Appendix C
166
C.4.4 Checking the insulation
Check the insulation of the motor and motor cable as follows:
1. Check that the motor cable is connected to the motor and disconnected from the drive output
terminals U, V and W.
2. Measure the insulation resistance between each phase conductor and the Protective Earth
conductor using a measuring voltage of 500 V DC.
For the insulation resistance of other motors,
please consult the manufacturer's instructions.
Note:
Moisture inside the motor casing will reduce the insulation resistance. If moisture is
suspected, dry the motor and repeat the measurement.
C.5 Breaker, electromagnetic contactor and leakage protection
switch
Due to the VFD output high frequency PWM voltage waveform, and the existence of distributed
capacitance between IGBT and heat sink in internal VFD and the distributed capacitance between
motor stator and rotor will cause the VFD inevitably generate high-frequency leakage current to
ground. The high-frequency leakage current will back flow to grid through the earth to
interference the leakage protection switch, thus causing the leakage protection switch malfunction.
This is due to the VFD output voltage characteristics inherent in the decision.
To ensure the stability of the system, it is recommended to use the VFD dedicated leakage
protection switch which rated residual operation current 30mA or more(for example, corresponds
to IEC60755 Type B). If you are not using the VFD dedicated leakage protection switch caused by
malfunction, try to reduce the carrier frequency, or replace the electromagnetic leakage protection
switch which rated residual operating current of 200mA or more.
It is necessary to add fuse for the avoidance of overload.
It is appropriate to use a breaker (MCCB) which complies with the VFD power in the 3-phase AC
power and input power and terminals (R, S and T). The capacity of the VFD should be 1.5-2 times
of the rated current.
Due to the inherent operating principle and construction of circuit
breakers, independent of the manufacturer, hot ionized gases may escape
from the breaker enclosure in case of a short-circuit. To ensure safe use,
special attention must be paid to the installation and placement of the
breakers. Follow the manufacturer's instructions.
