[DMCForum] Re: cooling fan follow-up

["checksix3" <jetjock11@xxxxxxxx>]

--- In DMCForum@xxxxxxxxxxxxxxx, Martin Gutkowski <martin@xxxx> wrote:
> Your penants will be to describe, using diagrams where appropriate, 
what the difference is between Series wound, Shunt wound and 
permanent magnet... :-)
> 
> More specifically, what are the advantages and disadvantages of 
each type?
>

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

I should have know better because there is no field wiring in the fan 
circuit and paralleling field with armature is bad form. 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). Shunt provides for good load and speed regulation 
while compound offers the best of both worlds, both starting torqued 
for the driven element and good load regulation once up to speed.

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). You'll need to excite the field on a shunt though, to use 
it as a generator.

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. 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?

Course this is all DC theory; with AC prime movers it's a different 
story altogether.

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