Let do some simple calculation..

A 250HP Motor = 250HP x 0.746 KW = 186.5KW

Total running current = P/(V x I x 1.732 x 0.85)

Current (I) = 186.5KW/ (415 x 1.732 x 0.85) = 305.28 Amps

Diesel Generator = 500Kva

Total Current at 100% efficiency = Kva /(V x I x 1.732)

Current at 100% efficiency (I) = 500Kva / (415 x 1.732)

Current = 695.62 Amps

Assuming the Diesel Generator is about 95% efficiency after some electrical and mechanical losses, therefore the maximum power capacity that you have is about 660.84 Amps

Now, for starting your 250 centrifugal blower motor, with Star Delta starter, the starting current will be 1/3 of the DOL Starter. Therefore, your staring current will be,

Star-Delta Starting Amps= (305.28 x 7.5)/3 =763.2 Amps (13.4 % over your Diesel Generator output).

The above simple calculation shows that you will trip the MCCB of the Diesel Generator Panel during starting your 250HP centrifugal blower motor.

Other starter like soft starter, you got to know which type of the soft starter to buy. Most of the soft starter is a current limit starter where it required about 2.5 time of the FLC motor current. That mean, the soft starter will start your 250HP motor with 763.2 Amp.

There are some new soft starter, that start the motor with zero amps and slowly ramps up to the motor Full Load Current.

For me, I would prefer to go for VFD.

Now my question is,

Is your centrifugal blower is direct couple to the 250HP motor or it is a belt driven by pulleys. If it is a belts driven by pulleys we need to have your pulleys ratio (the Dimension) couple to the motor and the centrifugal fan.

The starting amps will be slightly different.

ok. yes. a soft starter that gradually ramps up the current from zero to flc is good thinking. a vfd is better still.

but when we are talking about a blower the spoiler is that the start will be heavy and the heaviness will prevail for relatively longer duration.

ordinarily - the switchgear trip settings and vfd/ss settings will take care of the same - the overload trip devices can even be temprarily bypassed for a short duration - BUT i have reservations about the generator performance for such a start - suggest that the start conditions as foreseeable should be shared with the generator manufacturer before pressing it into use.

Thank you very much

The problem with soft starter you are loosing torque on the motor during start up and the motor may not overcome the inertia of the fan impeller. This will eventually result in a trip.

VFD is the best option, because you get full motor torque at 1.5 times motor current (this current changes from different types of VFD's). The VFD should have no problem to accelerate the fan impeller.

DGs all have what is called "reserve capacity" for strarting motors, which is much larger than the FLA capacity. You cannot answer this question definitively without knowing the reserve capacity of the DG.

That said, there is a 'rule of thumb" that works MOST of the time.

For DOL (Across-the-Line) starting, the DG kVA must be at least 3X the HP (no conversion) or 4X the kW. So if you have a 250HP motor, you will need a 750kVA DG to start the motor DOL.

For a Solid State Soft Starter, the kVA must be 1.5-2X the HP or 3X the kW, so in your case that 500kVA should be fine. By the way, you CANNOT start a motor with FLA using a soft starter. I know that many brands allow you to adjust the Current Limit down that low, but an AC motor will not accelerate with that.

For VFD starting, you can use 1X the DG size. A VFD can start the motor with FLA in most cases.

I never use Y-Delta for starting large motors, in most cases it is a misconception that it reduces starting current and it has the potential to make matters a lot worse, both mechanically and electrically.

It is true.. Starting big motor with Y-Delta has the potential to make matters a lot worse, both mechanically and electrically because there is two in rush current for this Y-Delta starter. In other words, the motor will induce two in rush torque, that is during the star point and during the changer over point to delta. Normally, you will not see the damages but for long run, there will problem to the motor and the fan.

When I was a child, a local bully used to offer me a choice: one big hit with his closed fist or 2 slaps with his open hand. Neither was an attractive choice.