Transformer Sizing for Large Motor

[kvramesh]

Hi,

First of all I apologize for not searching enough on the forum for this question is mundane and could have been answered several times.

We have two 1800HP motors at 4.16kV. Plan is to start them one after the other and that too via soft starter. The transformer is rated 8000kVA, 13.8kV-4.16kV, 5.75%, DYN1. The available short circuit on primary should be no more than 40kA. Can someone advise me if this size transformer is acceptable, not like a detail design just for quick understanding of how this is sized especially when running on soft starter

Thanks and appreciate your help

RK

 

[Jraef]

Quick and dity rule-of-thumb, assuming you want less than a 2% voltage drop at the transformer line side terminals, 5% on the load side and assuming one motor is already running at full load when the second one is started:

X-Line starting: XFMR kVA = 3-3.5 times the (2nd motor) HP, so for an 1800HP motor, 5400kVA + 2300kVA (approx FL motor kVA) = 7700kVA

RVAT starting: XFMR kVA = 1.5 times the motor HP + ...

RVSS starting: XFMR kVA = 2-2.5 times motor HP + ... (depending on the type of load, centrifugal pumps are easier)

So based on that, unless you have a lot of other loads besides the first motor on-line when you start the 2nd one you are fine almost no matter what.