I recently purchased a 60hp phase converter for a customer who purchased a 30hp air compressor. The phase converter company said 150 amp breaker feeding converter with 3/0 was sufficient. I can't get idler motor to start, just turns and trips breaker. Voltage drop at main panel is from 240 to 180. It's a single phase 200 amp panel residential fed from a 50kva transformer. Is there a soft start or autotransformer or something that would get this converter started. At start up it's drawing 800amps but when it's idle it should just draw maybe 20 amps. I know the utility rate their transformers for 80%. They said they would not upgrade transformer. Is there anyway around this to get phase converter started?
Assuming 240V single phase service and by "60HP" you mean it has a 60HP Rotary Phase Converter (RPC)? Forget all the stuff about idle current, totally irrelevant. You are wanting to start a large single phase motor that is rated 60HP as far as starting it is concerned, even if there were no load on it. That transformer is capable of delivering 208A continuous, likely about 300A momentarily without causing more than a 10% voltage drop. A 60HP 240V single phase idler motor is likely rated 135A at FLC, then 6x that on start (which by the way is your 800A). So you have a motor that is going to attempt to draw 800A from a transformer rated to deliver at most, 300A for a second or two. Can't happen.
Bottom line, your RPC vendor was being disingenuous in suggesting that a 150A 2P breaker would work. You need a 150A single phase
SUPPLY for continuous FLC, then the breaker, to avoid nuisance tripping, should be somewhere in the neighborhood of 200-250% of that. Your transformer would need to be that big as well, so I'd say 125kVA minimum.
No soft starter or autotransformer starter (not that there is one for single phase motors) is going to help you here. With a soft starter, it reduces current, but does so by reducing voltage, which cuts starting torque by the square of the voltage reduction. that means that even unloaded, you will still likely draw more current that the transformer can handle without itself dropping the voltage and on a single phase motor that has a centrifugal starting switch for the caps, you end up with too little torque to get it to the point where the centrifugal switch closes, so it never finishes accelerating. Those starting caps are, at the same time, causing damage to the SCRs in the soft starter, and the harmonics in the SCR firing are heating up the capacitor, so it ends up being a race to see who fails first unless the motor can accelerate in a matter of a few seconds, which is exactly the problem here.
If you are stuck with a 50kVA source transformer, the ONLY possible way I see that you are going to start that compressor is by using a 60HP VFD. The VFD must be over sized to handle the conversion from single phase to 3 phase, and a 50% de-rate is an absolute minimum, given ideal conditions. The conductors
feeding a VFD must be sized at 125% of the VFD input current. So because it is a 60HP VFD, likely sized at around 145A input, that means the conductors will need to be rated 181A, there is your 3/0 cable (most likely). The VFD can however accelerate the 30HP 3 phase load motor at 100% FLA if necessary, it's the only thing that can. So assuming an FLC of 80A on your 3 phase compressor motor, that looks like 138A on the single phase side so you have plenty of transformer available to get it up and running.
That said, VFDs and Reciprocating compressors can get problematic because of pulsating torque requirements, but that can often be overcome by using a Sensorless Vector Control capable drive. However compressors that use "splash oiling" are often required to accelerate in very short times so you might need more than FLC to do that. In this case you likely have enough head room in that transformer, as long as little else is running.
