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I do understand that in a Three phase Motor, a rotating magnetic field is produced in stator and the induced magnetic field in rotor tries to aligns itself with the stator field which makes the rotor rotate.

But in case of a single phase supply, there is only a pulsating stator field. So to start the motor, an auxiliary winding is used which has a field that lags the main field by 90 degrees.

After the motor starts, the auxiliary winding is disconnected and now we only have the pulsating field in stator which can not generate any torque. Then how does the motor continues to produce torque and rotates?

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    \$\begingroup\$ If you research Tesla's writings you'll find an explanation of the induction motor, which he invented. \$\endgroup\$ – Norm Mar 22 '18 at 15:00
  • \$\begingroup\$ the start windings are shifted in phase \$\endgroup\$ – Sunnyskyguy EE75 Mar 22 '18 at 15:04
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The stator magnetic flux can be visualized and mathematically described as two flux waves rotating in opposite directions. Because of the shape of the torque vs. speed curves and the effects of the rotor magnetic flux, once a direction of motion has been established, the forward torque is higher than the reverse torque as shown below. The solid line is the sum of the forward and reverse torques.

enter image description here

The above illustration is taken from Fitzgerald, Kingsley Umans, Electric Machinery 4th ed and the paragraph above is a summary of a 1-1/2 page explanation in that text. Also in that text, reference is made to another text for "an extensive treatment of fractional-horsepower motors."

Forward vs. Reverse Torque

The torque produced by the forward-rotating magnetic fields is higher than for the reverse because of the shape of the curves. The torque vs. speed characteristics can be determined by analyzing the equivalent circuit of the motor. That circuit for a single phase motor that has been started is shown below. A more complex version of the circuit may be required for an accurate determination of the characteristics.

Fitzgerald, Kingsley Umans, Electric Machinery 4th ed

Split Phase Starting and Running

A single-phase motor has two stator windings with different resistances and inductances or with a series capacitor in series with one of them. Split-phase motors have a phase displacement between the currents main and auxiliary windings. That creates an approximation of a two-phase motor. An analysis of a split-phase motor still makes use of the theory of two revolving fields. Another concept that is used is the symmetrical-component concept. Each of the two concepts has advantages and disadvantages.

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  • \$\begingroup\$ but why forward torque becomes higher than reverse? Can you refer me to a book where I can find the mathematical analysis? \$\endgroup\$ – Rupesh Routray Mar 29 '18 at 12:02
  • \$\begingroup\$ @Rupesh_Routray: The torque produced by the forward-rotating magnetic fields is higher than for the reverse because of the shape of the curves. They are the same, but rotated from each other as shown in the illustration. I will look for more a more detailed explanation and post it later. \$\endgroup\$ – Charles Cowie Mar 29 '18 at 13:19
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    \$\begingroup\$ @Rupesh_Routray: I revised my answer. The text from which I took the illustrations has additional information. Other texts that may be of assistance include Stephan J. Chapman, "Electric Machinery Fundamentals" and Charles Hubert, "Electric Machines" Fitzgerald et al refers to C. G. Veinott, "Fractional and Subfractional Horsepower Electric Motors." \$\endgroup\$ – Charles Cowie Mar 29 '18 at 16:42
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It's a bit like being on a swing. Once you get it going you only need to "push it" at the right time each cycle to keep it swinging.

enter image description here

The same goes for a single phase motor. Once the starter coil gets it going, the "pushes" from the AC power keep it circling in phase with the power supplied with a lag appropriate to the torque taken by the motor.

enter image description here

Too much torque and it will cause the lag to reach a point where the push is at the wrong point in the cycle and will stall the motor if it does not include a restart mechanism.

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  • \$\begingroup\$ Got it. but couldn't figure out how that pushing effect is created. The same field that is inducing a current in rotor, can't react with induced field to create a push capable of rotation. can it? If that would be the case, pots kept on top of induction cook-top would also rotate \$\endgroup\$ – Rupesh Routray Mar 22 '18 at 15:38
  • \$\begingroup\$ @RupeshRoutray it does create a field, but without a starter it would just hold it at the detent point. The magnetics are rather different on an induction cook-top and are vertical. \$\endgroup\$ – Trevor_G Mar 22 '18 at 15:46
  • \$\begingroup\$ picture represents a Permanent Magnet Synchronous Motor. In induction motor, shouldn't the induced poles stay near the stator poles, as that's the place where current is induced in rotor. The relative motion is only between the stator field and rotor conductors, I guess. \$\endgroup\$ – Rupesh Routray Mar 22 '18 at 16:18
  • \$\begingroup\$ @RupeshRoutray it does, but the field is induced at the same points as if it were a fixed magnet. \$\endgroup\$ – Trevor_G Mar 22 '18 at 16:30
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Also, despite what is often depicted in textbooks, if you look at the rotor cage of an induction motor you will see that the orientation of the rotor bars is skewed / slanted. This is so that the magnetic fields around the rotor bars are in constant contact with fields in the stator and the torque pulsations are minimized. enter image description here

In larger 3 phase motors that is generally not necessary because the mass of the rotor carries it through will little pulsation, so the the rotor bars are often straight.

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After the motor starts, the auxiliary winding is disconnected and now we only have the pulsating field in stator which can not generate any torque. Then how does the motor continues to produce torque and rotates?

enter image description here

Figure 1. A single-piston steam engine. Source: Kiddle.

This problem is not unique to electric motors. Steam engines had this problem and some locomotives would, if they started the wrong way, have to reversed quickly by the drive to run in the intended direction. (Controls were probably marked "One way" and "The other way" rather than forward and reverse.

Bicycles are also, in effect, single phase with a weak forward / reverse thrust available from the rider's legs. Most of the force available is vertical.

The trick in both of the above and in the case of the induction motor is to get the cycle started somehow, let the momentum carry you over top-dead-centre and then push hard.

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Once the motor begins rotating (with the aid of the capacitor and extra winding), it's purely inertia that keeps it rotating under mechanical loads and if the load is too big it will stall the motor and it will remain stalled even if the mechanical loading is removed.

If the "starting switch" is automatically reactivated upon speed being below a certain threshold, it will re-start.

It's like a bridge rectifier circuit and reservoir capacitor - the capacitor only gets charged for a short period each cycle and in between charging events, it's effectively free-wheeling. The load reduces the capacitor voltage until along comes another charging event. This isn't a problem and it doesn't have to be a problem for the motor under light (ish) mechanical loads.

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  • \$\begingroup\$ but then the periodic speed changes (slowing down to a certain speed and then re-starting again) would have been noticable. \$\endgroup\$ – Rupesh Routray Mar 22 '18 at 15:18
  • \$\begingroup\$ @RupeshRoutray Yes, noticeable if the loading torque were significant. \$\endgroup\$ – Andy aka Mar 22 '18 at 15:20

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