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Run 3 Phase motor from 240V service?

Home Improvement Asked by Carl Hatton on August 17, 2021

I just bought an older Delta Table Saw with a 3 phase motor. I have 240 Volt service. How can I wire the motor, or what do I need to do to make it work?

2 Answers

Your options are:

Replace the motor with a single phase motor (preferably also 240V)

Use a solid state variable frequency drive that can take single-phase input and provide 3-phase output. Be sure it it rated for the horsepower load, and DON'T exceed the rated speed of the motor (the drive may offer the opportunity, I've never seen a NON-variable speed solid state inverter drive, but if you make the motor go faster than it's rated for, it may mechanically fail.) You may also need accessories to keep the electrical noise generated contained.

Build a Rotary Phase Converter, generally done with a much larger 3 phase motor and a means to get it started.

For a tablesaw, the VFD offers little advantage (IMHO) unless you can do it for much less than a replacement motor. On a lathe, the VFD is a very desirable thing in its own right.

Answered by Ecnerwal on August 17, 2021

If the motor winding is connected in STAR and the star point is accessible as in the diagram, you can reconnect it in DELTA. That will make it 240V 3 phase. Connect a capacitor as shown to drive the 2nd phase. The torque characteristic won't be as good though. Use the rated current on the name plate & multiply it by √3 to calculate the capacitor value. Use a capacitor rated for motor run use, or power factor correction capacitor(s). Value=22.97uF/A (name plate current @ 415V). The reason for multiplying the current by √3 is because, by reconnecting the windings, the voltage has been reduced by √3, so the current is higher. For something like a pump motor, where the load is constant, use the nameplate current as a rough guide. Start off with a capacitor a bit below & tack some small 250Vac, or at least 400Vdc non polarised across it until the current through the capacitors is the same as the other phases, add up the total value, then get a capacitor the same value or close to it (or two to make up the value).

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Answered by Peter R. McMahon on August 17, 2021

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