Fuji Electric FRENIC-AQUA series, User Manual

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A-12
App. B Effect on Insulation of General-purpose Motors
Driven with 400 V Class Inverters
- Disclaimer: This document provides you with a summary of the Technical Document of the Japan
Electrical Manufacturers' Association (JEMA) (March, 1995). It is intended to apply to the domestic
market only. It is only for reference for the foreign market. -
Preface
When an inverter drives a motor, surge voltages generated by switching the inverter elements are
superimposed on the inverter output voltage and applied to the motor terminals. If the surge voltages
are too high they may have an effect on the motor insulation and some cases have resulted in
damage.
For preventing such cases this document describes the generating mechanism of the surge voltages
and countermeasures against them.
Refer to A.2 [1] "Inverter noise" for details of the principle of inverter operation.
B.1 Generating mechanism of surge voltages
As the inverter rectifies a commercial power source voltage and smoothes into a DC voltage, the
magnitude E of the DC voltage becomes about
2
times that of the source voltage (about 620 V in
case of an input voltage of 440 VAC). The peak value of the output voltage is usually close to this
DC voltage value.
But, as there exists inductance (L) and stray capacitance (C) in wiring between the inverter and the
motor, the voltage variation due to switching the inverter elements causes a surge voltage originating
in LC resonance and results in the addition of high voltage to the motor terminals. (Refer to Figure
B.1)
This voltage sometimes reaches up to about twice that of the inverter DC voltage (620 V x 2 =
approximately 1,200 V) depending on a switching speed of the inverter elements and wiring
conditions.
Figure B.1 Voltage Waveform of Individual Portions
A measured example in Figure B.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.