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 MikeCommented Jan 23, 2019 at 7:04
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$\begingroup$ @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. RotosCommented Jan 23, 2019 at 14:04
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$\begingroup$ not sure I would agree - usually because it is about repair or somesuch... $\endgroup$– Solar MikeCommented Jan 23, 2019 at 14:12
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$\begingroup$ See this for an example : electronics.stackexchange.com/q/271333/152903 $\endgroup$– Solar MikeCommented Jan 23, 2019 at 14:14
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$\begingroup$ @S.Rotos It depends on the question. Sometimes people do things like "I have a motor in a robot. How do I know how much torque I need to move the robot?" That's kind of question is a mechanical question. $\endgroup$– DKNguyenCommented Aug 10, 2021 at 22:11
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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. Search for 3 phase electrical machines or rotating magnetic field in google for more information:
Without naming each phenomena specifically (look for Lenz's law and Lorentz force if you are curious 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.
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1$\begingroup$ "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. $\endgroup$– user14407Commented Jul 21, 2019 at 6:19
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$\begingroup$ 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.... $\endgroup$ Commented Jul 21, 2019 at 14:11
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$\begingroup$ This is such a great write up but maybe I can add a little clarification. The torque of the motor is proportional to the difference between the rotation speed and the synchronous speed-- the speed of the magnetic field as a result of the supply voltage frequency. If the motor is stalled, the torque is maximum, as is the current. If the motor doesn't start, in most cases, it will burn up. In the US, four pole motors have a synchronous frequency of 1800 RPM, one half (since they are four pole) the 60 Hz line frequency. $\endgroup$ Commented Jul 24 at 1:19
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