Sizing 3 phase Generator

[Locus]

Hi Everyone

May be someone could shed some insight on the sizing and provide some suggestions:

3 phase Generators (400 V, 50 Hz) have the ability to sustain 250% kVA of its rated kVA at 30% voltage dip.

Suppose a machine needs to be powered by a standby generator during a power outage. It has 3 Nos. motors that start simultaneously as follows:
1 No. 11 kW (Start via VFD)
1 No. 2.5 kW (D.O.L)
1 No. 2.5 kW(D.O.L)

The Diversity Factor is 1


The Start kVA (11 kW motor) = 1.6 x (kW/ starting P.F) = 1.6 x (11/0.8) = 22

The Start kVA (2.5 kW motor) = 8 x (kW/ starting P.F) = 8 x (2.5/0.7) = 29 kVA

The Start kVA (2.5 kW motor) = 8 x (kW/ starting P.F) = 8 x (2.5/0.7) = 29 kVA

Total Starting kVA = 22 + 29 +29 = 80 kVA & Running kVA about 22 kVA (including lighting)

1. Since the Generator can sustain 250% of its rated kVA for 10 seconds, can a 50 kVA Generator rating be selected as all the motors will be started in less than 10 secs.

2. Someone who is used with generator sizing maybe could provide the multiplying factors for VFD, soft starter, DOL, and Star-delta, and a good insight of generator sizing for the above.

Regards

 

[James]

Before investing in a unit that big that will cost a lot of money, I would hire a 22kw unit ant try it out.

My gut feeling says it will be enough, the fuel usage of a 22kw is a lot less than a 59kw unit when running less than 16kw of load.

 

[pc1966]

Certainly worth looking at the cost (initial and running) as it might work out cheaper if you have problems to install soft-start units than to over-size the generator.
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Running diesel engines at small fractional load is not good for them either so that is another thing to consider (look up "wet stacking") but maybe not a big issue at ~1/4 load.

 

[snowhead]

Is this real life or theoretical / training?
Are the motors starting on full load or no load?

A generator is only as good as the engine driving it, one generator may have and engine with 50% more power compared to another, both with the same KVA rated generator.
You wouldn't want to rely on driving the generator at + 250% every time it starts up.

I can't help with the calculated sizing but can tell you that the 2 x 2.5 kw motors will be up to full speed before the 11kw Vsd controlled motor is hardly moving /taking current.

As above re pc1966 running at fractional load, but same also applies to running at near full rated load is not good for fuel useage either.

 

[Locus]

Is this real life or theoretical / training?
Are the motors starting on full load or no load?

A generator is only as good as the engine driving it, one generator may have and engine with 50% more power compared to another, both with the same KVA rated generator.
You wouldn't want to rely on driving the generator at + 250% every time it starts up.

I can't help with the calculated sizing but can tell you that the 2 x 2.5 kw motors will be up to full speed before the 11kw Vsd controlled motor is hardly moving /taking current.

As above re pc1966 running at fractional load, but same also applies to run at near full rated load is not good for fuel usage either.

Its a real application and all the motor starts on the fly and on load.
Good point - the smaller motors will reach full speed well before the VSD controller motor.