Can I convert single phase motor to three phase?

[Powertoolman]

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.

[KEN BANGERTER]

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.

[philserveng]

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

[tool_man]

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.

[KEN BANGERTER]

Originally Posted by Jim Estes 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.

[integerspin]

Originally Posted by tool_man 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.

[geezergeek]

Originally Posted by integerspin 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.