Is it possible to reduce back EMF in DC motors using shorting rings?

[ibuildstuff]

TL;DR Summary

Shorting rings are commonly used in linear motors to reduce back emf; could something similar be used to reduce back emf in rotational motors?

Shorting rings (also called Faraday rings) are commonly used in loudspeakers to reduce the back emf that is induced in their voice coils. Could something similar, i.e. shorted turns of wire that are placed in the same plane as each of the windings in a DC motor, be used to reduce the back emf?

And followup question, how would the magnetic field induced in the shorted windings affect things?

By the way, this is a purely theoretical question, I'm not trying to solve a problem.

Thanks.

 

[berkeman]

Summary:: Shorting rings are commonly used in linear motors to reduce back emf; could something similar be used to reduce back emf in rotational motors?

Shorting rings (also called Faraday rings) are commonly used in loudspeakers to reduce the back emf that is induced in their voice coils.

Could you supply a reference for that please? I'm not familiar with "shorting rings" other than using "shorting bands" outside of transformers to reduce external stray fields. Thanks.

 

[eq1]

Shorting rings (also called Faraday rings) are commonly used in loudspeakers to reduce the back emf that is induced in their voice coils. Could something similar, i.e. shorted turns of wire that are placed in the same plane as each of the windings in a DC motor, be used to reduce the back emf?

I'm also new to the term shorting ring. It looks fairly well described here:
http://diy-audio.narod.ru/litr/FaradayRingsVoiceCoilImpedance.pdf

Basically it looks like the purpose of the ring, at a very high level and ignoring why, is to remove flux from the coil. So ya, that would reduce back EMF. That would also reduce motor torque though. Whether or not that is a good trade-off is left an exercise to the reader. :)

Although not back EMF related, I suppose in some applications the shorting-ring could make the motor more linear. One can imagine that being a desirable outcome in some cases.

 

[DaveE]

Back EMF is how DC motors work (speakers too). Back EMF is a good thing in these motors.

Yes, you could ruin a DC motor by adding a shorted turn to the winding structure and reduce back EMF. This would be essentially the same as adding an additional secondary winding to a transformer and shorting it.

The "shorting rings" I think you've seen go around the entire structure and are intended to reduce the amount of magnetic flux (EMI) that escapes the object. They do not interfere with the function of transformers or motors by modification of the internal structure.

The use of shorting rings in speakers is a more complicated subject, they are used to linearize the response by modification of the induced fields and are carefully designed to only slightly "ruin" the magnetic fields.

 

[CWatters]

Not sure what's behind the original question but some people think that reducing or eliminating back emf is the key to making a motor more efficient (possibly more than 100% efficient but obviously that's not possible). This is wrong.

Some modern DC Brushless motors are incredibly efficient, as much as 94% efficient. The cause of the remaining 6% is pretty well understood and includes things like:

Winding resistance/copper losses
Eddy currents/iron losses
Windage/Air resistance
Bearing friction

These motors work like generators... when you apply power they accelerate until the back emf approaches the supply voltage (less IR losses). We can write..

Back emf = RPM / k

Where K is a motor constant (with units RPM per Volt) that depends on things like the strength of the magnets and the number of turns of wire in the windings.

If you try and reduce the back emf all you do is change the operating RPM and make the efficiency worse. Instead its better to use high power rare Earth magnets to increase the back emf because this allows you to reduce the number of windings needed to achieve the design RPM. That in turn reduces the resistance of the windings and the I^2R losses in them making the motor more efficient.

Aside: I have a high power brushless model aircraft motor that was designed many years ago to be very efficient. It has just 2 (two) turns of wire in the armature.

 

[sophiecentaur]

Yes, you could ruin a DC motor by adding a shorted turn to the winding structure and reduce back EMF.

A shorted turn in a DC armature can be the cause of a roasted winding. Can that ever be a good thing?
I could imagine a shorted turn in a loudspeaker voice coil could suppress mechanical resonances but that's not something I've ever come across

 

[DaveE]

A shorted turn in a DC armature can be the cause of a roasted winding. Can that every be a good thing?
I could imagine a shorted turn in a loudspeaker voice coil could suppress mechanical resonances but that's not something I've ever come across

I think the speaker thing is to improve the linearity of response vs. displacement of the coil by manipulating the fields with shorted turns a particular locations. But that bit of hand waving BS is the depth of my knowledge.

 

[DaveE]

Oh, yea, I forgot to add: Since speakers live in the audiophile world, there is also probably a large element of marketing BS involved. If you told me you did an in depth analysis of any part of speaker design and decided there was nothing to it, I wouldn't be the least bit surprise. Some of this stuff is to generate talking points and ad copy for market distinction purposes. Or, not, IDK, it's really hard to tell. People who are technically capable, truly objective, but still care enough to look into this stuff are a rare thing.