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Author Topic: Supercharging DC electric motors  (Read 874 times)
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Durham UK
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Hi,

I wonder if it is safe to "supercharge" a DC motor like in this attached picture using strong rare earth magnets.

Wouldn't it produce larger voltages and currents in the motor and cause it to overheat? These links helped me revise some school-learned theory to me:

http://www.magnet.fsu.edu/education/tutorials/java/dcmotor/index.html

http://www.magnet.fsu.edu/education/tutorials/java/electromagneticinduction/index.html

Maybe at a lower level of supercharging it would just shorten the life of the motor, so yes, it's safe but there's a down side?

Does anyone have any useful links / advice on this?

Cheers


* magnoMotor.gif (36.8 KB, 400x320 - viewed 7 times.)
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as far as i remember, it only would cause the motor bearing to rotate far faster then it design,
i cant remember anything about heating the coil and anything, The magnet work because, when current pass thru the coil it induce an electromagnet, so the permanent magnet is there to help repel the induce electromagnet, So the high the strength of the permanent magnet the higher the repulsion force thus making the motor rotate much faster. 
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Thanks ash901226

Should go faster, but when I look at the second tutorial I see the voltage needle moving a lot which made me think voltages are higher and changing quickly.

Could this have a detrimental effect on the coils not just the bearings being worn faster? So eg it's a 6v max  motor, and I run it at 6v then add some strong magnets could that be equivalent ('ish) of running it say at 12v? I'm not suggesting a linear relationship here, just trying to visualise.

Code:
E (volts)  = (d [magnetic-flux] / dt) * N

So the EMF (E) = rate of change of magnetic flux with time, multiplied by the number of loops in the coil.

In my motor N is fixed, so the magnetic flux can be increased by the rare-earth magnets, hence EMF (volts) must increase. Is this right?
« Last Edit: December 14, 2012, 08:22:08 am by trendski » Logged

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i cant say much about this topic. urm how about i make an experiment, report back later
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Thanks,

that's a great idea. I'll do that too, I've got a cheap 6v model motor somewhere, but I'll have to buy some magnets first.

cheers
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I think you will find the speed of the motor decreases (back emf is greater due to increased flux) but the torque capability increases.  If I remember correctly from my college days - many decades ago - the theoretical speed of a motor with zero flux is infinite but the torque is zero 
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You could also look into rewinding a DC motor (not for the faint of heart or patience). In the R/C racing world it used to be pretty common (not sure if it is still done with BLDC being more commonly used or not)...
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Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.
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If you increase the strength of the magnets, you will find that the no-load speed of the motor decreases for a given supply voltage, but the torque increases for a given current. You could increase the voltage to get the original speed back, but if you do that then you should current-limit the motor driver so that you do not feed it more current than it would have if it were running from 6V.
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Thanks guys,

I think I get it now. I've ordered some curved magnets from these guys in the UK: http://www.first4magnets.com/mitre--arc-magnets-94-c.asp

When they arrive I'll do some experiments.

Cheers
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If you increase the strength of the magnets, you will find that the no-load speed of the motor decreases for a given supply voltage, but the torque increases for a given current. You could increase the voltage to get the original speed back, but if you do that then you should current-limit the motor driver so that you do not feed it more current than it would have if it were running from 6V.

Absolutely - from simple physics of a conductor moving in a magnetic field, V = B x L x v  (magnetic flux density x "length" x velocity).
Increase B and v goes down.

However the value of B to use depends on the magnetic circuit - with ferrite permanent magnets the flux density is likely to be about 0.4T in
the magnets, but could be a lot more in the stator pole pieces because of their shape (perhaps 1T or more).  The maximum flux density in iron alloys is about 2T  (neodymium magnets are about 1.4T and are likely to saturate the iron).  I'd expect an increase in torque over ferrite magnets of a factor between 1.5 and 3 or so.

Actually the air-gaps can be a limiting factor to the flux density too - so the whole thing is rather complex and it would be interesting to
see what difference you get.
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Nice to see another physicist here!
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Thanks MarkT,

my background is in ecotoxicology and biology (some school physics), so I think I'll have even more physics questions when these magnets arrive!

How can I readily measure torque in my home office?  I have a hobby PSU which will measure supplied volts and current draw and hobby multimeter.

One website says I should measure motor torque rating at point when the motor is stalled, when lifting a mass. If I measure current drawn at this point, with and without the magnets, does this give me a fair comparison?

Cheers
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Should do - due to static friction I'd measure the current to start lifting the mass and the current at which it stalls again, take the average.
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