Switchgear Riddle No.12 - Drive fault
A 11 Kw drive having no I/C and O/G choke, at full load, measured current before drive and after drive are 2A and 30A respectively, why this abnormality? drive feeder is the outgoing feeder of a PMCC, incommer of this PMCC having capacitor bank for PF correction, is this because of capacitor bank or no choke, please discussed the matter.

#1
Wed, January 19th, 2011 - 15:03
The flow of current to a variable speed motor is controlled by the converter. The current is non-sinusoidal due to the non-linearity of the converter and the generation of harmonic currents. Generally forming of sneak circuit in power system including semiconductor devices such as diode and thyristor especially with higher harmonics is possible. Please note to form of tuned sneak circuit probability making with C1,C2 and L in following figure. You can check harmonic content of that leakage current (2A) with an oscilloscope and influence of external capacitor bank with temporary remove it.

#2
Sun, February 2nd, 2014 - 15:23
Dear Manirul islam,
pls ckeck the output  & Input voltage in drive terminal and input &output current . you see the voltage difference in input and output side.
the drive control the output voltage and output frequency to control motor flux(V/f Control)(Motor flux =motor torque).

Now you are doing simple mathmatical calculation.
VFD Input Voltage*Input current  = VFD Output Voltage*output current

And you see that power of VFD input and output side is aprox. equal.