Appendix C Effect on Insulation of General-purpose Motors Driven with 400 V Class Inverters
Appendix-21
C.3 Countermeasures against surge voltages
When driving a motor with a 400 V class inverter, the following are countermeasures against damage to the motor
insulation by the surge voltages.
[ 1 ] Using a surge suppressor unit (SSU)
A surge suppressor unit (SSU) is a newly structured unit using circuits based on the impedance-matching theory of
a transmission line. Just connecting the SSU to the surge suppressor cable of the existing equipment can greatly
reduce the surge voltage that results in a motor dielectric breakdown.
For 50 m of wiring length: SSU 50TA-NS
For 100 m of wiring length: SSU 100TA-NS
[ 2 ] Suppressing surge voltages
To suppress surge voltage, a method is employed which involves suppressing voltage rise and peak value.
(1) Output reactor
If wiring length is relatively short, the surge voltages can be suppressed by reducing the voltage rise time (dv/dt)
with the installation of an AC reactor on the output side of the inverter. (See Fig. C.3-1(1) .)
However, if the wiring length becomes long, suppressing the peak voltage due to surge voltage may be difficult.
(2) Output filter
Installing a filter on the output side of the inverter allows the peak value of the motor terminal voltage to be reduced.
(See Fig. C.3-1(2) .)
Fig. C.3-1 Method to suppress surge voltage
If the wiring length between the inverter and the motor is comparatively long, the crest value of the surge
voltage can be suppressed by connecting a surge suppressor unit (SSU) to the motor terminal.
For details, refer to Chapter 11 “11.13 Surge Suppression Unit (SSU)”.
[ 3 ] Using motors with enhanced insulation
Enhanced insulation of a motor winding allows its surge withstanding to be improved.
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