Re: [DMCForum] Re: cooling fan follow-up

[Martin Gutkowski <martin@xxxxxxxxxxxxxx>]

checksix3 wrote:

>
> No diagrams needed, I'd just confuse myself ;) Btw, you forgot
> compound wound.

Now my limit of "experience" was an Iskra catalogue which described what 
basically amounted to a motor with field and armature coils, but 
terminals to allow the independent control of both.They called it 
"series compound wound" This allowed it to be reversible where series 
would (and shunt wound if connected directly to the commutator) not. 
Also like the rather fun property of series wound motors whereby they 
just keep going faster....

> I should have know better because there is no field wiring in the fan
> circuit and paralleling field with armature is bad form. 

Why? (genuinely don't know)

> Fractional
> HP is normally PM while larger is usually shunt. (After all, there is
> a limit on magnet size and power). Larger still are normally
> compound. In a nutshell it's this: The main advantage of series is
> starting torque. Lousy speed control but very high torque from a
> stand still. (I'll give you one guess as to what kind of motor your D
> starter uses).

Not many starters that aren't series wound.....

> Years ago in school (when the instructor wasn't around) we used to
> open the field on large shunt wounds while they were running at rated
> speed. (I'm talking around 20 HP or more but it works with any shunt
> wound). Because of the rotating mass on the big ones it's impressive
> to witness what happenes if you let them go too far. Btw, I assume
> you know a shunt wound or PM and a genetrator are the same and in a
> pinch can be used for either purpose. It's that darn counter-EMF
> thing again. (It's also the principle behind dynamic or regenerative
> braking). 

Hoh yes, I know a thing or three about this. I remember when I was a kid 
my dad showing me a motor lighting up an LED if you spun it. 95% of 
fighting robots use a pair of PM motors in skid-steer and our new ones 
(2.4kW S1 pancake motors weighing in at 3.5kg each) allow a great 
demonstration of efficiency - if you connect the wires together, 
spinning the shaft on one results in the other turning *almost* as fast.

> Now here's one for you: Do you know what "neutral plane" is? It often
> causes problems for the unknowing when they try to rebuild DC prime
> movers. 

No and I don't know what you mean by a "prime mover"?! Just not a term 
I'm familiar with, but I'm wondering if it has anything to do with full 
four-quadrant control?

> Not usually a problem on fractional motors because of the
> fixed end bell but something you have to know about on larger
> machines. Then there is electronic commutation, the "inside out"
> motor...are you using that anywhere on your robot?

Our team isn't becuase the weight advantage is offset by having to build 
two extra (what effectively amount to) half-bridges in the speed 
controllers. That and the fact that the motors themselves are stupidly 
expensive at the kind of power outputs we're talking about. And rare too 
- care to point me in the direction of a cost effective lightweight 2+kW 
24v brushless motor? There's was a clever chap who built a robot with 
four alternators as drive motors, and associated controllers. Our new 
motors (see http://www.technobots.co.uk/en-gb/dept_203.html ) are 
stupidly efficient too making brushless a bit redundant. Plus it gives 
much more to go wrong

Martin

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