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

Reply to
pogo
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magnets; or modify the number of windings. Given that

torque is not even an issue ?

to stay with a 12 vdc power source.

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

Reply to
HardySpicer

magnets; or modify the number of windings. Given that

torque is not even an issue ?

has to stay with a 12 vdc power source.

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.

Reply to
HardySpicer

magnets; or modify the number of windings. Given that

torque is not even an issue ?

to stay with a 12 vdc power source.

Fewer (and thicker) armature windings = not easy...

Reply to
Peter Wallace

magnets; or modify the number of windings. Given that

torque is not even an issue ?

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.

Reply to
Ben Bradley

magnets; or modify the number of windings. Given

torque is not even an issue ?

to stay with a 12 vdc power source.

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

Reply to
pogo

magnets; or modify the number of windings. Given

torque is not even an issue ?

has to stay with a 12 vdc power source.

Can you actually provide an answer ? Thanks!

Reply to
pogo

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.

Reply to
Curt Welch

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).

Reply to
SucMucPaProlij

magnets; or modify the number of windings. Given

torque is not even an issue ?

has to stay with a 12 vdc power source.

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)

  1. 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

Reply to
Peter Wallace

So, you are saying the magnets are not needed to make a PM DC motor spin? That's absurd.

Reply to
Curt Welch

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.

Reply to
Curt Welch

of magnets; or modify the number of windings. Given

assuming torque is not even an issue ?

has to stay with a 12 vdc power source.

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!

Reply to
pogo

will increase so high that motor will explode (if

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 ?

Reply to
pogo

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.

Reply to
ŠućMućPaPro

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

Reply to
BobH

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

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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

Reply to
John Nagle

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

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

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!

Anyway ... thanks as usual!

Reply to
pogo

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.

Reply to
Curt Welch

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

Reply to
HardySpicer

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