Do It Yourself: How To Build A Rotary Phase Converter

Converting single-phase power into three-phase power is easier than you think by following these simple instructions.

The rotary phase converter is probably one of the simplest methods of converting single-phase power to three-phase power. Most of the major components may be acquired at your local salvage yard for generally pennies on the dollar.

The parts needed are as follows:

Delta wound three phase motor, a Wye wound motor will work but not as efficient. The horsepower rating of the converter should be no less than 25% of the total load you want to run from the converter itself. In other words a 10-hp converter can run up to 40-hp loads of multiple motors. It can only start 80% of its own power under a load. So this means that a 10-hp converter can start an 8-hp motor with no problem. It can run a bank of up to 40-hp in total motor loads i.e. 4 - 10-hp motors.

Non polarized electrolytic capacitors rated no less than 300 vac. I have found through my experimentation that a good rule of thumb is 40 mfd (micro farads) per hp is a good starting point and may be all the run capacitors you will need. You will need another set to start the converter and then pull them out after the converter reaches full rpm. You can build a bank of capacitors by simply placing them in parallel connections with each other until you achieve the proper rating for your system size.

Timer switch 0 - 2 min with a contact rating of 240 vac and 20 amps.

A solid metal electrical enclosure to place all the parts in for a nice installation. A 10-inch by 12-inch by 6-inch deep box with a removal cover works well for this installation.

Terminal strips capable of 30 amp loads or plenty of Red type 30 amp rated wire nuts.

Roll of UL rated electrical tape.

Enough footage of properly rated conductors with good insulation, remember you are running three-phase to each motor load now. This will require a total of four wires to each motor; this also includes the grounding wire. All motor load wiring should only be the braided type and make sure the type of conductor is rated for motor loads. Never use a solid type wire for any motor loads. Consult your local codes for proper amperage ratings.

A general rule of thumb for amperage ratings to wire size:

12 gauge = 20 amps

10 gauge = 30 amps

8 gauge = 40 amps

Fused Disconnect switch for starting and stopping your converter and the entire motor load system. The switch should be rated for the total load amperage of all the motors times 125%. Example if the total load is rated at 10 amps the switch should be rated to no less than 12.5 amps and fused as such. The converter is not calculated into this, as it does not pull a mechanical load. In fact never pull a mechanical load with the converter, it can damage the system.

The actual connections and mounting methods of your installation box I will leave to you, whether you use Red wire nuts or a terminal strip. The actual connection points remain the same.

1) You must confirm that the motor terminal connections are rewired for low voltage, 240 vac. Most three phase motors are set for high voltage, 480 volts. Each of the winding legs are identified by a metal tag or a number imprinted on the wire. The numbers range from t-1 to t-9.

Low voltage connection for Wye wound motor:

Wire nut or tie together t-4, t-5, t-6.

Wire nut or tie together t-9, t-3, this is c-phase

Wire nut or tie together t-8, t-2, this is b-phase

Wire nut or tie together t-7, t-1, this is a-phase

Low voltage connection for Delta wound motor:

Wire nut or tie together t-4, t-8, t-2, this is b-phase

Wire nut or tie together t-9, t-5, t-3, this is c-phase

Wire nut or tie together t-6, t-7, t-1, this is a-phase

Please note that the most important part of any motor is the nameplate check the data given on the nameplate before any connection is made. Some motors may vary or have been rewound since original manufacture.

You are going to run this system from 240-vac single phase. Remember to run from the bottom side of the fused disconnect to your motor converter. Of course as always you should have the main feed power off or disconnected whenever you are working with electricity. Never work on any electrical device while power is applied.

2) One leg of the 240-vac line will connect to the c-phase of your converter motor. The c-phase will feed the c-phase of your motor loads also.

3) The other leg of the 240-vac line will connect to the a-phase of your converter motor. This too will feed the a-phase of your motor loads. Connect one side of your run capacitor(s) to this a-phase line from the converter to the motor loads.

4) Connect the other side of the run capacitor(s) to the b-phase of the converter. This will feed the b-phase of your motor loads.

There, that's the basics. The converter may or may not start under its own power. This is where the timer switch comes for an extra punch from another capacitor.

5) Connect the starting capacitor in parallel with the run capacitors through the contacts of the timer switch. You will have to play with your time setting on this one as all systems may start differently. You may notice a heavy growl when the starting capacitor(s) are still in the circuit, this is okay as you are throwing some extra juice to start the converter.

6) Connect up your motor(s) using the 240-vac wiring describe above. Check your system for test runs. Does the motor run backwards? No problem, disconnect the power then just switch two of the terminal leads i.e. interchange a-phase with c-phase and visa versa. Reapply power, the motor should now be running in the correct direction.

You should now be up and running. Operations of three phase motors are the most cost efficient way for getting work out of electricity. You may also find that the more motors you run on your new system the more efficient it will become, at least to a point. You can never run over the 4 times rule with this converter system.

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