Some Opinions about Electronically-Controlled, Performance Motors Today

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.

The advantage of method (2) over method (1) above, is the fact that the whole system can be made less expensive, (a) over not requiring the special, rare-earth magnets that method (1) requires in the motor, and (b) in not requiring that the entire AC power input be rectified first, which also requires a massive capacitor in general.

A note about Permanent Magnets:

When I was young, permanent magnets were made in a fundamentally different way from how they are today. Essentially, today’s permanent magnets are frequently not the same type of object, which the old, traditional ones were.

In the old days, ‘The Way’ to make  permanent magnet was, to start with a hard piece of steel, or of other ferritic alloy, to heat that to a red heat in an oven, and then to switch on an applied magnetic field from an electromagnet. The applied field was kept switched on, while the alloy was allowed to cool. And only once the alloy was cold, was the applied field switched off. And thus, one had produced a ~permanent magnet~ .

One big problem with this type of magnet was, that it would easily lose its spontaneous field, especially if an opposing, oscillating external field was often applied, as is factually the case in some motor-types. In fact, steel-based magnets were often shipped with attached pieces of steel, which were called “keepers”, so that the applied field would replicate the spontaneous field, so that the magnet would not ‘get tired’ in storage and lose its field, on the way to a customer.

Advanced magnets today, have also become the common types of magnets, which are often based on an intermetallic compound, which sometimes use a rare-earth as one of their elements, and an example would be the ‘Iron-Neodymium-Boron magnets’ .  Note that in its pure form, Boron isn’t metallic, but as a part of this compound, it’s a part of a metallic or semi-metallic substance. This type was preceded by ‘Samarium-Cobalt’ magnets, in which the Samarium is actually a rare-earth. Neodymium may be less expensive than that, and may not even be a Lanthanide proper… I don’t recall.

Today’s permanent magnets have a very hard crystal structure, which has been defined by their intermetallic compounds, and not by any form of alloying. And so the new ones will keep their spontaneous field much better as well, and can be subject to oscillating applied fields, such as in some types of motors, without getting any weaker over time. But one fact which remains the same about them, is that above some temperature they will lose their spontaneous field. This temperature is called their “Curie Point”, and also corresponds to the temperature which the old alloy-magnets needed to be heated above, to be manufactured.

A problem with today’s high-intensity permanent magnets remains, that the higher Scientists push the magnetic field intensities they can keep, the lower their Curie Points become, and this can theoretically continue until we have a magnet that loses its field, just because it’s been placed into a vat of boiling water !

And so some of the Engineers that have designed cars have been criticized, for using Neodymium Magnets as the permanent, stator-magnets, of DC, brush-based starter-motors. While this produces excellent torque, those starter motors are frequently also mounted to the internal combustion engine in such a way, that they could get very hot and become useless.

Naturally, more recent engines started with Neodymium-magnet starter motors, that are still DC, brush-motors, have been designed with special attention to shielding their starter motors from intense heat, as well as to shielding some of their other components – from the heat of the engine itself.

But If a person just decides to crank the engine for 5 minutes, then I suppose this heat-shielding won’t help much.


(Note : )

Historically, permanent magnets played a crucial role in the development of early electrical motors. This generally led to DC, brush-motors, the rotors of which had armatures and coils, but the stators of which had these permanent magnets, with fields in a fixed position. This means that the advancement of stationary brushes along the armature, once kept the magnetic field of the rotor at a relatively fixed angle, while the stator field was also at a fixed angle – the combination of which gave rise to torque. Hence, neither component ‘saw’ the field of the other as oscillating, and the perms could keep their field. And, when the motor was not operating, the permeability of the rotor’s ferritic alloy, also acted as a keeper, for the stator magnet.

It was later observed that (A/C) squirrel-cage induction motors gave good torque, but their slip was sometimes seen as a disadvantage, especially since that type of A/C motor was often also used as a timing mechanism. And so fully-synchronous, A/C motors were eventually needed. But those did not start out as ones with permanent-magnet cores, until better permanent magnets were developed.

I know of two fully-synchronous motors, that existed Historically, and which may have been forgotten by now:

  1. The Reluctance Motor
  2. The Hysteresis Motor.

A reluctance motor used to have a magnetic core with a heterogeneous geometry, along which the rotating field was to fix itself, and was able to generate high torque.

A hysteresis motor used soft iron-alloys, often arranged as a cylindrical can-shaped sheet, the magnetic hysteresis of which would put a stop to the slip.

Both reluctance and hysteresis -type motors started-up as induction motors, because they could produce zero torque while not spinning, and as their rate of spin started to catch up with the applied A/C frequency, their reluctance- and hysteresis- design-strategies took over.

This meant that a hysteresis motor often also possessed a homogenous copper layer, while reluctance motors also typically had squirrel-cages, which became inactive when they were fully synchronous. One way to convert a squirrel-cage motor into a reluctance-motor, was just to drill cylindrical openings into its ferritic core, that were shaped according to how many magnetic poles the motor had, that were parallel to its axis of rotation, but that left the squirrel-cage intact.

If my memory serves me correctly, it was considered to be a good practice, to compute the diameter of these cylindrical wells such, that in total, they would remove 1/4 the volume of the rotor, 2 openings for a dipole, 4 openings for a quadrupole… And of course, for more than 4 poles, one would have started to position these cylindrical openings closer to the outer circumference of the rotor, than half-way between this circumference and the center.

(Edit 08/11/2017 : )

I should think that this would not work well for dipoles, where the diameter of the openings might better be computed, to remove 3/16 the volume ( = 0.306 relative diameter ), and their centers positioned +/- 0.6 outwards between the center of the rotor and the circumference.



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