More RPM = more windings or stronger magnets ?
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More RPM = more windings or stronger magnets ? pogo 09-07-2008
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Posted by HardySpicer on September 11, 2008, 2:46 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 be so weak 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.

Posted by pogo on September 9, 2008, 2:35 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).

Do you mean that you could actually remove the magnets to achieve higher RPM ?
If so, what would the emf (on the armature) push
against ?


Posted by HardySpicer on September 11, 2008, 2:44 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).

This is correct. Field wekening speed up a motor but r educes torque
since Torque proportional flux X armature current.

Posted by John Nagle on September 9, 2008, 9:37 pm


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.

Much confusion here. Start by reading this DC motor tutorial from MIT:

    http://lancet.mit.edu/motors

DC motor behavior is relatively simple. DC motors are also generators,
and if rotated, will generate a voltage. The unloaded voltage from the
motor being rotated at a given RPM is the "back EMF" some have mentioned.
When running as a motor, the motor still generates a "back EMF", and as
the back EMF approaches the input voltage, the motor goes no faster.
The speed a motor will achieve with no mechanical load is the "no load
speed", and output torque is zero at that speed, because, of course,
there's no load.

If the motor is not permitted to rotate at all, it will generate
some torque at zero RPM. That's the "stall torque".

On a graph of mechanical load vs RPM, the two points defined by
stall torque (max torque, zero RPM) and no load speed (zero torque,
max RPM) define motor behavior. You can draw a straight line between
those two points, and for a simple DC motor, that's its torque curve.
The MIT site has a great little lab apparatus for demonstrating this
and a demo video.

Driven from a constant voltage, current consumption will be maximum
at stall, and near zero at no load max RPM. Maximum output mechanical
(torque * RPM) is obtained at half the no load speed.

So those are motor basics.

The original poster wants their motor to go faster. The question
is whether their motor has any mechanical load on it. If it does, it
won't max out at no-load speed; it will max out at some lower speed
where the torque needed to move the load is equal to the motor's output
torque.

For the no-load case, going to fewer turns on the windings will
indeed increase the motor's top speed. But it will also increase
the motor's current consumption at low speeds. This may burn out
the windings. Large DC motors often have a resistance in series
with the motor during startup for this reason.

Note that DC motors are generally heat-limited. The limitation
is watts dissipated, not input voltage.

If the original poster wants their motor to go faster with some
mechanical load on it, putting fewer turns on the armature may not
be the answer. The motor may just overheat.

Realistically, nobody rewinds small DC motors. They're so cheap
it's better to find a motor properly rated for the job.

Or crank up the input voltage. A DC-DC power supply with current
limiting would be a good way to step up the voltage without burning
out the motor. A 12V motor can probably tolerate 50VDC provided
the current at startup is limited. Heat is the problem, not
insulation voltage limits.

                John Nagle

Posted by pogo on September 9, 2008, 10:23 pm


> pogo wrote:
>> Here's my "question of the week":

>> ... snipped by me ...

> The original poster wants their motor to go faster. The question
> is whether their motor has any mechanical load on it. If it does, it
> won't max out at no-load speed; it will max out at some lower speed
> where the torque needed to move the load is equal to the motor's output
> torque.

In hindsight, I can see where I should have specified no load. It might have
removed a good bit of confusion.

> For the no-load case, going to fewer turns on the windings will
> indeed increase the motor's top speed. But it will also increase
> the motor's current consumption at low speeds. This may burn out
> the windings. Large DC motors often have a resistance in series
> with the motor during startup for this reason.
> Note that DC motors are generally heat-limited. The limitation
> is watts dissipated, not input voltage.
> If the original poster wants their motor to go faster with some
> mechanical load on it, putting fewer turns on the armature may not
> be the answer. The motor may just overheat.
> Realistically, nobody rewinds small DC motors. They're so cheap
> it's better to find a motor properly rated for the job.

Yep - right again! I was just trying to establish a relationship between motor
speed vs. things you *could* change assuming a
constant voltage.

> Or crank up the input voltage. A DC-DC power supply with current
> limiting would be a good way to step up the voltage without burning
> out the motor. A 12V motor can probably tolerate 50VDC provided
> the current at startup is limited. Heat is the problem, not
> insulation voltage limits.

And this is what I will probably do in the real world application I have in
mind. I was trying to minimize the problem but I think
instead I just made it more confusing. Whoops!

> John Nagle

Anyway ... thanks as usual!


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