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Is an isolation transformer needed on a VFD input side?

edison123

Electrical
Oct 23, 2002
4,451
This week, we trialed a 125 KW VFD for testing LV motors in decoupled condition. During the trials, I noticed our LED lights started flickering even though they were switched off.

When asked, the OEM drive guru said there will be harmonics fed into the system, and we need to install an isolation transformer to prevent harmonics getting back into other loads.

Is this correct? If yes, what will be transformer connection, delta input/wye output, or delta/delta or wye/delta?

What will be the KVA rating of the isolation transformer?

If isolation transformer is required, can I go for 415 V input / 690 V output so that I can get a 690 V VFD output for testing some motors which are rated at 690 V?

Will a 690 V VFD give 415 V output (which will be the 95% of the motors we test) without any derating in current capacity?
 
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In general, drives only care about the current. This means a 690V-400A drive will happily operate at around 415V-400A. But the harmonic content on both the input and output sides of the drive will be somewhat higher than when operated at rated (690V) output.

Isolation transformers are (usually) rated 1:1 in terms of voltage, but there is no reason you can't go to 415/690 if that's what works for you. Given that the drive is on the output side of the transformer, it's probably a good idea to have the transformer secondary as a WYE connection.
 
Just a thought;
We have had posts that pointed out that while a 690 Volt drive will produce an effective voltage of 415 Volts, the peak of the individual pulses will still be based on 690 Volts.
The windings of the 415 Volt motor will see the same stress as if they were tested at 690 Volts.
If this is a concern, You may want to look for a 415:690 Volt transformer with 415 Volt taps so that you may test 415 Volt motors without the 690 Volt stress.
 

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