One fact which I’ve observed, is that some of my appliances today, that plug into the regular wall outlets, possess variable-speed motors, that are electronically driven.
I can think of two ways in which those could be designed:
1) The AC that comes out of the wall outlet can be rectified, resulting in pseudo-DC. And then this DC voltage can be applied in short pulses to the windings of a motor, the rotor of which has permanent magnets. The pulses need to be kept synchronized with the real rotation of the motor’s rotor, but essentially their switch-on duration will determine how much torque the motor generates. And the frequency of these pulses would actually be variable, determined by whatever rate the rotor is spinning.
I suspect that this approach gets used in my front-loading washing machine, especially since this type of motor can produce torque over a wide speed range, from the machine’s tumbling speeds, all the way up to the machine’s spin cycle. But at the higher end of the speed range, there would be some additional needs for the driver circuits to fulfill.
In particular, in order for such a motor to be efficient at the lower end of its speed range, the counter-EMF it produces needs to remain comparable to the applied voltages. And that more or less implies that the counter-EMF produced at higher RPMs can get quite high regardless of how it’s actually wound, and may even exceed the supply voltage.
But there are ways to use the reactive properties of the coils, as well as phase-shifts, to allow this type of configuration to produce positive torque, even if the actual voltage on the windings exceeds the supply voltage briefly and part of the time – i.e. at the high end of its speed range.
2) The AC that comes out of the wall outlet can remain AC at its original frequency, and a circuit similar to a dimmer can delay the time-point 2x during each cycle, at which the voltage switches on, which gets fed to the motor’s windings. The voltage would get switched off at the zero-crossing points of the current curve. This results in a variable-pulse-width control over the motor’s torque again, but at a frequency which remains equal to the AC frequency of the wall outlet.
The design of the motor would resemble a traditional squirrel-cage induction-motor, but designed especially so that it can still produce torque at high amounts of slip. It has generally been the case that induction motors would have some amount of slip, but this amount of slip was normally very slow in comparison to the overall RPM, set by the AC frequency. Well the old induction motors would not have been able to produce high torque, when allowed so much slip, that their real RPM was – only half that defined by the AC frequency – . If the old motors were asked to do that, they would simply have burned out, because from the rotating frame of reference of the squirrel-cage, the still-strong external field would change direction at too high a rate of rotation.
And yet I suspect that this is what happens in my refrigerator. There could be ways in which their induction motors have been redesigned, which I do not understand fully. The compressor of my fridge can work at 1/2 its nominal speed, but not at 1/10 its nominal speed, which my front-loading washing machine is capable of doing.