# Can I convert single phase motor to three phase?

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• 08-11-2006, 06:12 PM
Jim Estes
Can I convert single phase motor to three phase?
I have a compressor motor that is blowing the starting capacitor. I have checked the voltage and it is fine. I was wondering if I could just convert this motor to three phase and eliminate the starting and run capacitor.

I have searched and found tons of articles about converting three phase motors to single phase. I am sure it goes both ways, but I was wondering if someone else had already done this. I have three phase in my shop and converting this motor to three phase would be cheaper than buying a new three phase motor.

I have no idea why it is blowing caps, but I suspect that it is from too many starts per hour.

Any help is greatly appreciated.

Jim
• 08-11-2006, 06:23 PM
tobyaxis
Single Phase to 3 Phase?
I have been told by others (with electrical know how) that the reason your motor is blowing is because on an air compressor the motor is already under a load, hence more ware and tare during starting. Running 3 Phase should help you, but I have never heard of making a 3 phase from a single phase. To my knowledge (which is very little) a single phase motor does not have 3 poles. That is why it's called single phase. This conversion can't be done as far as I know.

• 08-11-2006, 06:34 PM
Jim Estes
It is my understanding that in a single phase induction motor (squirrel cage motor) there are all three poles, but the capacitors utilize the electricity generated by the rotation of the poles. The capacitor stores the electricity and then discharges it when the ghost phase (the missing phase between two and three phases of electicity) is doing the pushing. If I am running on 230v single phase, I actually have two out of three of the phases that I have when I use three phase.

With an induction motor, it would act as a generator when rotated, this is what allows them to be used as phase convertors, the leg that is not connected to line voltage, generates electricity and can be utilized.

Jim
• 08-11-2006, 06:40 PM
Jim Estes
Someone correct me if I am about to blow myself up please.

I have looked at some diagrams and this is what I came up with.

I can eliminate the starting capacitor by just disconnecting it and snipping the wires, I won't need the starting circuit at all. I should then disconnect the run capacitor and tie all three wires together. There is a jumper wire between two of the three connection posts, I need to remove this jumper and then connect three phase to the three connection posts and then I am done. Do I have a three phase motor or a melted hunk of copper?

Jim
• 08-11-2006, 07:07 PM
tobyaxis
Quote:

Originally Posted by Jim Estes
It is my understanding that in a single phase induction motor (squirrel cage motor) there are all three poles, but the capacitors utilize the electricity generated by the rotation of the poles. The capacitor stores the electricity and then discharges it when the ghost phase (the missing phase between two and three phases of electicity) is doing the pushing. If I am running on 230v single phase, I actually have two out of three of the phases that I have when I use three phase.

With an induction motor, it would act as a generator when rotated, this is what allows them to be used as phase convertors, the leg that is not connected to line voltage, generates electricity and can be utilized.

Jim

Your posted question actually helped me to understand a few things, Thanks! There are lots of guys on this forum that can help you (unlike me :o ). They will see your request in the new posts and come running. Thanks again for the info. :cool:
• 08-11-2006, 07:16 PM
Jim Estes
You are welcome. I learned about the theory of electric motors years ago. I know how things work, but I don't know anything about specific motors. This motor is a GE 230v capacitor start and capacitor run motor. What I am not sure of, is how to determine what is connected where. I know that the starting capacitor drops out of the circuit after a few seconds, and the run capacitor stays in the circuit always. I don't know how to determine if the start circuit is in series with something else or if it is in parallel and can just be left out. Another complication to this is that this motor is thermally protected and that circuit breaker might be in the mix somewhere between all these connections.

Jim
• 08-11-2006, 07:38 PM
Al_The_Man
A single phase motor is actually a two phase motor for starting as the capacitor together with the start winding gives it a rotating field, which if you connect the start winding to the run winding you will not have and it will not be able to rotate, unless you spin it manually to get it in to run.
A three phase convertor does not generate the third phase in the traditional sense, it is actually a rotary transformer and the third leg can be considered a secondary winding.
The cost of second hand 230v 3ph is cheap enough that it is not worth bothering with trying to wind three phases into a single phase motor.
The only way you would blow the capacitor is if the compressor keeps the rpm so low that the centrifugal switch does not operate, but in that case you would burn the start windings out.
Although you mention capacitor run so I assume it does not have a CF switch or maybe the contacts have welded together, as it is unusual to put a cap-start cap run motor on a compressor?
Al.
• 08-11-2006, 07:41 PM
NC Cams
You're failing to consider the internal speed sensitive switch that switches out the capacitor and redirects the wiring at a certain "run" RPM.

I botched this up and the motor let out a bunch of smoke afterwards.

You might want check the unloader valve on your compressor. This is a valve that seals the compressor outlet off from the tank. It also has a purge valve that bleeds off the air in the sytem between the pump and the check vavle.

IF this goes bad, you have to overcome all the air pressure load against the pistons which may be causing your hard start probelms.

DON"T CURE SYMPTOMS - FIX THE CAUSE!!!!
• 08-11-2006, 07:46 PM
Jim Estes
Are you saying that I am totally hosed here, that there is no third phase winding in the single phase motors? I could have sworn that I had this straight in my head.

This motor has two capacitors on top, one for starting and one for run. From what I read the starting cap is disconnected after a few seconds.

I will be checking out the compressor mechanicals and probably be getting a new motor.

Jim
• 08-11-2006, 08:02 PM
Al_The_Man
A single phase motor is made up of pairs of poles(windings) with the start windings straddling the run winding pairs, some times called a split phase motor the capacitor is either switched out fairly soon after starting or can be kept in, in a cap-run motor.
For heavy starting loads, like compressor, it is usuall to see cap start motors only. Are you sure there is no switch in this motor?
I see you mention the double cap motor :tired:
Is it the start cap you are blowing?
Al.
• 08-11-2006, 08:17 PM
Jim Estes
Yes it is the start cap that I am blowing, I replaced the cap and the first time it started, the cap blew oil everywhere. I looked and there is a centrifugal switch on the rotor. I also looked at the compressor, and there is no valve between the compressor head and the tank. There is just a line going to the pressure switch. I checked the safety and it wasn't stuck, and it has never popped open so I don't thing I have an over-pressure situation. My first thought was that maybe my shop is just too hot, but when I put that new cap on, it blew the first time it started, and the shop was pretty cool it rained last night so it was fairly cool.
I rotated the compressor by hand, and it feels like it turns pretty easily, I can hear the compression cycle and that seems fine.

Is there a positive and a negative side of the capacitor? It wasn't marked on either connector. Could I have connected it backwards? I know the small electrolytic caps will blow when connected backwards but those are marked.

Thanks for setting me straight on the single phase (split phase) motors, come to think of it I had read about that, but must have skipped over it.

Jim
• 08-11-2006, 08:26 PM
Al_The_Man
If the cap is blowing right off the bat, it is probabally due to a start winding problem/short circuit. Or the switch is welded shut.
The caps are not polarized, (or shouldn't be).
Al.
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