Alcoholic discussion - ac versus dc motors

[The Natural Philosopher]

Er, no, sinusoidal is not mentioned anywhere in the *definition* of AC.

[The Natural Philosopher]

No, three commutator segments spaced equally.

[The Natural Philosopher]

Well that's what the electronic controllers do.

Three wires coming out. Hard to connect DC to that and get it to work. Put a scope on any two pairs and you will see AC. Put a scope on all of them and you will see three phase AC, albeit with a lot of trash due to throttling and commutation, but motor inductance smooths that out somewhat..

Yes, in the sense that later on the current gets fed backwards to the winding that was forwards last time.

[The Natural Philosopher]

Its general to have an even number of fields or fixed magnet poles, and windings as a multiple of three.

So it depends what we re talking about.

The normal cheapest electric motor you might find in e.g.an electric drill will have two magnet poles and three wound armature segments.

Some of the motors I have have 12 wound poles wound as three phases, and

14 magnet poles. For example.

[Kevin]

we were talking brushed dc motors not brushless

[Kevin]

a 5 pole motor has 2 magnet poles and 5 windings & 2 brushes so bang goes 3 phase theory

[Bob Mannix]

Oh well, that's all AC current theory out the window then, so that's OK!

[The Natural Philosopher]

No, see above. You were talking permanent magnet motors. No mention was made of brushed or brushless.

[The Natural Philosopher]

Well then its a 5 phase AC motor ;-)

Unusual, but possible with a commutator.

[The Natural Philosopher]

Not at all. Sinusoidal merely makes it easy to do calculations based on single frequency operation.

In te real world, its almost never seen ourtside of ultra low distortin oscillators.

You have to distinguish between 'electrical theory for dummies' and 'electrical engineering, an advanced course'

[Bob Mannix]

Indeed

[Rob G]

Any chance of getting you lot back On Topic and answering the original question.

Bosch claim that this 36v driven machine will mow 300m^2 on one battery

- that's at least 30 minutes continuous running I reckon, or 1.8kwh at

50A, based on the poster mentioning model aircraft motors.

Rob

[The Natural Philosopher]

36v at 50A for half an hour? thats a tough call: You would need a 25Ah battery.

Not impossible, but does it have one?

You could get all that in a couple of pounds weight of motor probably, if done well. 5lb done cheaply.

Motors I use range from about 60% to 90% efficiency. Thats not such a huge variation in running time. It is a huge variation in waste heat though, which is why the less efficient motors need to be bigger to be able to dump the heat.

As I said, mutatis mutandis, there is no real theoretical efficiency difference between mains and battery DC powered motors.

Both CAN do better than 90%, most generally do 60-70%, as that is fairly easy to achieve. The devil is in a lot of tiny details..iron quality and shape, pole geometry, magnet quality - if PM - quality low friction bearings..commutator friction and brush design (if brushed) or switching losses in electronic commutators (if not)...etc etc..

[Fred]

Most motors have far more than 3 commutator segments!

[Fred]

Agreed!

To get the advantages of 3 phase, each waveform has to be close to sinusoidal.

[Fred]

But most dangerous of all is a little bit of knowledge, then they think they know everything!

[The Natural Philosopher]

Depends. Most cheap ones have three. Larger motors have more, but generally at least 5 are in contact with the brushes at any time.

[The Natural Philosopher]

No it doesn't.

3 phase as far as a motor is concerned, simply means it knows which way to go round.

[Fred]

What on earth are you talking about? Are you seriously suggesting that it doesn't matter what the waveform is for each of the phases, that you can still tell me which way the motor will turn? Wow.

[Fred]

So the brushes are in contact with more commutator segments than the motor actually has?