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Posted by Curt Welch on September 8, 2008, 8:11 pm
> > On Sun, 7 Sep 2008 23:40:12 -0400, "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.
> > Rewinding and/or changing magnets is a pain. It's easier to buy a
> > DC-to-DC converter to get more voltage out of that 12vdc power source
> > and into the motor.
> That's good info - thanks!
> But I'm asking a basic, theoretical question here. Let me put it a
> different way:
> Given the same voltage supply; torque not being an issue; what
> results in more RPM ? Stronger magnets or more windings ? both ?
> Thanks!
> JCD
I don't understand the theory well enough to answer your question, but I
suspect the question isn't valid - I think it might be too simplistic to
have an answer. There's no such thing as "torque not being an issue". PM
DC electric motors have a torque/RPM curve. You trade off one for the
other as a function of the load you have placed on the motor. The speed
the motor will actually turn in any given application is a function of the
load you put on the motor. At low speed, the motor will produce more
torque, and at high speed, the motor will produce less torque. How fast it
ends up spinning is a function of where the application will stabilize
along the torque/RPM curve of the motor.
The two commonly specified points on the curve are the stall torque (the
point where the speed is zero and the torque is at it's max), and the no
load RPM (the point where the torque is zero, and the RPM is at a max.
Changing the voltage, or the magnets, or the windings, will change the
curve. Increasing the voltage, I believe, will move the curve up so that
both torque and max RPM will increase.
But changing the windings, or the magnets, might change the shape of the
curve, causing one end to increase while decreasing the other (just a guess
on my part). So it might increase the RPM in the low torque range of the
curve while decreasing the RPM in the high torque side of the curve. If
that was so, the change might both increase and decrease the RPM (if you
try to pretend the torque doesn't matter).
So, I wonder for example, if increasing the magnet strength might increase
the stall torque, and increase the RPM when under high torque load, but
could actually decrease the no load RPM rating of the motor because you
have increased the back EMF as the motor is spinning.
If this is the type of dynamics that actually happen in motor design, your
question has no simple answer because it would depend on the specifics of
the motor in question and the application it was used for (how much load it
was placing on the motor at different RPM levels).
Then of course the winding question is also ignoring wire resistance. If
you change the windings, do you also get to change the wire so the total
winding resistance stays the same? Or are you talking about adding or
removing windings without changing the gauge of the wire (which of course
isn't for the most part practical because a given motor only has so much
space for windings so you can't just add more without reducing the wire
size which is yet another unspecified part of your question).
And then there's the 12V issue. Real 12 V sources like batteries have
power limits and their voltage will drop as you draw more current from
them. Are you assuming infinite power with a fixed 12 volt supply?
I would guess that in real PM motors, the resistance of the winding will
play a very important role in the no load RPM speed of the motor if you
have a fixed voltage infinite current supply. So again, you probably need
to be more specific about whether you question is purely theoretical and
applies to an example where the we pretend the wires have no resistance and
the armature has no friction or whether you question is more real-world in
nature.
Or maybe, none of this is important because like I said, I don't understand
motor theory well enough to answer the question. :) Maybe John, who knows
everything, and knows a lot about electric motors, can shed more light on
it. :)
My guess however, is if there is one simple correct answer, is that if you
increase windings while keeping total winding resistance the same you will
get more torque and higher RPMs. And if you use a stronger magnet, you
will likewise get more torque and higher RPMs. But this would be contrast
to what the other person posted who thought you needed to reduce windings
to get higher RPMs.
--
Curt Welch http://CurtWelch.Com/
curt@kcwc.com http://NewsReader.Com/
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Posted by =?ISO-8859-1?Q?SucMucPaProlij? on September 9, 2008, 3:39 am
> But I'm asking a basic, theoretical question here. Let me put it a different
> way:
> Given the same voltage supply; torque not being an issue; what results in
> more RPM ?
> Stronger magnets or more windings ? both ?
Weaker magnets and less windings. If you remove magnets from running motor RPM
will increase so high that motor will explode (if your power supply can give
enough power).
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Posted by Curt Welch on September 9, 2008, 1:24 pm
> > But I'm asking a basic, theoretical question here. Let me put it a
> > different way:
> > Given the same voltage supply; torque not being an issue; what
> > results in more RPM ?
> > Stronger magnets or more windings ? both ?
> Weaker magnets and less windings. If you remove magnets from running
> motor RPM will increase so high that motor will explode (if your power
> supply can give enough power).
So, you are saying the magnets are not needed to make a PM DC motor spin?
That's absurd.
--
Curt Welch http://CurtWelch.Com/
curt@kcwc.com http://NewsReader.Com/
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Posted by =?iso-8859-2?B?qXXmTXXmUGFQcm9 on September 9, 2008, 3:31 pm
> So, you are saying the magnets are not needed to make a PM DC motor spin?
> That's absurd.
No, this is not what I as saying.
This is simplified formula:
Voltage of power supply = Number of windings * RPM * Strength of magnetic field
+ current * ohm resistance
Removing the magnet is a process. During this process strength of magnetic field
goes to zero. If power supply can produce enough power, according to the
formula, RPM goes very high (if resistance is low). After you removed the
magnet, strength of magnetic field is zero, formula is U=I*R (Ohm's law), torque
is zero and motor will (if still in one piece) eventually stop running.
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Posted by HardySpicer on September 11, 2008, 2:45 am
On Sep 10, 5:24 am, c...@kcwc.com (Curt Welch) wrote:
> > > But I'm asking a basic, theoretical question here. Let me put it a
> > > different way:
> > > Given the same voltage supply; torque not being an issue; what
> > > results in more RPM ?
> > > Stronger magnets or more windings ? both ?
> > Weaker magnets and less windings. If you remove magnets from running
> > motor RPM will increase so high that motor will explode (if your power
> > supply can give enough power).
> So, you are saying the magnets are not needed to make a PM DC motor spin?
> That's absurd.
> --
> Curt Welch http://CurtWelch.Com/
> c...@kcwc.com http://NewsReader.Com/
I showed the formula earlier. Any electrical engineer will verify
this. If you reduce the field so much, the torque will eb so weka that
the motor will not spin. It is possible to explode a motor. A series
connected one is the classic on no-load or a differential compound one.
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