Two phase motor?


Thread Starter

Bill Sturm

A coworker asked me if I have ever heard of this motor. It says two phase on the nameplate. It has 4 wires coming out, but 2 are twisted together. I think it is a small spindle motor for an old grinder. I was told that it is not a 3 phase motor.

Any ideas?

Bill Sturm
Responding to Bill Sturm's query:

It is normally connected to a two phase supply, where the phases are 90º of phase. Under certain conditions it can be connected to a single-phase source. The current lead arrangement is probably for the single-phase case.

If interested in detail, please provide nameplate data.

Phil Corso, PE
Trip-A-Larm Corp
(Deerfield Beach, FL)

Guy H. Looney

Sounds like a stepper motor to me. Most steppers are 2 phase motors (A & B windings, where each winding has a pair of wires). A spindle motor is typically a servo of some type, which would indicate either a brushed arrangement (2 wires and ground) or a brushless (3 wires & ground).

What amplifier is running it? That would probably tell you a lot about the motor.

My 2 cents,

Guy H. Looney
Sales Engineer

Regan Controls, Inc.
475 Metroplex Dr.
Suite 212
Nashville, TN 37211
phone: (615) 333-1940
fax: (615) 333-1941
email: [email protected]

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Further to my earlier response:

You can also connect it to a three-phase source. However, I don't want to mislead you. The ability to connect a two-phase motor to either a single or three-phase source depends greatly on the motor's rating, ie, voltage, Hp, & speed.

Phil Corso, PE
Trip-A-Larm Corp
(Deerfield Beach, FL)

Vitor Finkel


The two phases ought to be 90 degrees apart.
The common wire is the neutral. The other two are the phases. Rotation change from clockwise to counter-clockwise when you invert phase A and B for the two remaining wires.

You may generate that from any regular three phase system, if you use capacitors for changing the phase vectors acordingly. In fact, you may start with only 2 phases and neutral ( star connection ) out of the three phases. It's been along time since I calculated the phasors ( vectors ) for the capacitors. Maybe the easiest way is to do it graphically on milimetric paper.

Another way to generate
the 2 phases is wiring a small 3 phase transformer, delta-star insulated windings, in such a way that you obtain a phase angle right between the angle of two regular phases. Call that the zero degrees phase. Put two other windings in series, to make another phase, so that the new "phase" will be 90 degrees apart from the zero phase.

Connect one wire from each phase to the other to obtain the common "neutral", and see if you can change Xformer taps or use a Variac to get both voltages equal, and preaferible equal to required
motor voltage.

If you derate the motor voltages acordingly, using regular 2 phases and neutral ( 120 degrees apart ) might make it run on lower torque, warming up a bit. Speed will remain nominal, but power ratings somewhat smaller.

It has been quite some years since the last time I saw this kind of motor. It's "synchronous" speed for "N" poles, is somewhat faster than for a 3 phase "N" poles motor.
I remember something like 2400 RPM nominal speed
( that's 2/3 of the usual 3600 RPM for 3 phase motors ) and it was for a grinder. Apparently grinders with certain diameters run better at 2400 RPM. It was a french motor, by the way.

Tell me what happened next as you try to operate such a silly motor.

Vitor Finkel [email protected]
P.O. Box 16061 tel (+55) 21 285-5641
22222.970 Rio de Janeiro Brazil fax (+55) 21 205-3339

Craig Dahlquist

I worked with two phase motors about ten years ago. They were real old (1940 -1960). There are two single phase windings that are fed by two (in my case)230 VAC pairs of wires. I don't recall the angles between the two phases. To change rotation direction, I believe that both feeds were switched with both windings.
A 2 phase motor requires a "Scott T" connection of a transformer. I don't know if these can be purchased, but you can connect two single phase transformers in the following way:

The main has a center tap. The teaser is connected to the center tap of the main. The teaser has an 86.6% tap. The start and finish of the main and 86.6% tap are all connected to the 3 wires of a 3 phase supply. The secondaries of the main and the teaser produce the two phases that have voltage waveforms 90 degrees apart from each other.