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

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.

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

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

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:

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

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.

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!!!!

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

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.

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

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.

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

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.

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

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 :confused: Weird

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.

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.

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.

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

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.


Actually a RPC uses single phase to produce three phase.
There is 180 degree difference with respect to centre tap or Neutral, which is still single phase (with a centre tap).
Not to be confused with two phases. There is no 180° difference between the 240v, because it is 1 phase.
Also neutral is not involved with WRT to a RPC.
Think of a motor fed by single phase as an auto transformer, which it is actually what it is, the three phases 120° apart will be produced, without the introduction of capacitors, capacitors are used to improve the power factor.
There were some good articles published in the late 90's, this is one link
http://www.metalwebnews.com/howto/ph-conv/ph-conv.html
Also the Metalworking.com drop box 1998 files FRW- et al.
show the wave forms in detail.
Al.

Get a size O or 1 motor starter off ebay and use one pole of your pressure switch to pull in the coil. It does not take long for all the silver to be worn off the contacts and then they are toast, those little pressure switches are barely rated for your starting current most probably.
Steven

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

This is where I would put my money. If you don't here the head vent (psssss) after the compessor cycle. The next time it goes to start the head will be under pressure and impossible to start.

Darek

Don't know why I forgot that darn center tap issue. Stuck in a lab all day without windows can play with your mind - :cheers:

edit>

I just got done scanning over those pics from the Fitch Williams rotary converter. Is there any other info out on the web that you know of about this that I might go and look at? I poked around for a while and haven't found anything yet. I like the idea of placing the motor on a cart for mobility. I hopefully getting a 7.5hp motor and may give it a try. Although I admit I'm a bit skeptical about having that much motor ready to do a flip from rotational torque. Do you see any problems down the road with that?

edit>>

I found it Al, it took me a while! Seems like Fitch Williams hasn't posted anything lately on the forums. However I found
someone that saved his work in .pdf format on Practical Machinist. Here's the link:

http://www.practicalmachinist.com/FitchWConverter.pdf

If anyone would like it email me if the above link fails to load.

The most common reason for blowing a start capacitor, especially repeatedly, is that the motor is starving for power at startup. The first thing you should be looking at is the circuit size for the compressor. Just because the voltage is fine does not mean the circuit is sized or wired correctly.


Whoaaa! I just realized that you never told us what type of 3-phase power you have in the shop. If this is a comercial-type building, then you likely have 120/208, but if it is light industrial, then it is likely 120/240 hybrid. This is important!

If your supply is 120/208, then all of your single-phase 240 tools are running at low voltage (208 volts). In this case, you need a buck/boost transformer to bring your supply voltage up to the 240 volts it wants to see. You don't do this for the entire shop, just the 240 volt single-phase tools.

On the other hand, if the shop is the hybrid 120/240 3-phase, then you need to check the size of the circuit feeding the compressor. If the circuit wire is undersized, then the voltage during startup will drop too low, the motor will start slower than normal, and the cap will blow.

I am running late and have to wrap this up. In the mean time, why don't you provide more specifics about the compressor and the wiring it is connected to.

IF YOUR AIR COMPRESSOR UNLOADER IS NOT WORKING, YOUR MOTOR IS TRYING TO START AGAINST THE COMPRESSOR LOAD. CHECK YOUR UNLOADER FIRST, IT HAS TO RELEASE THE PRESSURE BETWEEN THE COMPRESSOR OUTLET AND THE RECEIVER INLET CHECK VALVE TO ALLOW THE MOTOR TO START UNDER LIGHT LOAD.

NO, NO, NO. unless you have it rewound for 3 phase, but then it would be easier to BUY a 3 phase motor.

I have converted 1 phase,2 pole single phase motrs to 3 phase for building phase converter.The reason I did this is that the 1 phase motor has a governor/start switch that I used through a heavy DPDT relay,to start it.The problem is that you have to strip the windings out of the motor,take dimensions of the slots,core length,ect.I fed all the stator data into the EASA computer engineering program library ,and it gave me the data (span,#of turns,,connection,ect.)to wind it as a 3 phase motor.However,unless you can rewind the motor yourself or,you have a friend that can do it for you,your out of luck.My suggestion is to make certain the check valve and unloader valve are working.When your compressor shuts down,the check valve closes,keeping the pressure in the tank and the unloader valve releases the pressure from the check valve to the compressor head.If it does not,the motor is trying to start under a load.It is my bet,that is where your problem is.

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

I HAD A SINGLE PHASE MOTOR DRIVEN COMPRESSOR THAT HAD A HARD TIME STARTING BECAUSE OF INSUFFICIENT START CURRENT CAPACITY IN THE BUILDING WIRING. I REPLACED THE MOTOR PULLEY WITH A CENTRIFUGAL CLUTCH SO THE MOTOR STARTS UNDER NO LOAD.

BEING A SINGLE CYLINDER COMPRESSOR, IT DID CAUSE SOME LIGHTS ON THE CIRCUIT TO DIM SLIGHTLY ON EACH COMPRESSION STROKE. I ADDED A FLYWHEEL RIM TO THE DRUM OF THE CENTRIFUGAL CLUTCH TO HELP THE MOTOR OVER THE COMPRESSION STROKE AND ALMOST TOTALLY ELIMINATED THE LAMP FLICKER.

AS I UNDERSTAND IT, THE FLYWHEEL EFFECT OF THE MOTOR, AS SEEN AT THE LOAD, IS INCREASED BY THE SQUARE OF THE REDUCTION RATIO, SO THE FLYWHEEL EFFECT WAS MUCH GREATER THAN IF THE ADDITIONAL MASS HAD BEEN ADDED TO THE COMPRESSOR FLYWHEEL, AND A LOT CHEAPER.

FROM THAT EXPERIENCE, I HAVE SINCE MADE 2 OTHER INSTALLATIONS USING 3450 RPM MOTORS INSTEAD OF 1750 RPM MOTORS AND FOUND THE HIGHER SPEED MOTOR STARTS THE HEAVY LOADS BETTER THAN THE LOW SPEED ONES, PRESUMABLY BECAUSE THE REFLECTED LOAD INERTIA IS REDUCED BY THE SQUARE ROOT OF THE REDUCTION RATIO.

ON PORTABLE COMPRESSORS THAT ARE DIRECT CONNECTED TO THE MOTOR PREVENTING THE ADDITION OF A CENTRIFUGAL CLUTCH, I SIMPLY INCREASE THE VOLUME OF THE PIPING CONNECTING THE COMPRESSOR TO THE RECEIVER SO THE COMPRESSOR HAS SEVERAL REVOLUTIONS TO BUILD ENOUGH PRESSURE TO OPEN THE TANK CHECK VALVE. THIS ALLOWS THE MOTOR TO COME UP TO SPEED WITH REDUCED LOAD. STARTING CURRENT TIME IS REDUCED, MAKING A SAVING IN ELECTRICITY AND ALLOWING OPERATION ON EXTENSION CORDS.

ON ONE DIRECT COUPLED PORTABLE COMPRESSOR THAT HAD A STARTING SWITCH FAILURE BUT HAD TO BE USED IN THE FIELD THAT DAY, I TEED A PRESSURE SWITCH INTO THE COMPRESSOR DISCHARGE LINE TO OPEN THE START CIRCUIT AFTER PRESSURE BUILT UP AND STARTING TORQUE WAS NO LONGER NEEDED.

IN EVERY CASE, THE PRESSURE SWITCH, CHECK AND UNLOADER VALVES WERE VERIFIED TO BE IN GOOD WORKING ORDER.

When your compressor shuts down,the check valve closes,keeping the pressure in the tank and the unloader valve releases the pressure from the check valve to the compressor head.If it does not,the motor is trying to start under a load.It is my bet,that is where your problem is.

Usually you will hear the pressure valve, a loud hiss as when the motor shuts down.
A lot of the little compressors I have played with havn't had any sort of unloader, the ones that did had the unloader built into the auto on/off switch.

Usually you will hear the pressure valve, a loud hiss as when the motor shuts down.
A lot of the little compressors I have played with havn't had any sort of unloader, the ones that did had the unloader built into the auto on/off switch.
The pressure switches that have the additional connection for a small tube to connect to the compressor pump side of the check valve are the first thing to fail over time since they are a mechanical overly complex design. Most use a diaphram that operates a plunger that operates the actual air release valve. An alternative to these devices is a unloader called a "genie unloader valve" sold be grangers, no tubes from the pressure switch, no diaphranms, it is about the size of a check valve installing inline between the pump and check valve. It is preset to unload pressure via a small orafice. Never had a problem with one whereas I've has the pressure switch activated unloaders either not release, not shut off, or simply wear out the activation mechanism in the pressure switch and fall apart....the only catch is that the "genie unloader" costs about 20 bucks, which I consider a small price to pay for the increased reliability it offers. In addition to this I usually add a solid state relay for the switching of the motor power, the contacts of the pressure switch activate the "coil" of the solid state relay (less than one amp usually), the "load" side of the solid state rely then switches power to the motor at the point where the supply voltage crosses zero, minimizing noise spikes and the pressure switch contacts will usually last longer than the rest of the components in the system as a result. The solid state relay will usually require a small dc voltage to operate the "coil" via the pressure switch contacts, I usually adapt a "wall wart" dc power supply of the proper voltage from radio shack or similar type store, ebay usually has solid state relays (new) for extremely low prices. Make sure the voltage and current ratings are sized like you would for a start or run cap, more is better. Finish off the system with a automatic drain valve, power twist link belts and synthetic compressor oil in the crankcase of the compressor pump and there is a good chance the only time you will be looking at your system is when giving a shop tour! Unless of course you enjoy the challenge of discovering you have no air at just the time you need to use it the most, I hate those kind of surprises. Hope this helps, needless to say my suggestions all were as a result of having to deal with a failed component that would usually decide to go "belly up" sometime after 3pm on Friday afternoons....maybe you have better luck. Best regards, Joe.