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Posted by Curt Welch on September 11, 2008, 3:08 pm
> On Sep 10, 4:28 pm, c...@kcwc.com (Curt Welch) wrote:
> > > Curt Welch wrote:
> > > >> On Mon, 08 Sep 2008 16:27:42 -0400, pogo wrote:
> > > >>>>Sep
> > > >>>> 2008 23:40:12 -0400, pogo wrote:
> > > >>>>> Here's my "question of the week":
> > > >>>>> Suppose I have a DC motor that I can either modify the strength
> > > >>>>> or number of magnets; or modify the number of windings. Given
> > > >>>>> that I can only use 12 VDC, what would be the best way to
> > > >>>>> increase RPM - assuming torque is not even an issue ?
> > > >>>>> Stronger or more magnets ? More windings? Less windings ?
> > > >>>>> Remember that is has to stay with a 12 vdc power source.
> > > >>>>> Thanks!
> > > >>>>> JCD
> > > >>>> Fewer (and thicker) armature windings = not easy...
> > > >>> Can you actually provide an answer ?
> > > >>> Thanks!
> > > >> I thought I did...
> > > >> If you have an existing motor the you need to run faster at a
> > > >> constant input voltage, you can:
> > > >> 1. Reduce field strength as others have mentioned (but this is bad
> > > >> for efficiency)
> > > >> 2. Reduce number of windings
> > > >> Since you are talking about a PM motor the windings are armature
> > > >> windings, so you would need to unwind the current armature
> > > >> windings and rewind with fewer turns of thicker wire.
> > > >> Peter Wallace
> > > > So as far as I can tell, the question has no correct answer. It
> > > > depends on the motor and it's load. For a motor operating under
> > > > heavy load, you have to increase windings and increase magnet
> > > > strength to make it run faster. And for motors operating under a
> > > > light load, you have to do the inverse. Without knowing all the
> > > > math I think that's the best answer I can deduce based on how much
> > > > I currently understand about PM DC motors.
> > > Curt,
> > > Notice that they said reduce the number of turns AND use heavier
> > > wire. The heavier wire will reduce the resistance and allow you to
> > > push more current through the motor, giving you the same field
> > > strength that you had with the higher turn count.
> > > Other than missing the wire guage change you pretty much got it.
> > > BobH
> > Yeah, that sounds good. I did miss that. If you increase the wire
> > thickness so that the reduced resistance gave you enough current gain
> > to get back the field strength lost by the fewer winding, you would
> > still have all the same torque but less back EMF over the entire curve
> > giving you increased RPM over the entire range.
> > However, as a question of whether it's possible, the size wire you
> > would have to switch to might make the windings too large to fit in the
> > current motor making it impossible to actually implement. :) But the
> > theory sounds like it might work. That's cool, I've learned a little
> > more by this.
> > --
> > Curt Welch
> > http://CurtWelch.Com/ c...@kcwc.com
> > http://NewsReader.Com/
> I don't think thats right. Changing armature current only effects
> torque. You would reduce armature resistance too and the back emf
> would go up
As I understand it, because you have reduced the number of windings, back
EMF goes down. Back emf is not a function of resistance, it's only a
function of the number of windings cutting the field as the motor rotates
as I understand it.
> and the speed woudl not change.
The speed would increase because you have both increased torque, and
reduced back EMF while keeping the field strength the same. The increased
torque, acting against a reduced back emf, causes the motor to reach an
equilibrium point at a higher RPM. At least that seems logical to me.
> You must change the
> armature voltage or the field -field is teh flux or magnetic strength.
--
Curt Welch http://CurtWelch.Com/
curt@kcwc.com http://NewsReader.Com/
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