Hi
I am working for a Brown field project, where we are adding two 75kw motors (pumps) on the existing 415V LV siwtch board.

This switchboard (bus rating 600A) is fed by 300KVA Generator and 300KVA Transformer.

Generator is for emergency operation.

We have exixting (base) load of 140kw. and we need start one of these new motors.

When I do calculations for motor starting voltage drop with DOL mode, the voltage dip is around 10% at motor terminals when transformer is in operation But 37% with generator operation.

So it is not safe to start 75kw motor in Direct mode during generator operation.

hence, i am planning to propose a soft starter.

Please advice me, 1) does this a right option?
2) Whether a soft starter can work all right during Geneartor in operation?
3) If yes what parametaers, i need to specify for soft starter.

I dont have complete details of load inertia, etc at this stage, as we are in still design stage.

Motor - 415V, 3phase, 50Hz, 75Kw,
Applicatiopn - Cent Pump.
Generator is 300KVA, 415V, 50Hz, 1500rpm diesel generator set and i didnt get details of generator XD", Inertia etc, since it is 15 years old one. (some how data got missed).

Please advice me what are the typical values of inertia for 300Kva Generator and 75 Kw motor.

Thanks in advance

VSR

Hi.

From my experience, using Soft-Starter to start motors generate alot of harmonics during starting.
A 300kVA Generator will have very poor fault level and it will thus amplify the harmonics distortion.
A generator by it self has a lot of harmonics.
You need to be careful.
It may not be advicieable to use softstarter because it would be difficult to set the starting parameters.

I would advice you consider VFD instead.

...................

Did you consider the frequency fluctuation during motor starting?

I am now working on another project that will require starting a 150kW motor on DOL. The system impedance is high. I am required to size the gen-set.

Do you have any simple free software for such simulations?

Regards,
JimC.

Hi Power2020,



Is there only one motor run at a time? If it is, for the Power generator capacity, soft starter or inverter would be the pretty option. If cost would not be a problem to think of, I suggest you to try with inverter (the biggest cost) but inverter bring us many advantages over the soft starter. Because Inverter is easy for operation, saving power device…


If you have to keep your eyes on every “penny” of the project, then, I would like to suggest you to try with Star/delta starters. The cheapest and most economic one. With the starting current is (1/sqrt3)xInrush current of the motor. For one motor starts at a time, the Power generator is enough to supply power at the input of the motor. I think that this would be worth for you to sit down and consider about.


2. If you insist on the Soft starter, you just need to buy one soft starter provides the power to the 02 pumps.

Remember that the starting current using soft starter is 3 to 4 times the rated current of the motor. Is = 4x[75000/(1.73x415)] = 417A. The current supplied by Generator: Ig = 300000/(1.73x415) = 417A. This is normal start (pump application). The soft starter capacity must be equal or ideally one size higher than 75KW. With one soft starter, we can reduce the investment cost for the pumps starters.



Regards,

hi
1) Yes it is a right option.
2) Soft starter will be out of service after starting
please check cable size to meet max. 15% voltage drop during starting