Permanent magnet motors...

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Externet
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Permanent magnet motors...

Post by Externet » Sat Jan 17, 2004 12:43 pm

Hi fellows.<p>A motor with typical ferrite permanent magnets, fed by a finite power supply (fixed voltage, limited current) can perform a certain work W.<p>The same motor, fitted with very strong samarium rare earth magnets instead, ¿will it perform a greater work W under the same power supply ?<p>Miguel
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Re: Permanent magnet motors...

Post by josmith » Sat Jan 17, 2004 5:52 pm

Not more work just more torque at a lower rpm.

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Re: Permanent magnet motors...

Post by russlk » Sat Jan 17, 2004 6:33 pm

E X I = power input. Can't put more power in, can't get more power out.

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Chris Smith
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Re: Permanent magnet motors...

Post by Chris Smith » Sat Jan 17, 2004 8:57 pm

The stronger the magnetic field, the more efficient the work out put, per "given" magnetic field.<p>"Given" is the magnetic field. <p>All things have a loss and or / inefficiency, and this is improved, not increased per see, as per the stronger the magnetic field.

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Joseph
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Re: Permanent magnet motors...

Post by Joseph » Sun Jan 18, 2004 2:07 am

Chris is onto something. The stronger magnetic field does raise efficiency, but how much depends on things like RPMs, gap, etc. Think of the case where the strength of the magnetic field approaches zero. Then the efficiency approaches zero also. In that case, all the power is lost in the commutation and field winding resistances.

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Re: Permanent magnet motors...

Post by josmith » Sun Jan 18, 2004 2:06 pm

Ideally power out should be power in minus IR loss.(I squared R). Efficiency on a p.m. motor increases as current (torque) approaches zero and counter emf (rpm) approaches the applied voltage. Unfortunately maximum horsepower occures at the point where counter emf is half of applied voltage. At this point efficiency is 50%.
Changing the field strength by a reasonable amount changes the rpm at which maximum power occurs. More field shifts it to a lower rpm.<p>Reducing the field to zero will produce a motor that doesn't run at all much less efficiently.

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Re: Permanent magnet motors...

Post by Isenbergdoug » Tue Jan 20, 2004 4:06 pm

The "stronger" magnets will cause the magnetic flux (phi) to change. This will cause a different torque to be seen given a specific armature current. The equation for this is;
t=k*(phi)*Ia
where t is torque
Ia is armature current
and k is the motor armature constant
k=Na*p/(pi*a) where p is poles, Na is windings, and a is number of parallel paths of windings. <p>Motors are energy converters, they convert electrical energy into mechanical energy or vice versa in the case of a generator. You must put energy in to get energy out. And you can't put energy in with just permenant magnets. It's true that energy must be put into magnetizing a magnet so maybe energy can be extracted from demagnetization. The magnet would have to loss some energy for there to be more work created by the motor, which likely doesn't happen. After all, work=energy.

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Re: Permanent magnet motors...

Post by Dean Huster » Tue Jan 20, 2004 4:54 pm

I would think that a motor from a defunct cordless power tool that used rare-earth magnets would make a rather robust dc generator, assuming that it used a standard mechanical commutator vs. being brushless.<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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Re: Permanent magnet motors...

Post by josmith » Tue Jan 20, 2004 6:30 pm

The answer to your question Miguel, is Yes.

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Re: Permanent magnet motors...

Post by Externet » Wed Jan 21, 2004 9:07 am

Thanks, gentlemen.<p>Then, in other words, stronger magnets will increase the motor efficiency and nothing else.<p>Being more efficient, can perform some more work that would had been wasted as heat otherwise.
Then, a rare earth permanent magnet motor in theory will run cooler while performing the same work and consuming the same power.<p>I think I got the concept, thanks,
Miguel
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Re: Permanent magnet motors...

Post by Will » Wed Jan 21, 2004 2:03 pm

Miguel,
You probably didn't mean it as you said it but if you believe the efficiency is the only thing which will change with using stronger magnets then you haven't got it yet. There may be some marginal change in efficiency but the most predominant change will be the speed which will change in inverse proportion to the field strength. Once a motor armature is turning it operates as an electric generator and generates an armature voltage (Back e.m.f) which is proportional to both the speed of the armature conductors and the strength of the magnetic field and opposes the voltage applied to the armature. When a motor is started on light or zero load then its speed runs up to the point where the back emf is almost equal and the slight difference produces enough current to take care of the armature losses i.e. windage, magnetization and copper (I^2.R) losses. As load (Torque) is apllied to the motor shaft the speed reduces so that the difference between applied voltage and back emf creates enough armature current to supply the work load (The current is directly proportional to the load torque). If you inncreased the magnetic field strength by (say) three times then the armature would only have to run up to one third of the required speed (That of the lower magnetic field value) in order for the back emf to reach equality with applied voltage less armature IR drop. One cannot say what the effect on the torque would be because that is a characteristic of the load. For instance, if the motor were driving a centrifugal pump then the torque at the lower speed (Higher magnetic field) would be reduce because, in such a pump the torque is proportional to the speed (The developed pressure is proportional to the square of the speed and the work done/power consumed is proportional to the cube of the speed - consequently, in such a theoretical pump the power consumed at the lower speed would be approximately 1/27th of that at the higher speed.
If the load was a reciprocating coompressor working into a fixed pressure the the torque at both speeds would be equal consequently the work done/power consumed/compressed product produced would be 3:1 high/low speed.
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