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Posted by HardySpicer on September 8, 2008, 2:30 am
> On Sep 8, 3:40 pm, "pogo"
> > 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
> For a given torque the Power developed at the armature is WT where W
> is the speed and T is the developed torque.
> Is this a shunt,series or separately excited machine?
> Anyway, to get more speed you need more power and the back emf E =WT.
> Hence E must be increased.
> However E is proportional to flux X W so that if the flux is increased
> E will increase and hence the speed.
> So you need more magnets! This will also increase the torque since it
> is proportional to flux X armature current.
> Simply a bigger motor (with larger magnetic field) has more speed
> unless you increase the armature voltage which could damage the motor.
> Hardy
Did you believe that?
No? Well you are right. Here is teh true story.
V-E =Ir where r is armature resistance V is armature voltage (fixed)
and E is back emf.
Now E =const (sayk) X flux X W
hence speed W directly proportional to (V-Ir)/flux
Hence you need to REDUCE the flux to increase the speed. This is known
as field weakening and with a shunt connection you can do it easily by
adding a resistor in teh field. I suppose you can do something similar
if you have a separately excited field. This sounds counter intuative
but it is the case. It will increase the losses in teh field and
reduce the efficiency of course.
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> Suppose I have a DC motor that I can either modify the strength or number of