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 Peter Wallace on September 12, 2008, 2:03 pm


On Thu, 11 Sep 2008 20:16:02 -0700, HardySpicer wrote:

>> On Wed, 10 Sep 2008 23:48:20 -0700, HardySpicer wrote:
>> >> On Mon, 08 Sep 2008 16:27:42 -0400, pogo wrote:
>> >> >> On Sun, 07 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
>> > That will not change the flux. It will only reduce armature resistance
>> > and hence increase teh armature current. You will get more torque for
>> > sure but not more speed.
>> Wrong!
>> The original question asked about increasing the speed
>> "assuming torque is not an issue"
>> If torque is not an issue you are running the motor in
>> a mode where its speed will be limited by BEMF, Reducing
>> the number or armature turns means the motor will have to
>> rotate faster to generate the ~12V of BEMF where motor
>> current (and therefore torque) are in equlibrium with
>> friction/windage etc.
>> Think of it as making your 12V motor into a 9V motor but
>> still running it on 12V...
>> Peter Wallace
>
> Show me the equations first. This up a bit down a bit stuff is for
> amateurs.
> Motor current and torque are in equilibrium? Torque is proportional to
> armature current.
> Please show your equations to back up the claims. A steady-state model
> will do.
>
> hardy


Please read what I wrote a little more carefully
Maybe then you will understand what I said
but then again, maybe not...

Peter Wallace






Posted by HardySpicer on September 14, 2008, 5:36 am


> On Thu, 11 Sep 2008 20:16:02 -0700, HardySpicer wrote:
> >> On Wed, 10 Sep 2008 23:48:20 -0700, HardySpicer wrote:
> >> >> On Mon, 08 Sep 2008 16:27:42 -0400, pogo wrote:
> >> >> >> On Sun, 07 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
> >> > That will not change the flux. It will only reduce armature resistance
> >> > and hence increase teh armature current. You will get more torque for
> >> > sure but not more speed.
> >> Wrong!
> >> The original question asked about increasing the speed
> >> "assuming torque is not an issue"
> >> If torque is not an issue you are running the motor in
> >> a mode where its speed will be limited by BEMF, Reducing
> >> the number or armature turns means the motor will have to
> >> rotate faster to generate the ~12V of BEMF where motor
> >> current (and therefore torque) are in equlibrium with
> >> friction/windage etc.
> >> Think of it as making your 12V motor into a 9V motor but
> >> still running it on 12V...
> >> Peter Wallace
> > Show me the equations first. This up a bit down a bit stuff is for
> > amateurs.
> > Motor current and torque are in equilibrium? Torque is proportional to
> > armature current.
> > Please show your equations to back up the claims. A steady-state model
> > will do.
> > hardy
> Please read what I wrote a little more carefully
> Maybe then you will understand what I said
> but then again, maybe not...
> Peter Wallace

Means you have no idea!! The design of electrical machines has been
well defined by simple equations for at least a century or more and yo
ucannot state your point. I take it you are not an electrical
engineer.


Hardy

Posted by Peter Wallace on September 14, 2008, 2:53 pm



>> >> Wrong!
>> >> The original question asked about increasing the speed
>> >> "assuming torque is not an issue"
>> >> If torque is not an issue you are running the motor in
>> >> a mode where its speed will be limited by BEMF, Reducing
>> >> the number or armature turns means the motor will have to
>> >> rotate faster to generate the ~12V of BEMF where motor
>> >> current (and therefore torque) are in equlibrium with
>> >> friction/windage etc.
>> >> Think of it as making your 12V motor into a 9V motor but
>> >> still running it on 12V...
>> >> Peter Wallace
>> > Show me the equations first. This up a bit down a bit stuff is for
>> > amateurs.
>> > Motor current and torque are in equilibrium? Torque is proportional to
>> > armature current.
>> > Please show your equations to back up the claims. A steady-state model
>> > will do.
>> > hardy
>> Please read what I wrote a little more carefully
>> Maybe then you will understand what I said
>> but then again, maybe not...
>> Peter Wallace
>
> Means you have no idea!! The design of electrical machines has been
> well defined by simple equations for at least a century or more and yo
> ucannot state your point. I take it you are not an electrical
> engineer.
>
>
> Hardy

I take it you are unable to read

Please read what I wrote again (slowly)









Posted by Ben Bradley on September 8, 2008, 1:11 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.

>Thanks!
>JCD


Posted by pogo on September 8, 2008, 4:26 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


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