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IndustryArena Forum > CNC Electronics > Phase Converters > Can I convert single phase motor to three phase?
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  1. #1
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    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

  2. #2
    Moderator tobyaxis's Avatar
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    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.

    If I'm wrong about this someone will correct me.
    Toby D.
    "Imagination and Memory are but one thing, but for divers considerations have divers names"
    Schwarzwald

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

    www.refractotech.com

  3. #3
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    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

  4. #4
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    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

  5. #5
    Moderator tobyaxis's Avatar
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    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 ). They will see your request in the new posts and come running. Thanks again for the info.
    Toby D.
    "Imagination and Memory are but one thing, but for divers considerations have divers names"
    Schwarzwald

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

    www.refractotech.com

  6. #6
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    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

  7. #7
    Community Moderator Al_The_Man's Avatar
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    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.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
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  8. #8
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    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!!!!

  9. #9
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    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

  10. #10
    Community Moderator Al_The_Man's Avatar
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    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.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  11. #11
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    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

  12. #12
    Community Moderator Al_The_Man's Avatar
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    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.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  13. #13
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    I replaced the cap (with one slightly smaller and with a bleeder resistor-scrounged) and started the motor with the belt off the compressor, it starts and runs fine. The switch it kicking in and out like it is supposed to, and the cap is not getting hot.

    I am now starting to wonder is maybe the motor is fine but the compressor is toast.

    I am wondering if I should tear into the compressor pump to inspect the cylinder wall for galling.

    Thanks for all you advise guys and I am sorry for being hosed up about the single phase motors tobyaxis.

    Jim

  14. #14
    Community Moderator Al_The_Man's Avatar
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    Put the cap on the without the bleeder and disconnect the motor from the compressor, if it starts and runs fine then it could well be due to the loading problem, not allowing it to come up to run speed.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  15. #15
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    OK, this is what I did. I cut the resistor off the cap, started the motor and it ran fine. I checked the hoses from the pump to the tank, and took it all apart to check to see what's what. The check valve is working and isn;t stuck, the sensor line is clear and seems to be fine. I oiled everything and put it back together. I installed the belt and then tried the motor, everything was working fine, of course, the tank is empty so there is no load. I decided to cycle the motor a couple of times, and when I turned it on the second time it smoked and I popped the cap, but I caught a glimpse of what was really causing the problem. When I turned the switch off to kill the motor before I really messed things up, I notice a big arc from the contactors in the pressure switch. I had to replace the switch a few years ago, I think it's time to replace it again.
    Just to be sure I tried the motor one more time, this time I pushed the switch really hard, and the motor started right up. New pressure switch, new cap, and some TLC should get me back up and running.

    Thanks again,

    Jim

  16. #16
    Moderator tobyaxis's Avatar
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    Put a call out in a new thread. Call For Al-the-Man, this is his speciality

    Intresting enough This posted instead of sending a PM to Al-the-Man Weird
    Toby D.
    "Imagination and Memory are but one thing, but for divers considerations have divers names"
    Schwarzwald

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

    www.refractotech.com

  17. #17
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    Hi Jim, seems like you've solved your problem. The real problem is that small compressors are a compromise because they rely on the tank getting full and then cutting the motor out.
    If you put a big demand on the air supply then it will start cycling on and off at a fair rate.
    Single phase motors, no matter how they are made just do not like to be switched on and off too much, and if a load is present then this is just adding to the problem.
    Some years ago the firm I worked for had a similar problem with a turret lathe that had a three speed motor fitted. The operator used to flick the speed control switch down to slow and stop the machine. This resulted in the motor going "to sleep" on a regular basis. A long story short, I bought the same machine (1930 vintage, 1-1/4" Taylor capstan lathe),for my garage workshop and tossed the motor in favour of a 3 HP single phase motor.
    I also fitted an electric clutch out of an automobile air conditioner, to the countershaft. This allowed the motor to continue to run but the spindle was stopped and started by the electric clutch.
    These electric clutches run on 12 volts and only draw 2 amps when engaged, so a transformer and rectifier does the trick of controlling it.
    I've got a home workshop type of air compressor, 2-1/2 hp direct drive, and this will only let you have so many on offs before you get problems.
    I'm thinking of redesigning the drive to allow an electric clutch to be fitted, controlled by the pressure cut-off valve, and an air solenoid valve (one day)
    Ian.

  18. #18
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    Wink Similar problem I had years ago - but solved as well

    I had the same situation years ago on my compressor. It runs a 5hp Century (I think that's the brand) motor with both a start and run cap. Problem was a partially worn slip ring surface on the centrifugal switch. That lead to a high current load on start and poof! Man did that make a mess - LOL. I ended up giving the motor to the local rebuild shop for a good going over and bearing replacement as well. The price was well worth it (something around $60) and I grabbed a couple of extra caps as spares before I walked out.

    On the subject of motors and phases mentioned in this thread no one ever seems to mention that on a single phase system both phases are 180 degrees apart due to transformer coupling at the pole (residential) and not the 120 degrees I think many are assuming and might lead to some confusion. I was told this, researched it and verified this many years ago by doing a phase measurement with o-scope on the home ac line. (measured in my home but that could be different elsewhere.) On a true three phase system you'll have that 120 degrees of separation between phases.

    On a rotary phase converter you start with two phases 180 degrees apart (from residential power) and produce the third phase close to the others by adjusting the power factor with capacitance. Ultimately you will loose efficiency in the conversion but make up the difference in cost savings from not having to purchase industrial power. I spent much time researching phase converters and I could still have it wrong. (hope not!) so I invite someone to set me straight if need be. I should have a 7.5hp 3PH motor in my grasps to play with shortly down the road. The entire concept of RPC's just seems fun to get involved with and a lot cheaper than a frequency drive of similar hp at the moment.

    Sounds like you solved your motor issue for sure. Just thought I would had a couple of words while reading the posts I missed over the last month or so.

  19. #19
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    Good to see.

    Now just FYI in reference to your original question about converting the motor, no you cannot do that with US made motors. In IEC countries they don't really use 1 phase too much other than for small household appliances, so they sometimes just modify 3 phase motors by using the caps as you described; that may be where you got that info. Where we often see that end up over here is on submersible pumps made in Europe. They don't want to bother making 1 phase motors so they just sell a cap box along with their 3 phase motors to make them work on 1 phase supplies over here. But motors made in the US for the US 1 phase market are NOT inclusive of the 3rd set of windings. The cost to have one rewound would exceed the cost of buying a motor on eBay.

  20. #20
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    Is the head pressure being relieved in the line from the pump to the tank? If not, the pump (motor) maybe trying to start up under a load. Check your check valve and unloader - make sure they are doing their respective jobs!

    Scott
    Consistency is a good thing....unless you're consistently an idiot.

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