A-18
Figure C.2 Measured Example of Wiring Length and Peak Value of Motor Terminal Voltage
C.2 Effect of surge voltages
The surge voltages originating in LC resonance of wiring may be applied to the motor input terminals and
depending on their magnitude sometimes cause damage to the motor insulation.
When the motor is driven with a 230 V class inverter, the dielectric strength of the insulation is no problem
since the peak value at the motor terminal voltage increases twice due to the surge voltages (the DC
voltage is only about 300 V).
But in case of a 400 V class inverter the DC voltage is approximately 600 V and depending on the wiring
length, the surge voltages may greatly increase and sometimes result in damage to the insulation.
C.3 Countermeasures against surge voltages
The following methods are countermeasures against damage to the motor insulation by the surge voltages
and using a motor driven with a 400 V class inverter.
[ 1 ] Method using motors with enhanced insulation
Enhanced insulation of a motor winding allows its surge proof strength to be improved.
[ 2 ] Method to suppress surge voltages
There are two methods for suppressing the surge voltages, one is to reduce the voltage rise time and
another is to reduce the voltage 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. (Refer to Figure C.3 (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 a peak value of the motor terminal voltage to be
reduced. (Refer to Figure C.3 (2).)
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