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I understand that for magnetic lines of the rotating magnetic field to cut the windings of the rotor (and induce current), the field has to rotate at a slightly higher rate than the rotor. But if we assume the motor starts from zero RPM, at what speed does the field rotate at this moment in time? Do we have to somehow sense the rotating speed of the rotor and adjust the frequency of the 3-phase input voltage somehow? I think this is not what happens, since such a requirement seems somewhat complicated to achieve, especially given that induction motors have been around for quite a long time. So what am I missing?

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  • $\begingroup$ Better on the Electrical Engineering Stack? $\endgroup$ – Solar Mike Jan 23 at 7:04
  • $\begingroup$ @Solar Mike Sometime ago I saw an electric motor question on EE stack closed as off topic because it wasn't a "question regarding electric circuits"... $\endgroup$ – S. Rotos Jan 23 at 14:04
  • $\begingroup$ not sure I would agree - usually because it is about repair or somesuch... $\endgroup$ – Solar Mike Jan 23 at 14:12
  • $\begingroup$ See this for an example : electronics.stackexchange.com/q/271333/152903 $\endgroup$ – Solar Mike Jan 23 at 14:14
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Induction motors are amazing machines - they utilize several physical principles in a very elegant matter.

Your statement "The field has to rotate at a slightly higher rate than the rotor" is correct, however, there aren't any sensors. The physics is the only thing that makes it happens. Lets deal with the classical case of a three phase motor, without using any variable-frequency drive. The induced magnetic field angular velocity around the stator axis is the same as the power supply frequency. Seek for 3 phase electrical machines or rotating magnetic field in google for more information:

rotating magnetic field illustration from WIKIPEDIA

Without naming each phenomena specifically (look for Lenz's law and Lorentz force if you are carious about it), lets try to describe what is going on under the motor's hood: The rotation of the magnetic field exposes the rotor to a time varying magnetic flux, which in turn induces electrical current along the rotor winding. Next, since the winding now conducts electrical current and also subjected to a magnetic field - a force is exerted on the winding, creating a torque around the rotation axis and causing the rotor to accelerate.

This is where the beauty of the motor design really come into play. As the rotor accelerates, its relative angular velocity with respect to the rotating magnetic field is gradually reduced. Is it clear? just to remind you that the magnetic field rotation speed is constant and dependent only on the supplying voltage frequency. As the relative velocity is getting smaller, the induced current also decreases and so is the accelerating torque. For each external load (torque) acting on the rotor, a different constant angular speed would be reached. At that speed the relative velocity between the magnetic field and the rotor will result in an internal torque equals to the external one. This is why an induction motor will never rotate at the exact magnetic field speed (and therefor called asynchronous motor). If it was able to rotate at that speed, the relative velocity would be zero so no torque would be applied on the rotor.

1 1: https://i.stack.imgur.com/bANJf.png

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    $\begingroup$ "As the relative velocity is getting smaller, the induced current also decreases and so is the accelerating torque." That is true if the working point of the motor has already passed the pull-out torque. Otherwise what you say doesn't quite add up. $\endgroup$ – Sam Farjamirad Jul 21 at 6:19
  • $\begingroup$ I don't see what you try to say. This statement is always true. A decrease in the accelerating torque still ends up in an accelerating shaft, it just accelerates slower.... $\endgroup$ – Yaniv Ben David Jul 21 at 14:11
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    $\begingroup$ Sorry i can't follow the comment, I don't know what the statement "decrease in the accelerating torque" means. Let's go back to your original statement. Smaller relative velocity means smaller slip so the armature current decrease, finally the main flux, according to the Lenz's law, the rotor current increase to oppose the change. A proper grammar and of course an insight into the subject is necessary to stop wasting peoples time. imgur.com/a/ie1lWNV $\endgroup$ – Sam Farjamirad Jul 21 at 14:57
  • $\begingroup$ First you gave a very lazy answer to @S. Rotos, who obviously asked to have a little feeling about the mechanics of the motor. Next, you became a "wise guy" by bothering my wording and finally you even try to insult. Shame on you dude, it seems you are the only one wasting peoples time here... $\endgroup$ – Yaniv Ben David Jul 21 at 18:32
  • $\begingroup$ In my two previous comments, i mentioned and later explained why your statements is wrong. The proper grammar is essential, if you're interested in communicating with people. Your first comment does't make sense at all. Insight into subject is absolute must. Your statement is wrong and i explained why. I kept it totally professional and didn't insult you at all, but you did! And after all I'm not a dude. I'm a lady, yes you get beat up by a girl. $\endgroup$ – Sam Farjamirad Jul 21 at 18:58
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But if we assume the motor starts from zero RPM, at what speed does the field rotate at this moment in time?

The synchronous speed (rotating field speed) $\Omega_{sy}$ is independent of the rotor speed. If we feed the primary windings (stator) with a three phase supply then the frequency of the rotating field is the same as the frequency of the supply (50/60 Hz) during the whole time.

The frequency of the induced emf in the rotor, at the very beginning is equal to the frequency of the rotating field. If the rotor is free to rotate with speed $\Omega_m$ then, finally it reaches the synchronous speed $\Omega_{sy}$. And the induced emf is then zero, because there is no relative movement between the rotating field and the rotor so no emf. Duo to mechanical losses, the rotor decelerate, then the process repeat itself on and on.

We don't have to manipulate the frequency of the supply. The induction machines are also considered as the frequency converter. The input frequency of the supply is converted to the frequency of the induced emf in the rotor. Think about what I explained above. The frequency of the induced voltage of the rotor at standstill is equal to the supply frequency, as the rotor rotates, then the frequency of the induced voltage decreases, so eventually it reaches the zero.

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