3 Phase Power - Making 3 Phase Electrical Power from Single Phase
Everybody knows that 3 phase power is expensive to get, and most homes only have single phase (110 or 220) power. But 3 phase motors run smoother, because power is constant, so they would be nice to have. You can even get a hold of them used for free/next-to-nothing because almost no one has 3 Phase Power in their home.
You can run a 3 phase motor from standard 220 Single phase power. Really. It is an old technique, known mostly only to old tinkerers and the like. If you are looking for a modern day solution, have a look at our AC Drive control panels that can also run 3 phase motors on single phase power including 120VAC wall outlets.
First, you get the 3 phase motor turning it (manually, or better, with a small 110 v motor), and THEN turn on the 220 (connected to two legs) it will run. It will not run at rated power, or smoothly, but it will run (at speed). This is OK for some machines. Now for a magic trick.
Take a second (free, same size or smaller) 3 phase motor, and connect it to the first (three leg switch, and zing!, the second motor turns on instantly, and both motors run smoothly!, with more HP than with just 220. The first motor is acting like a "generator" to provide the third leg. This is not "full three phase power", but it works quite nicely. The 220 supplies power to both motors, with one more wire to connect them. You can can connect additional motors also.
This works best if the first motor is a larger HP (2 vs 1), or higher speed rating (3400 vs 1750) than the slave. Older, "beefy" motors are preferable, due to their mass. A 3 HP, 3400 RPM motor works nicly. The reason for this is that if you try to start a bigger load than your master motor, you may reverse the rotation direction of the master (3 phase motors can be wired to run either direction). A large/fast motor will have enough momentum to resist changing directions. The more motors you get running, the more stable the system becomes. The limit may be the amount of power drawn through the third leg of any given motor, the wiring, or your switch, etc.
A real world, functioning system:
- Master motor: 3 HP, 3 Ph., 3400 RPM
- Starter "pony" motor- 110, 1/2 HP, single phase washing machine motor or eq.
- Slave Motor- 3 Ph, 1750 RPM punch press or lathe. (Insert your machine here)
The Washing machine motor and the master are mounted to a board/bench, and their shafts are connected with a flexible coupling. Power to them is switched with a Double Throw, Double Position, CENTER OFF switch. The 3 legs of the master also go to a 3 pole switch, to the slave machine.
- Start with Switch 1 OFF.
- Make sure that Switch 2 is OFF.
- Turn on the 110 V "start up" motor with switch 1. (as drawn) This will drive the master motor.
- Once running, flip Switch 1 so that power now goes to the Master Motor. It will now run at its speed, and drive the 110 motor. (note that power is NEVER connected to both at once)
- You may now turn on Switch 2, which will start the machine motor.
- Turn off BOTH switches when done.
Additional motors must have their own switch (of course), wired to the master, and should only be started one at a time.
A note to the reader: When we were moving articles to the new site this one almost didn't make the cut since economical AC drives that can run on single phase were thought to have "obsoleted" this technology. We were shocked when we started looking at the numbers and discovered that this was one of our most popular electrical articles. So it is an oldie but goodie. Do you have a topic you would like to see us write about? Send an email to firstname.lastname@example.org.
Go to the Motors, AC VFD Drives, and 3 Phase Power Training to select your next lesson. There are also many other Lesson Series on PLC Programming and Industrial Automation.