@Marky Mark posted:The only way to measure the real in rush is to put a scope on it. Rule of thumb, in rush without a softstart is 7 to 10 times running current. A softstart works much the same as an inverter. SCRs fire to slowly allow the voltage to ramp up while limiting the current. An inverter, because it is mimicking full sine way can only allow about a 200% current inrush. Softstarts don't provide clean AC, just square wave pulses. They can go 7 times locked rotor current. Locked rotor is the same as normal FLA (full load running) amps.
Thank you for the best explanation of how a soft start works that I seen. I am curious now, if the softstart is using a square wave during start-up, does it also modify the sinewave once the compressor is running even though there is no current reduction?
My crude method for trying measure starting current consisted of a Fluke 87iii meter with min/max and fast peak settings along with a Fluke current transformer and let it trap the max current on a number of starts. The highest was 30.84 amps with the softstart. The compressor has a FLA =8.75 (compressor only) and a LRA = 56 amps which is about 7 times so I am thinking the softstart cut starting current nearly in half.
I think your last sentence might have intended to say: 'Locked rotor amps (LRA) is the same as normal inrush starting current amps (i.e. without a soft start)'.