Modify Single Phase Motor to Run on 3-phase Power (yes, that way around!)

Hi all,

I've got a lathe with a single phase, dual capacitor, 1HP motor.
For years I've been meaning to upgrade it to a 3 phase motor with a VFD, so that I can have easy speed control on the lathe. Now is the time.
I have two VFDs, one cheapo chinese one which I have successfully used in the past to run my 3ph bridgeport motor off single phase power, and another Danfoss drive that I acquired through work. I do have 3 phase power available (one outlet only) but it is being used by another machine so I think it will be more convenient to run the VFD off a single phase outlet which is right near the lathe.

I did acquire a suitable 3phase motor, with the intent that I'd install it, but it has a different shaft diameter, and I'm now thinking there's a much simpler way, by using the existing motor.

So, my question is, can I modify the existing 1ph motor to run on 3ph power?
Below are photos of the motor nameplate and internals. To me, the windings look identical to that of a 3phase motor (3 pairs of wires: red/yellow/black), so I'm thinking that I should just be able to pull the capacitors out and wire it up like any 3phase motor. Is that correct?

Thanks,
Hugh.

A single phase motor generally expects 90 degrees phase shift between the windings, vs a 120 degree shift in three phase systems.

There are single phase output VFDs available, but only low quality chinese ones.

Given that three pairs of IGBT choppers should be able to generate any waveform on each of the three outputs, I am not sure why no manufacturers have firmware that allows a 90 degree phase shift to be output.

You could try connecting it to a three phase drive, but you will likely need to disable phase imbalance protection, run it in scalar mode (you could try vector, and see if the drive adjusts enough), and oversize the drive. The motor will likely run a bit rough with lots of torque imbalance as the fields will be fighting each other.

Not unless you rewind it, single phase motor is a 2 phase winding with a 90 degree phase shift ,But a time rated 2nd phase of less than a min, or if it is cap run the 2nd winding is still very wimpy... just change the motor 1hp on ebay are very cheap....Phil

Simple answer is "no that will not work".

But...

With "real" three phase, you can ALWAYS run a single phase motor on it, just use 2 of the wires, and set the motor up for the right voltage. Done.

With a VFD, there is a problem. The single phase motors have a huge starting surge, because they MUST be started at full voltage, even the ones that you CAN use with VFD (such as PSC motors)

The VFD, unless WAY oversized, like 3 to 5x oversized, simply does not have the current capacity to supply the surge.

At low voltage and low frequency, the way a VFD does the start, the single phase start circuit does not supply the torque to get the motor started on 2 wires. The phase shifting means is frequency dependent, and produces poor results if any.

VFDs are set up to operate 3 phase, and do not have the needed phase shift to develop torque decently with the start winding, which is nominally a 90 deg shift, but in a practical sense a bit short of that.

Yes, a motor with a full sized start winding (none have that) could use a suitably re-programmed VFD to run that winding, but such VFDs do not exist.

There is a possibility that a proper setup of transformers, both wye-delta and straight voltage changing, might be able to supply something to do it at a usable phase shift, but as a practical matter you just do not want to mess with it.

In general, the difference of a three phase and a single phase motor is the winding. If you change it to operate on 3 phase you really end up making a 3 phase motor out of it (there may be issues with slot count, rotor bar count, etc, the above is general).

Get a 3 phase motor and don't look back.

hmcleay said:

I did acquire a suitable 3phase motor, with the intent that I'd install it, but it has a different shaft diameter...

Click to expand...

3P 1HP motors can be found cheap on ebay. You can probably find something nearly identical if not exactly the same dimensions of your current motor. Bolt right in with zero mods.

Electrical Motors - Frame Dimensions

Thanks all,

I think that clearly answers my question.
Sounds like it'll be much easier to just find a 3phase motor that fits.

Cheers,
Hugh.

hmcleay said:

I did acquire a suitable 3phase motor, with the intent that I'd install it, but it has a different shaft diameter,....

Click to expand...

New motor shaft too small, or too large?

If too small, you buy a split sleeve to adapt your pulley to the new motor.
If too large, you bore out the pulley to fit your new motor.

Just to cover the bases, everyone seems to be thinking of a capacitor run motor, while it is more likely to be capacitor start with a centrifugal switch. The drill would be to start at 60 cycles and once up to speed with the capacitor out of the circuit, vary the frequency. Of course, the motor might not like to run at a reduced frequency and if you go too slow, the centrifugal switch may close. I concur, get a three phase motor.

Bill

It's reasonably common for a motor in a "product" to have a non-standard shaft for it's frame size. If the required pulley is too small to take the shaft of the motor, the shaft size is reduced. In machines, there can be a lot of reasons for the "special" design, but usually it ends up being about size limits and required ratio.

You will do much better finding a motor that is the right size, and then either modifying the pulley, or the motor shaft end to fit. It is unlikely that you will easily find a motor that was ordered with the same special size that you need.

There are some single phase input VFDs that will run single phase motors with a 90 degree phase shift. Typically to be effective they remove the capacitor(s)/centrifugal switch and use the start coil to generate the 3rd phase. Not ideal, I have not seen any information as to motor performance or how well they work. Better off to get a decent 3 phase motor and a decent VFD.
Single Phase Output VFD, Control 1 Phase Motor Speed | GoHz.com
2 hp (1.5 kW) Single Phase Output VFD | GoHz.com

On one occasion I clamped a complete larger motor to my lathe bed and turned the shaft down under its own power. So just adjusted my tool height to suit wherever the motorshaft centre ended up.
I've also (on 3 occasions) taken the fan shroud off a motor that fitted in my lathe intact and put it in between centres, driving the shaft with a dog between fan blades. The motor housing doesn't spin, since it has bearings, only the armature and spindle spins.
That may we'll work for you.

Cheers
Joe

Yes you can run it on 3 phase.

Wire the motor for 240v (two 120v coils in series).

Connect the capacitor start winding to the midpoint of the main coils, and the third phase of the vfd.

Turn it on and measure the current through both windings. If the start winding is too high, use an auto transformer to buck it down. Or boost it up.

Pm me i can explain it better over the phone

Edit: you have a 240v only motor, you will either need to tap the midpoint of the coils oryou will need an additional 120:240v transformer

Motor makers being cheap, they generally use the least space possible for the start winding. That usually means a small wire, and as few turns as they can get away with while still working. Putting current through such a winding is usually going to either do very little, or overheat something, depending on current. They are not a problem for starting, because the start is quite short. You may have seen motors that had the start winding fried by a stuck switch, or shorted capacitor.

If the motor is one that uses both a start and a run capacitor, then no issue, knock your socks off. That is fairly good proof that the winding is intended to carry some run current.

Adjusting the voltage would allow a suitable operating condition.

But, a run capacitor type motor actually gives decent performance without 3 phase, so I'd not bother much, unless you just had to have variable speed. Any capacitor in series will not allow a typical soft start, so you want 3 phase for that and/or variable speed.

The scheme involved is basically a Scott connection. I had not been considering that, which is odd, as I had thought of a similar scheme some years back. Nice.... No reason it would not work with a motor that allows reasonable "help" at a current the winding can take long term