More RPM = more windings or stronger magnets ?

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
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On Sep 8, 3:40 pm, "pogo"

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
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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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On Sun, 07 Sep 2008 23:40:12 -0400, pogo wrote:

Fewer (and thicker) armature windings = not easy...
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Can you actually provide an answer ? Thanks!
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On Mon, 08 Sep 2008 16:27:42 -0400, pogo wrote:

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
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Ok, so I'm trying to figure out the theory that makes this work and I'm not getting very far.
The field strength of the windings when the motor is stalled (not spinning) it seems to me will be a function of the current times the number of turns in the windings. And the stable state current will be limited only by the coil resistance if you have a true constant voltage supply.
These factors combined with the strength of the permanent magnets would directly define the stall torque (along with the physical size and shape of the armature and magnets of course).
Now, from what I grasp in the other posts, when it starts to spin, a back emf is generated in the armature which creates a voltage to offset the supply voltage. So the faster it spins, the more back EMF, that reduces the voltage across the windings and reduces the torque.
So for a constant torque load, the motor will reach a steady state RPM value where the back emf reduces the voltage and hence the current, and hence the torque until it equals the torque of the load. So the RPM of the motor will be defined as the speed at which the back emf reduces the torque to match the load.
So, if we reduce windings, what happens? The back emf is reduced so that would imply a greater RPM value for the steady state. But the torque is also reduced, so the means the motor will reach equilibrium at a lower RPM. But coil resistance is also reduced if the windings are all series wound, and that will increase current and increase torque.
So we have at least three factors at work here trying to push the steady state RPM value in different directions. With out the exact math of how these three factors work against each other, it's not at all obvious to me which will win.
Same thing seems to apply for the other option of reducing the strength of the PM. Back EMF is reduced, but so is torque. Oen effect would tend to cause higher RPM steady state, and the other would cause a lower RPM steady state. The question is which one wins. But I suspect the answer to that depends on where alone the RPM torque curve the previous steady state was located.
Lets look at the end of the curve near the stall torque (0 RPM). If we reduce magnet strength, we have reduced torque. Because the motor isn't spinning, there is no back emf so that as no effect at this end of the curve.
So, if the motor was operating very a very heavy torque load at a very slow RPM, reducing the field strength could drop the stall torque to below the torque of this load, at which point, the motor would simply stop spinning. So at this end of the curve, where back EMF has very little effect, reducing magnetic strength will reduce RPM as I see it.
But, if the motor was operating at the other end of the curve, near the no load RPM end, the only torque on the motor is from the frictions in the bearings, and the primary factor controlling RPM will be back EMF, not load torque. So if you reduce the magnetic field in this case, I could believe that the no load RPM would increase because the reduction in in back EMF has a much larger effect than the reduction in torque. So that would push the equilibrium point to a higher RPM.
But this only works as long as the torque is small compared to the back emf. As the magnet gets weaker and weaker, the torque becomes a larger and larger factor in determining the equilibrium point. Once the torque is as much a factor as the back emf, decreasing the magnetic strength further will simply lower the RPM, instead of increasing it. At least that what it seems to me like what will happen here.
So, even without knowing the exact math that applies here, I'm pretty sure that the answer as to whether you want to increase or decease windings, or increase or decrease magnet strength will depend on what part of the torque RPM curve the motor is operating in. Changing either will change the shape of the curve - my guess is that it mostly changes the slope of the curve - making one end go higher and the other go lower. So for a fixed torque load, the new RPM value will depend on which part of the curve the previous equilibrium point was located.
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.
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Curt Welch wrote:

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
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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.
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On Sep 10, 4:28 pm, snipped-for-privacy@kcwc.com (Curt Welch) wrote:

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 and the speed woudl not change. You must change the armature voltage or the field -field is teh flux or magnetic strength.
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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.

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.

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Yes you did - Thanks! I just didn't "get it" at first. My mistake.

Cool. This clears up a lot and I appreciate it very much! Thanks again!
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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.
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On Wed, 10 Sep 2008 23:48:20 -0700, HardySpicer wrote:

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

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Please read what I wrote a little more carefully Maybe then you will understand what I said but then again, maybe not...
Peter Wallace
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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
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I take it you are unable to read
Please read what I wrote again (slowly)
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On Sun, 7 Sep 2008 23:40:12 -0400, "pogo"

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.

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wrote:

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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