132kw 3-Phase Induction Blower Motor

Reading the output current of a VFD with standard hand-held multimeters is a pointless exercise, the readings you get are virtually meaningless. It takes a very expensive meter to be accurate with the harmonic rich output of a VFD. it's best to trust the output current reading of the VFD display, it is already required to calculate that correctly in order to accomplish the motor protection.

No idea why someone would use two inverters. On really large systems there are some inverter mfrs that use parallel output power sections to attain the necessary current, but 132kW is nowhere near large enough to require that. Are you sure you are seeing what you think you are seeing? Are there really two inverters, or are you maybe looking at a 12 pulse inverter with two input transformers and rectifier systems? Much more likely. If someone tried to use parallel inverters using two separate VFDs, that is destined to fail in short order. There is no way to absolutely guarantee that the output firing patterns were identical unless they are both being controlled by a single firing system..

i) The input current is sinusoidal and you get an RMS Value of this wave form as the current reading in your incoming ammeter. The output current is not sinusoidal but a PWM Wave and hence its RMS would definitely not match with that of the input. Hence, you get two different redaings. Go for a true RMS Meter and measure currents at the input & output. They should match (or input current will be slightly higher than the output current).

ii) It could be two 6-Pulse Inverters. This is to reduce the Harmonics. Two 6-Pulse Inverters fed from a three winding Star-Delta-Star Transformer - nullifies the 5th & 7th Harmonics.

But on a 12 pulse inverter drive, it is the converters (rectifiers) that are duplicated, not the inverters. They still only have one inverter. That's why I mentioned that he might be mistaken about what he is seeing.

thanks for the detailed explanation.will pass on the electrical drawing for the inverters as soon as i get hold of them fro the factory.i am sure you will be able to find out the clue to my problem

I am attaching the start up method i.e. inverter for 132 kW motor. the drawing clearing shows that two inverters are used. Hope you will guide me accordingly.

Input current should not follow voltage on a PWM Drive: Current will flow only as needed to keep the DC Bus charged. In light load condition, current will flow only at input voltage peaks. As the drive digs in, current will flow at lower (closer to zero volts) points on the input voltage sine wave.

Agreed! Thanks for correcting me!

will electrical analyzer AR5 or you will suggest some other testing analyzer thanks

Another possibility, do you know if both inverters are on line at the same time? Being that you did not state the function of the motor, I cannot say for sure, but could one inverter be a redundant backup? Where I work, we build many tactical systems for the military, where failure is not an option. So, everything has a redundant backup system. May not apply, just brainstorming.

the function of the motor is used for humidified air for the automobile factory.

capacity (air flow) 4000 meter cube/m. static pressure 1100pa. RPM 530.

shaft power 114 B.kw.

air density 1.198kg/cube meter.

total pressure 1200pa.

total efficiency. 77%

starting method INVERTER.

frequency control range 6 hz-50Hz(50Hz base)

rating continuous'

rated current 250 amp

inspected the site today and noticed that one inverter was on and the 2nd one was not displaying Frequency on the frequency meter which shows that 1 inverter whose frequency is displaced on the panel

yes i agreed with Jreaf.