Alcoholic discussion - ac versus dc motors

[The Natural Philosopher]

As long as they are 120 degrees apart, it doesn;t matter whether they are sine, square or spikes. It will go round

[The Natural Philosopher]

It as you that said it had more segments than three, not me.

[Fred]

It was you who said that "generally at least 5 are in contact with the brushes at any one time"! You also said that most cheap motors had 3 segments.

[Fred]

If we're measuring degrees then it implies a sinusoid, though in practise there would be harmonics of the fundamental.

If we apply an arbitrary waveform to each phase of a 3 phase motor, I can assure you will not be able to tell which way it will turn.

[Kevin]

you could if they followed each other like a normal 3 phase wave form take 3 pulses as below (if it works)

_[]____[]_____ ___[]____[]___ _____[]____[]__

[Fred]

I agree, the fundamental for each phase is a sinewave with odd and even harmonics added to each to make the pulse like the waveforms you sketch. They aren't arbitrary waveforms.

[The Natural Philosopher]

wrong.

as long as teh three wavefortms are poahse related, whether sinusoid or nnot, it will turn.

There is a lot of difference between sinusoid, repetitive non sinusoid with a common fundamental frequency harmonically and phase locked, and 'arbitrary'.

YOU have merely set up a straw man by saying arbitrary. I never claimed the signals applied to the poles were random and arbitrary, merely that they don't need to be sinusoidal.

[The Natural Philosopher]

thank you.

[The Natural Philosopher]

NO, the fundamental for each phase is a square wave.

Stop weaselling.

The point at which you have to introduce an infinite Fourier series of sinusoids is the point it ceases to be worth talking about them.

[Kevin]

I still dont think a simple dc motor is three phase though :-) despite it having three wound poles and two magnets

[Bob Mannix]

AC motors are motors designed to work on an "AC supply"

An "AC supply" is an electrical supply system supplied by an alternator

An alternator provides a sinusoidally varying voltage/current (it has to)

AC theory allows design of such alternators and motors to be effective and takes as a given that the supply is sinusoidal

The fact that a sawtooth waveform alternates in direction does not make it an "AC supply"

The fact that some AC motors might "go round" if supplied by a sawtooth waveform is irrelevant

A "DC motor" is not an "AC motor"

OK?

[The Natural Philosopher]

Thats semantics.

This arose out of the comparison of 'AC' and 'DC' motors. I was merely pointing out that at the core of whats happening inside them, they aint so very much different, and that for analytic purposes a DC motor is an AC synchronous motor with a mechanical DC to AC convertor called a commutator.

OK?

[Bob Mannix]

No, what I said was engineering, what you said was semantics - pushing the term "AC motor" far away from its accepted engineering definition to merely make your own point!

I'm OK, if you are!

[Andy Dingley]

Something that has already adopted the cost and complexity of Li-Po may well be using a brushless motor too. As the extra cost of going brushless remains largely constant, while the cost of vaguely robust brushgear goes up with power, then this makes even more sense for powerful motors. Once you have brushless control and some vaguely smart control gear, you can get a lot more initial starting torque for little effort.