DG Set Rating

DG set capacity should be 125kva capacity.

DG set capacity should be 125kva capacity.

we have tried 125 kva dg set.

neighter star delta starter nor soft starter has worked.Now we are thinking of vfd

Allen bradley soft starter is not indicating any tripping signal.

A quick and dirty "rule of thumb" for DG set sizing for a single motor is for the kVA to be 2 times the kW if using a soft starter, 3.5 times the kW if starting DOL. For those of us in N. America, that's having the kVA being 1.5 times the HP for a soft starter, 2.5 times the HP if Across-the-Line. This is a gross estimate because it does not factor in the transient reactive capability of the alternator, the recovery capability of the engine governor, the type of load connected etc. etc. It's just based on what tends to work out for most common applications. So in your case, if using a soft starter the DG set should be no less than 150kVA. Being that your 125kVA failed to work, I'd say that is a good bet.

VFDs are a lot easier on the DG set from a starting standpoint because theoretically you can start the motor with no more than 100% FLA if you need to. So for sure your 125kVA would work, maybe even a 100kVA, but I would'nt do that. VFDs also come with harmonics issues so sizing the DG set too closely may end up causing it to overheat. Allow for some extra mass in the alternator to deal with the harmonics by keeping the size at 125kVA.

I have no idea how it would work for Star-Delta, I would never use that in a million years! Star-Delta is really a fake "reduced voltage starting" method, so I would assume you may need to use the DOL/Across-the-Line values to be sure of starting successfully. That's because at some point you need to transition from star to delta and when that happens, the current transient is just as high, if not HIGHER, than DOL anyway!

JRaef, what is that symbol between the capacitor and AC supply? in reference to your picture/avatar

That is my logo, and that symbol is not a capacitor, it is a contactor with an overload relay below it. My logo represents the 3 fields of AC motor control experience I offer; Soft starters, as depicted by the SCR symbol; electro-mechanical controls, as depicted by the contactor and overload symbol; and VFDs, as depicted by the transistor symbol. I understand that these are US symbols and not necessarily universal to the rest of the world, but I don't really do work outside of the US and Canada any longer.

Cheers...

Yeah, that symbol you use for a contactor is the universal symbol for a capacitor. Interesting, I find myself in a situation where I have to write a decrementing power sequence from the source supply to compensate for a shunt circuit. Which in a sense is an overload relay. except, I'm using direct circuitry to split and channel the shunt.

I'm just having one problem, I'm using Axial capacitors that only has a negative symbol on one side of the capacitor and an arrow pointing to the other side to indicate the positive and negative poles of the capacitor. The problem is, the arrow moves away from the negative, so that I'm not sure which side is which when it comes to wiring it. right hand rule says the arrow should point to the negative. have you ever used the Hitano capacitors (the ones with the arrows i.e.(-)==>)?

my multimeter isn't sensitive enough to pick up pico or micro farads, so I can't even test it out.

General thumb rule is starting current of the drive load should not be greater than the 3 times the full load current of the dg set.supposing the Dg is already preloaded the load current to be reduced for calculation.125 kva is not suitable for 75 kw motor.

In this case since u are considering a star Delta starter we can presume that load is a low inertia one .!50KVA will do. How ever if load is a high inertia load with FI >2 then acceleration time is likely to be longer and generator has to meet the starting KVA for a longer duration.So u need to calculate the TVD (Transient Voltage dip)which if less than 80%of rated volts motor may stall.Motors are designed to run for 1 minute at 70% Voltage.There is a good paper Root ABC of Starting from Westinghouse if I am right which may be helpful.Its a very old article dates almost 15years .I had a copy and some one borrowed it with no intention of returning.

Further I also presume that there is only one load that is 75KW motor and no other load on the DG.