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MITSUBISHI ELECTRIC
NAGOYA WORKS
INVERTER ENGINEERING SECTION
TECHNICAL SHEET
V-A5-02
SOFT-PWM CONTROL
1. Purpose
The following explains the control method of Soft-PWM control that is performed to improve
motor audible noise while suppressing the increase of inverter electro-magnetic noise.
2. Reducing the audible noise of the motor
When an inverter drives a motor at a low carrier frequency of about 1 kHz, annoying
metallic audible noise is generated from the motor. To reduce this audible noise, the carrier
frequency was increased to 10 to 14.5 kHz in conventional models in order to move the
audible noise to a frequency range where it could not be heard by human ears. However,
although the audible noise issue was improved by setting a higher carrier frequency, the
amount of inverter generated electo-magnetic noise and the leakage current increased,
leading to adverse effects such as malfunctions of other devices.
3. Soft-PWM control
To resolve this problem, [Soft-PWM control] has been developed. It disperses metallic
audible noise elements without increasing the carrier frequency. As a result, it achieves
the reduction of motor metallic audible noise while suppressing the increase of generated
electro-magnetic noise.
Table 1
Relationships among carrier frequency, motor audible noise, and electro-
magnetic noise
Conventional models (without Soft-PWM control) With Soft-PWM control
Condition
Low carrier frequency
(approx. 1 to 2 kHz)
High carrier frequency
(approx. 10 to 14.5 kHz)
Low carrier frequency
(approx. 1 to 2 kHz)
Item
Motor audible noise
Large
Small
Small
Electro-magnetic noise
and leakage current
Small
Large
Small
4. Control method of Soft-PWM control
A triangular wave comparison method is used to achieve the PWM control of three-phase
inverters. Figure 1 shows the triangular wave comparison method.
Triangular wave signal
(A cycle denotes a carrier frequency)
Phase voltage command
Phase voltage output
(Command of output transistor)
Figure 1 Triangular wave comparison method
This method compares the phase voltage command of a sine wave with a triangular wave
signal. It then determines the switching operation of an output transistor with phase voltage
to output the voltage based on the magnitude correlation between the triangular wave
signal and the phase voltage command.
TECHNICAL SHEET : E (V-A5-02)
1
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MITSUBISHI ELECTRIC
NAGOYA WORKS
INVERTER ENGINEERING SECTION
TECHNICAL SHEET
Triangular wave signal
Figure 2. Carrier frequency and output voltage wave
TECHNICAL SHEET : E (V-A5-02)
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MITSUBISHI ELECTRIC
NAGOYA WORKS
INVERTER ENGINEERING SECTION
TECHNICAL SHEET
Figure 2 shows the switching timing of each phase using the triangular wave comparison
method when the command voltage is set to a constant value.
The voltage for the N point on the DC bus negative polarity side of each phase obtained
from the triangular wave comparison is the pulse voltage shown by (1) in Figure 2, and the
line voltage is the pulse voltage shown by (2) in Figure 2. This pulse-type voltage is applied,
and its time average value will become equal to the line voltage value of the command
voltage.
Here, the line voltage is determined, for example, by the pulse width t 0 in which voltage is
output in the case of voltage Vu-v between U and V. The output voltage will be the same
even if the output timing of this pulse voltage is changed.
Therefore, frequency elements are dispersed by changing the output timing on a time basis
(i.e., changing the distribution of t 11 and t 12 on a time basis while maintaining the total time
value of t 11 + t 12 ) without changing the pulse width t 0 of this pulse voltage. This enables to
change the sound quality of the magnetic noise from the motor.
This method that changes the output timing of the pulse voltage on a time basis is called
Soft-PWM control.
Figure 3 shows examples of noise data with and without Soft-PWM control.
With Soft-PWM control
Without Soft-PWM control
Noise level
Noise level
12k
10k
8k
6k
4k
2k
0
12k
10k
8k
6k
4k
2k
0
4
4
2
2
Time (s)
Time (s)
0
0 2k 4k 6k 8k 10k 12k 0
0 2k 4k 6k 8k 10k 12k
Frequency (Hz)
Frequency (Hz)
Because frequency elements are dispersed, magnetic
audible noise is reduced and not unpleasant to the ear.
Because frequency elements are concentrated,
magnetic audible noise is disturbing to the ear.
Figure 3 Examples of noise data
TECHNICAL SHEET : E (V-A5-02)
3
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