3.6MW Weg Motor Autotransformer Start
3.6MW Weg Motor Autotransformer Start
(OP)
We have had issues starting a 3.6MW Squirrel Cage motor with an 85% auto transformer tap. The inrush current is higher than the DOL inrush! We measured the inrush as 1050A with a 15% volt drop. The DOL inrush should be around 850A with a lower volt drop. The manual shows a 1140A inrush for DOL but thats obviously for infinite bus starting. We dont have infinite bus as we are starting at the end of a long 70mm2 ohead line. We are also using 5KVAr of caps to help boost the voltage as the motor starts (start banks). The caps switch out when the motor is up to speed. We were expecting a 20sec start up time for a 85% voltage start. What could be causing the inrush to be higher than that expected for DOL?
Kevin Bosch
Rainbow Technologies






RE: 3.6MW Weg Motor Autotransformer Start
Is the "expected" DOL value of 850A was ever measured or just calculated? Assumptions for the calculated may not be accurate. You really need to compare two 'actual' values at two different taps to verify if the auto-transformer setup in effective or not.
Also what is the actual voltage at the motor terminals vs. rated? Remember the rated motor voltage is typically less than nominal system voltage to begin with.
Try 65% tap, which should be more of a norm than 85%. If the motor is stronger (actually capable of delivering of more than power 'nameplate' or torque produced at 85%V is more than enough), it will tend to draw more current at 'slight' lowering of the voltage as long as the source can deliver it.
Comparing the 65% value to 85% value will also tell you, if the auto transformer arrangement is working or not.
Rafiq Bulsara
http://www.srengineersct.com
RE: 3.6MW Weg Motor Autotransformer Start
As Rafiq says, you really need something to compare it to.
David Castor
www.cvoes.com
RE: 3.6MW Weg Motor Autotransformer Start
RE: 3.6MW Weg Motor Autotransformer Start
It is always prudent to do so, or alternatively request a full performance test including heat run. This size of motor
usually warrants taking this precaution.
Unacceptable inrush current for your system will show up on the factory test.
Good Luck
RE: 3.6MW Weg Motor Autotransformer Start
How are you determining switching time from Autotrans to across the line?
Obviously the lower the voltage tap the longer the acceleration time and possibly the more thermal capacity that gets eaten up during the start.
As Rafiq says if the system nominal voltage is larger than the nameplate voltage (eg. 4kV motor on 4.16kV system) this will lead to higher LRC than nameplate or datasheet.
RE: 3.6MW Weg Motor Autotransformer Start
It is not unusual for OEM "calculated" values of locked rotor current to be less than actual, which is why user should ask for actual (or reduced voltage locked rotor test that was mentioned).
I have seen people fooled by dc offset component of current was was mentioned. Also by the difference between peak and rms. In some cases the person feeding you the info uses the terminology differently. For that reason I prefer to see a time waveform current trace.
For that matter, voltage trace would go a long way as well (is the autotransformer stablishing the voltage you think it is).
I had a thought that maybe your caps were boosting your voltage, but then I saw it was 5kvar... drop in the bucket.
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RE: 3.6MW Weg Motor Autotransformer Start
I would expect about 5MVAR would be needed to make much difference to the starting current of a 3.6MW motor.
I have read about this technique but this is the first actual instance of an installation that I have heard of.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 3.6MW Weg Motor Autotransformer Start
Kevin Bosch
Rainbow Technologies
RE: 3.6MW Weg Motor Autotransformer Start
Kevin Bosch
Rainbow Technologies
RE: 3.6MW Weg Motor Autotransformer Start
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter