Estimated kva we must provide


Thread Starter


I want to ask how to determine kva that we must provide for 500kw motor, 3 phase, 380 Volt. As we know, motors have big starting current. About 4 times the nominal. Should we ask for 2000kw or 2500kva from our state electric provider? Thank you.
There are two different parts of your problem.
First is to decide KVA rating of the motor & second to ask from Elec Board for KVA demand.

1. As you have mentioned, induction motors take very high current to start. And in your case 500 KW, 380 V motor, even if it has a PF of 0.80 at no load, will draw approx 1200x6 = 7200Amp while starting on DOL. But it will depend upon the type of load you put on the motor, which decides up to what time (full accelearation of motor) it will continue. For loads like Pumps or Compressors it may be for 10 to 15 secs whereas for Reciprocating Compressors / Crushers / Rolling M/c or Centrifuges it may be in minutes. So how much amp at what PF will help you to decide the KVA rating of the motor.

2. Usually Electricity Boards put a two or three part tarriff meters which count your MD (Max. Demand) in several steps of time. It may vary from 10 mins to 30 mins. So you have to ask for a MVA demand 10% higher than the usual KVA Rating of the motor to keep margins for process / machine's short time overloading. But while starting the motor, you may have a precaution to wait for the MD counter to reset after its one count cyle, so that while starting a heavy load, you do not cross the MD limit for that specified time period of MD counter to pay heavy penalty. If the load is continuous type, it will work well, but if starts/stops are very frequent, it is better to reduce starting current by putting some soft starter & demand MD according to starting current.

I hope it clarifies the matter.

In Australia, most supply authorities do not permit the DOL starting of motors if it is over about 150kW (200hp). I would hazard a guess & say that the motor you speak of will be started under current limiting conditions (resistive, soft starter etc) but as Pachant said, the starting current will be determined by the mechanical load & the limitations imposed by the supply authority. In the meantime, if you can find out the locked rotor current of the motor, you can use this figure to calculate the supply KVA.