Appendix C Effect on Insulation of General-purpose Motors Driven with 400 V Class Inverters
Appendix-18
A measured example in Figure C-2 illustrates the relation of a peak value of the motor terminal voltage with a
wiring length between the inverter and the motor.
From this it can be confirmed that the peak value of the motor terminal voltage ascends as the wiring length
increases and becomes saturated at about twice the inverter DC voltage.
The shorter a pulse rise time becomes, the higher the motor terminal voltage rises even in the case of a short
wiring length.
Figure C-2 Measured Example of Wiring Length and Peak Value of Motor Terminal Voltage
C.2 Effect of surge voltages
The surge voltages originated in LC resonance of wiring may be applied to the motor terminals and depending on
their magnitude sometimes cause damage to the motor insulation.
When the motor is driven with a 200 V class inverter, the dielectric strength of the insulation is no problem even 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.
Excerpt from Journal of IEEJ, No. 7, vol. 107, 1987
For IGB T,
tr is equivalent to 0.1 to 0.3 µs.
For the bipolar transistor,
tr is equivalent to 0.3 to1 µs.
In case an output reactor or a filter is inserted,
tr is equivalent to 1 µs or more.
Surge voltage magnification factor:
Magnification factor for DC voltage (E)
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