45kW screw compressor on a star-delta starter. The timer relay and the delta contactor cooked last week and plant air is on the backup unit while we sort it out.
Three quotes on my desk now. One says put a new star-delta in, cheapest of the lot. One says soft starter. One says VFD and promises 30 percent off the energy bill.
The compressor loads and unloads on a pressure switch, runs fixed speed, maybe 8 to 10 starts an hour. Motor is 45kW, 480VAC, nameplate 67 A. I've checked the motor and the cable and both are fine, nothing else tried yet.
I'm leaning soft starter but the VFD number keeps coming up in meetings. On something that runs flat out and loads off a pressure switch, is that 30 percent real or am I right to bin it?
The VFD quote. What's the compressor controller doing, load and unload on a switch and nothing else? A VFD on a load-unload screw with no way to modulate the screw is a soft starter with a fan and a bigger invoice.
Pressure switch and the unload solenoid, that's the whole controller. There's nothing on it that could take a speed reference, and the compressor vendor wants real money for one that could. Not sure I'd get that past the budget anyway.
Fixed speed, no demand control, that's a soft starter job. It does what the star-delta was there for, limits the inrush, without the open-transition bang at the changeover, which is what probably killed your delta contactor. Bypass contactor built in so it isn't making heat while running. Size it for 10 starts an hour, most 45 kW units are fine with that but check the spec sheet, that's the number that catches people.
The VFD earns its money when the compressor runs below full speed to match demand. Yours can't, it has a pressure switch. Star-delta again is just waiting for the next contactor.
The starter versus drive part is written up here: https://plctr.com/utilizing-plc-for-motor-control-and-drive-systems/
Agree. One caveat: if a second compressor ever turns up and one becomes the trim unit, that's where the 30 percent lives, and that one gets the VFD. Not this one, not today.