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Given a three-phase asynchronous AC motor ("induction motor"), I tried figuring out whether lowering speed / torque by lowering the supply voltage should work, similarly to how it works with DC motors. From all that I could learn from Wikipedia, voltage-control is not possible, but one has to control the supply frequency to control the speed.

Here's my understanding of a asynchronous AC motor: the three-phase supply current induces a rotating magnetic field in the stator, the stator's field. The rotor rotates slower than the stator's field (the speed difference being the "slip"). This means that the stator's field moves relative to the rotor, which induces a current in the conductive rotor. This current in the rotor creates a magnetic field (the rotor's field). The rotor's field rotates with the same speed as the stator's field, but phase-shifted. The two phase-shifted fields push away from each other, which creates torque, driving the rotor.

Now, if I lowered the voltage across the stator's coils, nothing should change but the strength of the magnetic fields - right? And that should lower the torque. It would probably also lower the slip (given a fixed torque), but would that be a problem?

A slight hint that such a speed control should be possible is that (according to Wikipedia), larger asynchronous AC motors are started using a "YΔ-Connection" (translated from German, could not find that in English), where the motor is run in a star connection at start, putting 230V (in the European system) across each of the stator coils, and switched to a delta connection afterwards, putting 400V across the coils. Thus, a lower voltage seems to be possible…

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  • \$\begingroup\$ Yes. slip would increase with decrease in voltage. But the sensitivity of the speed to the load will also be higher for lower voltage. This may be why frequency control is preferred compared to voltage control. Try to find if your motor has a load versus speed curve for different voltages. \$\endgroup\$ – AJN Oct 15 '20 at 12:00
  • \$\begingroup\$ @StainlessSteelRat: D'Oh. That DC should have been an AC, sorry, my bad. Corrected in the question. \$\endgroup\$ – Lukas Barth Oct 15 '20 at 12:43
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    \$\begingroup\$ Running such a motor with a huge slip isn't done. If you want to vary the speed you have to rectify to DC and synthesize a new AC waveform of a different frequency. Such VFD drives are common, small ones are a few hundred dollars and a common retrofit to machine tools, etc. Running very far from designed speed needs a motor with more iron specd for VFD usage. \$\endgroup\$ – Chris Stratton Oct 15 '20 at 12:51
  • \$\begingroup\$ Does this answer your question? Speed control for PSC induction motor \$\endgroup\$ – Charles Cowie Oct 15 '20 at 13:42
  • \$\begingroup\$ Stator Voltage Control of an Induction Motor \$\endgroup\$ – StainlessSteelRat Oct 15 '20 at 16:20
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The issue with controlling ONLY the voltage is mostly of torque and load. An AC asynchronous motor will lose torque proportional to the voltage reduction, but also lose PEAK torque at the SQUARE of the voltage reduction. Peak torque, typically around 200% of Full Load Torque, is what the motor utilizes to maintain speed as load changes. So if you cut that down by the square of the voltage change, your motor becomes less able to react to any change in load. The result is an increase in slip (not a decrease) and when slip increases, the motor draws more current, but in this case without producing enough torque to get back to design speed, so it overloads and possibly stalls.

IF however, the LOAD is reduced at the same time, the motor may run fine. For example if the load is a centrifugal (quadratic) machine like a pump or fan, lowering the voltage and dropping the torque results in a lower speed. But in that type of load, the load on the motor drops at the CUBE of the speed change, so the pump/fan couples less with the load and it might run like that. But notice that the torque drops at the square of the VOLTAGE, and the load drops at the cube of the SPEED, they are not the same thing and the complex dynamics makes this a very tricky situation to control accurately, then if anything in the load changes, the motor may not be able to deliver more torque and stall.

May people try to take advantage of the Wye-Delta motor wiring issue you mentioned, by purposely leaving the motor in Wye, which effectively applies 58% voltage to the windings. This reduces the running torque to 58%, but reduces the PEAK torque to 33% (.58 squared). 33% of the 200% peak torque is 66% of FLT. So the running torque is 58% of FLT, but the ACCELERATING torque is only 66% of FLT and the motor can be easily stalled if there is any change in the load.

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