Start circuitry for a single phase air compressor motor.

When I purchased my Centec 2b, it was fitted with what looked like a large single phase motor. I fitted a 3 phase motor and VFD and put the single phase motor to one side.

I have been sorting and testing all my electric motors over the past month and this particular motor puzzled me, it has two separate windings and nothing else, no centifugal switch or any capacitors although it has a large (empty) box on top of the motor. One coil was connected to the mains feed when it was taken out of the mill.

Subsequent research reveals that it is a 3HP single phase motor designed for air compressor use. There is no manufacturers plate and the clue was given by some lettering on the aluminium casting.

Seems a waste of a motor and goodness knows how the last owner thought it would run in the mill! I am not bothered about the starters used in air compressors. I was thinking of wiring it up as a capacitor start motor, with a suitable starting capacitor. Not sure which of the windings would be the start winding, but should be able to figure this out. There is room for a centrifugal switch with some modification.

My concern is that these air compressor motors may have some unusual feature that might prevent such a simple minded approach from working. Not being familiar with them, makes me a little cagey. Googling for some while left me non the wiser.

Andrew.

Edited By Andrew Tinsley on 03/01/2021 19:23:58

Single phase motors with no centrifugal switch to control the start windings aren't uncommon in larger sizes.

The necessary control gubbins and contactors would have been in the equipment. Some of the air conditioner motors can be a bit unusual. Think Wye-Delta three phase starter equivalent for permanent capacitor, pseudo two phase, start and run motors. Something to do with power factor and efficiency I imagine.

I have a Brook capacitor start 4 hp motor with that configuration sitting on the floor. Too good to bin. I used that one to run a compressor with two contactors and two start buttons. Just held the buttons down until the compressor was up to speed. I'd used a normal centrifugal start switch "compressor duty" motor previously which was subject to occasional stalls on start up if acceleration was slower than normal for any reason. Presumably not enough momentum to carry it over the torque drop when the starting winding dropped out.

If using VFD drives its often necessary to set the torque boost parameter when used with compressors. I found that out with a Hydrovane 502 which would stall out, or half speed, on the first start after standing for a while in cold weather.

Clive

Air compressors can take some 'grunt' to start but 3hp should cope with that.

You really need to establish whether that second winding can remain in circuit , or not.

It may be an induction start, ie you don't need a capacitor.

Air compressors dump the pressure when they stop via the unloader valve, the tank non-return valve keeps the tank at pressure. So the motor always starts with the pump at atmospheric pressure and off load.

I think that the usual configuration for these motors are capacitor start capacitor run, rather than a simple capacitor start system. I don't have a clue as to the capacitance values required for a 3HP motor. Anyone any idea of rough values?

Andrew.