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Transformer size requirement for motor starting

Transformer size requirement for motor starting

Transformer size requirement for motor starting


For starting large motors connected to transformers what considerations need to be taken into consideration when sizding the transformer.

For instance lets say I have a large motor connected to an isolation transformer that only serves the motor.  Could it be possible that the transformer needs to be increased in size well above motors FLA kVA rating to deal with inrush current?  Would the transformers voltage requlation need to be considered in this case?  

Are transformers with lower %impedance usually selected for motor starting applicaions to help with voltage drop?

RE: Transformer size requirement for motor starting

Probably not but you need to check it out.

What is the starting current of the transformer( or starting method).
What is the full Load current.
What is the rating for the transformer.

Then look at the transformer damage curve and the Motor starting curce, If they dont; cross you fine.

RE: Transformer size requirement for motor starting

In addition to Sparky's advise above, voltage dip may be a concern.  Your motor and contactor should have no problem dealing with 25-35% voltage dip during starting, but you may have other loads on the secondary side of the transformer that are more sensitive to voltage dip.

Calculate the voltage drop across the winding of the transformer based on the impedance of the transformer and the motor starting current.  To get an accurate number, you may have to take the transformer's X/R ratio and the motor starting power factor into account (which should be in the range of 0.2-0.4).

You are correct that a transformer with a lower impedance will help lower voltage drop.  Larger transformers will have lower impedance, as will transformers with a lower temperature rise rating.

RE: Transformer size requirement for motor starting

In IEC-land contactors with AC coils are guaranteed to pull in at 85% nominal and to hold in down to 75% nominal. Below those values it's pot luck what happens.

Larger transformers typically have higher impedance, in p.u. terms at least. The reason the impedance is higher with bigger transformers is to limit the fault level to a value which circuit breakers can handle and to limit the electro-dynamic forces during faults.

If we learn from our mistakes I'm getting a great education!

RE: Transformer size requirement for motor starting


What is the best way to calculate voltage drop across transformer during starting.  Would it be to take the transformer impedance and calculated the acuatal impedance referred to the primary and secondary and then calculate the voltage drop across the primary and secondary seperately using the starting current?

What about transforemr size for long starting times.  If a motor has a long acceleration time lets say 20s or so will a transformer sized close to the full load kVA of the motor be able to take the 6-8x starting current for this long duration without damage?

RE: Transformer size requirement for motor starting


Your motor and contactor should have no problem dealing with 25-35% voltage dip during starting

NEMA contactors are required to stay pulled in down to 85% voltage IIRC.  You may get away with something less than that, but you should not count on it.  

Also, as you probably know the motor starting torque varies with the square of the voltage.

The captive transformer acts a lot like a reduced voltage starter during motor starting.   

David Castor

RE: Transformer size requirement for motor starting

 an overcurrent for 20 seconds is more of an issue.

What size is your motor, If it is a large size I would have expected it to be a Star Delta. If the start time is 20 seconds it might be worth using a soft starter.

But Again you have to look at your transformer damage curve, and see what current it can withstand for how long.
However if your motor starts regularly you may need to talk to you transformer manufacturer as repeated starts may reduce the expected lifetime of the transformer.

Don't forget to check your cables as well.

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