Single phase 240v motor wiring

paratim · Jun 7, 2014

Please help. I have recently fitted a new pump, pressure switch and a 2.2kw(3HP) single phase 240v, 2 pole electric to my compressor. However I am at a loss at how to wire it. I have been trying to google it for 3 days, but am getting conflicting answers. It is a start capacitor and start run motor which runs anti clockwise. I have had it running, but when the compressor almost reaches cut off pressure the motor reset button trips. It is connected via a blue 16amp plug and a 20 amp RCD. I have attached some pics. I currently have it wired as follows: live(brown) to the reset switch and neutral (blue) to Z2/W2. Thanks for looking
.

[ElectriciansForums.net] Single phase 240v motor wiring

Marvo · Jun 7, 2014

Was the original motor a 2-pole? Most of the large single phase domestic compressors I've come across are 4-pole motors. Reason I ask is that the symptoms sound like you could be mechanically overloading the motor. I'll check out your wiring later when time permits.

What test equipment do you have? First step would be to check the on-load running current of the motor and the on-load voltage

paratim · Jun 7, 2014

Thanks for your reply Marvo. I assume it's a 2 pole as it's rated at 3000rpm. The original motor was 3 phase and I've just swapped it for a single phase. Also when under load the start capacitor gets very hot and there was a small amount of smoke coming from the reset switch.

Marvo · Jun 7, 2014

What was the RPM rating of the original 3-phase motor?

paratim · Jun 7, 2014

2850 rpm

paratim · Jun 7, 2014

It's now running really slowly and the reset switch is smoking. It's kaput I think. I think it's just a cheap, nasty Chinese import. I think I'll have to buy a decent one!!!! This is the motor I purchased:

AIR COMPRESSOR ELECTRIC MOTOR 240V 3HP SUITABLE 100-200 LTR | eBay

Marvo · Jun 7, 2014

The idea would be to unplug it before the damage gets too bad

If you're lucky you might just have damaged a capacitor and/or the reset button. I take it you're not an electrician, do you have or have access to an insulation tester and a clamp on ammeter?

paratim said:
It's now running really slowly and the reset switch is smoking. It's kaput I think. I think it's just a cheap, nasty Chinese import. I think I'll have to buy a decent one!!!! This is the motor I purchased:

AIR COMPRESSOR ELECTRIC MOTOR 240V 3HP SUITABLE 100-200 LTR | eBay

telectrix · Jun 7, 2014

are you trying to run a single phase motor off a 3 phase supply?

Marvo · Jun 7, 2014

In his opening post he says he's wired live and neutral to it although if the supply was 3-phase the blue could be another phase and not a neutral.

Marvo · Jun 7, 2014

TBH I'm tempted to suggest it's time to phone an experienced sparky. First step is to establish what damage is done by testing the caps and the motor windings. Second step is to test the supply and make sure you're wired to 230v and not 400v. Finally wiring and commissioning the motor using an ammeter to check the run current and adjusting the pulley ratios if necessary to reduce the shaft power to within the capability of the motor. If you're unsure what you're doing it could work out a lot more expensive to carry on than getting a professional to help you.

paratim · Jun 7, 2014

Thanks for your replies chaps, but I'm not an idiot. It's a 240v, single phase motor. It's in my workshop and I do not have a 3 phase supply. Initially all I wanted to know was where to put the live and neutral wires. I think I have got them correct (Live to the reset switch and neutral to U2). It worked fine up to about 90 psi and then sounded like it was really struggling and kept tripping the reset switch. As you can see from the link, I think I have purchased a piece of cheap junk. My fault for being tight! I'll have to invest in a decent motor. Thanks anyway.

Darkwood · Jun 7, 2014

What was the KW rating or the HP rating of the original 3ph motor this would be the power size your new 1ph needs to deliver..!!!

Marvo · Jun 8, 2014

paratim said:
....but I'm not an idiot. ......Initially all I wanted to know was where to put the live and neutral wires.

If you insist you want to carry on then after half an hour of my life I won't get back here's your schematic of how your wiring should go. I'd strongly suggest you commission this or any other motor with a clamp meter to ensure you don't exceed the maximum run current stamped on the plate of the motor otherwise you're likely to destroy motors like they're going out of fashion.

Please note the Z1 and Z2 denote the centrifugal switch and this setup will give you clockwise rotation if your looking at the motor front on.

paratim · Jun 8, 2014

Thanks for that Marvo. The motor now runs perfectly at full speed. However it now runs clockwise and I need it to run the other way. Any advice is much appreciated. Thank you.

Marvo · Jun 8, 2014

Okay, when I made the drawing I was being lazy and hoping you weren't going to ask this. The drawing I gave you was electrically correct for clockwise rotation but the way I'd laid it out made it topographically difficult to easily show the way you change the direction so I've amended it slightly. Just check that the capacitor wires and overload wires conform to the exact locations they're in as shown on the amended drawing above.

If you need to change the rotation direction on the internal wiring, ie not using an external reversing starter, then you basically remove both the brass links and reinstall them facing the other direction as show below.

I've added a circuit drawing with the legs of the winding labeled so you can see the difference between the two wiring schemes electrically. Also please note that the centrifugal switch I've shown on the new drawings is actually built into the motor so you won't see it as an external component.

Single phase 240v motor wiring

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