sized, and properly installed)
2. Use a Variable Frequency Drive
Saving Energy with Variable Speed Drives
By Mark Gmitro
Baldor Electric Company
PWM drive signals produce currents with harmonic distortion
T = KT I
T = ∑ K Ti cos iθ ⋅ ∑ I j cos jθ
i
j
Only harmonics in the current waveform which correspond to harmonics in the torque
function contribute to useful work. A higher switching frequency enables lower distortion.
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Image from: http://www.screenlightandgrip.com/html/emailnewsletter_generators.html
The combination of Inverter Drive and Motor creates
power loss in both locations.
P
Inverter
M
Drive
Q
Each harmonic component in the drive waveforms has a “power factor” of it’s own:
Real power (P) is delivered to the motor,
Reactive power (Q) returns to the Inverter Drive.
Except for useful work done at the fundamental frequency, the real power serves to heat
the motor.
The reactive power serves to heat the inverter (becomes real power in the Inverter and is
dumped as heat to the heat sink)
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Specific example: energy temporarily stored in cable and
motor capacitance, dissipated in inverter. P = 3⋅ 1 CV 2 f s
2
Lumped total parasitic motor capacitance (common‐mode and differential mode)
I = CdV/dt
M
Inverter output
cable
Ground represents a simplified union of all possible return paths.
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Switching loss occurs when Id and Vds are
nonzero simultaneously.
Id
+
Vds
Vdc
Iload
Current through the
switching device (Id)
Voltage across
the device (Vds)
t
Switching loss
Power
t
Rapid rise time to reduce switching loss=motor stress. Increasing switching
frequency reduces loss at other harmonics, but increases loss at fs
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Commercial Si inverter, DC power in,
no motor connected
25
Pin (W)
20
15
10
no cable
with cable
5
output off
0
0
5
10
15
20
fs (kHz)
Increased switching loss due solely to a 26ft cable that adds external
capacitance. Plotted is DC input power versus PWM switching frequency.
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Commercial Si inverter, electrcial power loss,
driving a 1hp motor at full load, rated speed
40
loss (W)
35
30
25
20
15
0
5
10
15
20
fs (kHz)
Increased switching loss in the inverter due to motor capacitance.
Plotted loss versus PWM switching frequency. (short cable)
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A solution: high switching-frequency Inverter Drive
with output filter.
Only low‐frequency excitation outside inverter
M
filter
cable
Illustration of using a filter to isolate the inverter from external capacitances.
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Testing a GaN three phase inverter drive.
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Three-phase GaN module
GUH
GVH
GWH
KUH
KVH
KWH
U
V
GUL
GVL
GWL
KUL
KVL
KWL
W
P
Module spec:
• 6 in 1 switches
• 600V, 14A capability
at TC = 100oC
Three-phase GaN inverter, including filters
Electrical efficiency - Pac(out)/Pdc(in) –
up to 2hP driving a resistive load
Loss due to the output filter is included in the efficiency calculation.
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Electrical efficiency - Pac(out)/Pdc(in) –
Driving a 1hP AC Induction Motor 60Hz/1800rpm
GaN inverter efficiency: PAC(out)/PDC(in)
99
27
98
24
97
21
efficiency
95
18
loss
94
93
(W)
(%)
96
15
92
12
91
90
200
300
400
500
600
700
800
9
900
3‐Phase Electrical Ouput Power (W)
Loss due to the output filter is included in the efficiency calculation.
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Electromechanical efficiency - Pmech(out)/Pac(in) –
for a 1hp AC Induction Motor 60Hz/1800rpm.
Motor was driven by a GaN Inverter Drive, and by a commercial, silicon-based drive.
The GaN Inverter Drive includes output filters; the silicon inverter drives the motor
directly using 16 kHz PWM frequency.
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System efficiency - Pmech(out)/Pdc(in) –
Of a 1hp AC Induction Motor 60Hz/1800rpm.
Standby power for the commercial silicon drive is subtracted out.
Motor was driven by a GaN Inverter Drive, and by a commercial, silicon-based drive.
The GaN Inverter Drive includes output filters; the silicon inverter drives the motor
directly using 16 kHz PWM frequency.
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Further System Efficiency Test Data
Part of the development supported by funding from ARPA-E, contract number DE-AR0000115
Rapid rise and fall times can lead to EMI problems.
EMI requires
1. A source
2. A coupling means
3. A receiver
Minimize the coupling means and the source is less likely to offend.
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Myth Buster:
Faster Switching ≠ Higher EMI*
Superjunction Silicon
Att 10 dB
dBµV
100
1 MHz
RBW
9 kHz
MT
10 ms
PREAMP OFF
vs.
Marker 1 [T1 ]
54.00 dBµV
150.000000000 kHz
Att 10 dB
dBµV
10 MHz
1 PK
MAXH
80
2 AV
MAXH
100
RBW
9 kHz
MT
10 ms
PREAMP OFF
Marker 1 [T1 ]
51.11 dBµV
150.000000000 kHz
1 MHz
10 MHz
90
90
1 PK
MAXH
GaN (TPH2002PS)
TDF
70
2 AV
MAXH
80
TDF
70
EN55022Q
EN55022Q
60
60
1
PRN
EN55022A
PRN
1 EN55022A
50
50
6DB
6DB
40
40
30
30
20
20
10
10
0
0
150 kHz
30 MHz
Peak
Average
150 kHz
30 MHz
Conducted noise: Converter (PFC + Dual Flyback), 90 Watt, Vin = 230
Vac , Fsw = 60 kHz;
ton & toff of GaN is 0.33 toff of Silicon, so the GaN is switching faster
without increasing the EMI.
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*Courtesy of Fairchild Semiconductor company
Conclusion
GaN power transistors switch very fast with little loss,
enabling high PWM frequency.
High PWM frequency can increase system efficiency by
1. Enabling high-fidelity (low distortion) drive
waveforms
2. Enabling small-size filters which isolate the real
and reactive impedances of the motor and cable
from the switching signal.
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Contact information
Jim Honea – Applications Engineer
Transphorm Inc.
115 Castilian Way
Goleta, CA 93117
(805) 456 1300 (jhonea@transphormusa.com)
Richard Welch Jr. – Consulting Engineer
Welch Enterprise
7819 – 31st St N
Oakdale, MN 55128
(651) 777 – 6066 (welch022@tc.umn.edu)
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