3.6MW Weg Motor Autotransformer Start

[THD]

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

 

[rbulsara]

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

 

[dpc]

Motor starting current can include a fair amount of dc offset current for a few cycles in addition to the normal locked rotor current. Your measurement may be picking this up.

As Rafiq says, you really need something to compare it to.

David Castor

http://www.cvoes.com

 

[rcw retired EE]

Did WEG provide motor test data? Their test data sheet usually includes a measurement of locked rotor current at reduced voltage which could give you another data point to consider.

 

[gattie]

Did you request a Type Test certificate from the manufacturer before placing purchase order?
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

 

[rockman7892]

If you are switching from the Autotransformer to across the line too early the starting current could be as much or maybe slighly more then DOL starting current. Are you necessarily looking at LRC or starting current as a whole?

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.

 

[electricpete]

Lots of good comments.

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[waross]

We are also using 5KVAr of caps to help boost the voltage as the motor starts (start banks).

Is this a typo?
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

 

[THD]

TYPO!!! Its 5MVAr of caps for the start. We have used this method with great success. We boost the voltage by about 1000V to help her start, ie offset the motors volt drop.

Kevin Bosch
Rainbow Technologies

 

[THD]

Im thinking that Im connected to the wrong tap and am actually BOOSTING the voltage to the motor. The guys on site never did a ratio test... 15min extra on commissioning could of saved so much time, not to mention caning the windings of that poor machine... :/

Kevin Bosch
Rainbow Technologies

 

[waross]

Thanks for the information.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter