Motor Start Voltage Drop-Transformer Selection

[cosfi]

Dear Friends,

Suppose that we have 400 V LV switchgear.And 2000 kVA transformer supplies it.while starting a motor which is supplied from this switchgear , a voltage drop occurs. Can we select transformer LV side 420 V at NO LOAD ? To prevent the voltage drop. But transformer power is the same (2000KVA).
Is there any disadventages selecting LV side 420 instead of 400 V, like insulation or current drop effects?

 

[dpc]

Changing the transformer tap to increase the LV voltage will not eliminate voltage drop - it just raises the starting voltage.

The main problem is risk of overvoltage during light load conditions.

David Castor

http://www.cvoes.com

 

[wareagle]

There is no disadvantage choosing 420 volts as long as it is not too high during minimum load periods.

 

[IndianTechEngg1001]

I think following post will be helpful to you

http://www.eng-tips.com/viewthread.cfm?qid=268359&page=2

 

[7anoter4]

First of all I agree with wareagle as I know a lot of transformers with no-load voltage 420 V[some where I saw even 433 V]. The lighting bulbs life will be shorter, may be. Let's say usual life is about 3000 hours for 230 V .For 420/sqrt(3)=242.5 V will be (230/242.5)^4*3000= 2470[if the voltage will be all the time 420 V indeed].
I agree also with dpc. But since what it is important is the remaining voltage at motor terminals the no-load of 420 V instead of 400 will be welcome.
As thumb rule the maximum motor power for DOL start could be 1/3 of transformer rating that means 2000/3=667 kVA corresponding to 450 kW [600 hp].
Let's say the motor pf =0.8 for rated load and 0.4 for start, the motor starting current [DOL] will be 6.5 time the rated and the pf=0.8 for the other load [of 1375 KVA]. Let's say the 2000 KVA transformer short circuit impedance will be 6% and 22 kW copper losses. Total current when motor starts will be7653 A across xfr then the voltage at transformer terminal will be 364 V [91%] if no-load voltage will be 420 and 344 V [86%] if the no-load voltage will be 400 V. This difference may state if you could start the motor or not.
If the total duration of the motor starting is less than 10 sec and the motor will start no more then twice per day then the transformer life will not be reduced[the total current is 2.7 times the rated!]. A cooling pause of at least 3-4 hours is required between motor full load and restarting.