# Thread: need speed control for single ph motor

1. ## need speed control for single ph motor

I have a 1.5 h.p., 110volt, 17amp, 60hz., single phase motor to drive my milling machine spindle and need a speed controller. All I have been seeing is 3-phase output drives. Can I use three phase? Any links? Im not sure about "phase" compatability. Actually the plate says 110/220v @ 17/8.5amps.

2. You don't say what kind of motor, so I assume it's an asynchronous motor.
Then you may be lucky that it's a 3ph. motor converted to 1ph. by adding a capacitor. (My Emco millhead is one example of this). Then just toss the cap and add a VFD. Check it by comparing resistance over the 3 terminals. If it is the same regardless of which of the 3 terminals you measure between, you can be almost sure it's a 3ph. motor.

A 1ph. + start winding will have a higher resistance over the start winding, and the resistance between 2 of the terminals will usually be the sum of the 2 windings. In other words a delta winding with one winding missing and the remaining 2 are different.

The VFD's are always 3ph. out. Smaller units are 1ph input.

3. Your best bet may be to look for a surplus 4 pole 3phase 1.5 or 2 hp 220v motor, and use a VFD with single phase 220/240v in. You can program it for max 120hz and get up to 3000 rpm range.
Al

4. thanks everyone, I'll try to get more info about the motor. Al, I'm not sure why the best bet woud be to find another motor. There must be something I'm missing about the motor I already have. I don't know a whole lot about motors other than volts and amps, brushes. So ES, what your saying is that I may have is a 3ph. motor converted to 1ph. by adding a capacitor. I'm kinda confused about the whole phase thing and compatability between vfd.

5. If you have a standard single phase ac induction motor, the motor by itself would not rotate because there is no rotating field, so a second set of windings are wanted at 90deg to the first set and are fed by a capacitor (starting windings), essentially producing a 90deg phase shift and a 2phase motor, these windings are sometimes left connected or are switched out by a centrifugal switch when the motor is up to speed, because now the motor is rotating a rotating field is produced.
It is hard to find a single phase induction motor controller, as they can stall suddenly if the speed is dropped a little too low on load.
The reason I mentioned using a three phase motor is they are fairly cheap on the used market and they are much more efficient than a 1ph one of similar dimensions, and also allows you to use a VFD on single phase. There are very sophisticated vector drive VFD's that operate with high torque down to low speed, nearing the control of a DC motor they are used in crane hoists and elevators where the low speed high torque demands are high.
Al