# Understanding the rotating magnetic field in a single phase motor

#### Just Another Sparky

Joined Dec 8, 2019
245
This is one of those questions that just sort of pops into my head out of the blue once in a while. I've always been told that a single phase induction motor generates it's own rotating magnetic field once it is up to speed. I've sort of taken that statement for granted without really questioning how or why until now. If anyone has this knowledge and would care to enlighten me I'd enjoy the discussion.

#### crutschow

Joined Mar 14, 2008
34,719
It's not an easy question to answer.
One explanation is to look at the single-phase AC as generating two equal and opposite rotating fields when the rotor is stalled.
That, of course, generates no net rotation force on the rotor.
But if you spin the rotor, the induced field generated in the rotor windings (or squirrel-cage) generates a counter rotating field that suppresses the undesired rotating field and allows the desired field to generate a net force on the rotor and strengthen the rotation in that direction.

Here's a further discussion of that.

Joined Jul 18, 2013
28,782
It generates the rotating stator field immediately at switch on at the frequency of the supply, the squirrel cage rotor consist of shorted turns, representing a shorted turn secondary.
So at the point of switch on, the motor is actually a transformer, with a shorted turn secondary, hence the very high initial current, as the rotor starts to turn, (the field in the induced secondary is attracted to the rotating primary field).
As the rotor starts to accelerate the phase difference between the primary and secondary start to move towards synchronization. Therefore the current starts to decrease in both primary and secondary, as this attempt at synchronization occurs.
In this type of motor the rotor can never achieve synch as there would be zero current in the rotor, so the nearest it can come is within 3-5 cycles of the supply frequency.

In the case of a 1ph version, the motor would not rotate without providing another phase, or shifted a second phase of the 180° supply.
IOW, The single phase 180° does not provide rotation.
This is often done with a second winding that is phase shifted using a capacitor. Usually called a split phase motor.
With a 3ph motor, this is automatic.

Max.

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Joined Jul 18, 2013
28,782
Yes, the shaded pole motor is usually used in simple applications such as fans, as they do not do well under load and are the least efficient (~20%) of all induction motors.
Max.

#### Just Another Sparky

Joined Dec 8, 2019
245
I am speaking in terms of an ideal single phase induction motor consisting solely of one run winding, with no consideration given to starting provisions.

In other words, a capacitor start induction run motor... or a repulsion start induction run - both of which run on single phase power with only a single run winding once the starting provisions have been cut out of the circuit. Or I suppose an unloaded three phase motor being supplied by only a single phase. (I.E. rotary phase converter, etc.) No shading windings, permanent split capacitors, etc.

Crutschow pointed out an interesting theory for modeling the effect, but I still don't quite grasp the particular *mode* by which the effect manifests. I.E. how and why the phenomenon actually occurs. For instance, whether the reluctance, etc. of the rotor laminations combined with the relative motion between rotor and stator might be 'bending' the flux lines in a particular manner, etc., so as to have some sort of canceling or complimenting effect within the motor. (Forgive my loose usage of magnetic terminology - the field is not a strong suit of mine.)

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#### crutschow

Joined Mar 14, 2008
34,719
I still don't quite grasp the particular *mode* by which the effect manifests.
Well, I can't say that I do either.
All I understand is that the field generated by the rotating rotor tends to cancel the backward rotating field and be attracted to the forward rotating field.
It's obviously a very strong effect as evidenced by the high torque of a single-phase induction motor.

#### nsaspook

Joined Aug 27, 2009
13,446
One intuitive (not 100% correct) view of the single-phase motor is to see two counter-rotating (direct and induced opposed fields) magnetic fields (phasors) that coincide at 180 degree points as they complete each 360 rotation at X period of time and provides a 'bump'.

Because the 'bumps/torque' cancel with alternating polarity , they don't give rotational torque (or stored energy in the mass of the rotor) in either direction. If we spin (in either direction adding energy) the rotor manually we slip (change timing, shift phase) the two counter-rotating magnetic fields into a offset that causes asymmetrical 'bumping torque' with one side seeing more bump than the other. This bump slip offset transforms into a rotation torque that causes the rotor to spin, adding even more energy to the rotor to keep it spinning, causing addition asymmetrical 'bumping', etc ...
https://learnengineering.org/working-of-single-phase-induction-motors.html

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