# Commutator Bars



## Guest (Sep 19, 2009)

I would like to know the full benefit of having more commutator bars vs fewer? Would the motor with more com bars use less amps per revolution than the motor with less? More torque? What?

Example:

What would be the difference between the same brand and size motor.

One with 37 com bars:
One with 65 com bars:

Both motors 9" and same brand and both series motors.

Pete


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## Coley (Jul 26, 2007)

More torque would be my guess.

At the same time, more amps used also.

I haven't ever heard it discussed before.

Good point.


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## Guest (Sep 19, 2009)

Coley said:


> More torque would be my guess.
> 
> At the same time, more amps used also.
> 
> ...


Well then I guess it is time to discuss this topic. I know many motors have either lots of narrow com bars or fewer wide bars. 

Pete


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## major (Apr 4, 2008)

gottdi said:


> I would like to know the full benefit of having more commutator bars vs fewer? Would the motor with more com bars use less amps per revolution than the motor with less? More torque? What?
> 
> Example:
> 
> ...


Hi Pete,

Actually the number of armature conductors is the important factor in the performance of the motor. But the motors you speak of use a single turn wave wound type armature. So the number of conductors in the armature is twice the number of comm bars, or one turn per bar.

Eg = K * Wa * Flux

Tem = K * Ia * Flux

Eg = generated voltage

Wa = rotational speed (rad/sec)

Tem = electromagnetic torque

Ia = armature current

The motor torque and voltage constants are equal in metric units.

K = (p*N)/(2*pi*a)

Where:
p = # of poles
N = # of conductors
2 = 2
pi = 3.14
a = # of current paths


So for your example motors, p = 4, a = 2 (wave wound), and N = # of comm bars times 2. You can then see how the 65 to 37 ratio affects the speed voltage and current torque relationships from the top 2 equations. The higher number increases the torque/amp and decreases the RPM/volt.

Also note that the number of turns in the armature will influence the resistance and the current carrying capacity. The higher number of turns having higher resistance and reduced current capacity.

There you go.

major


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## major (Apr 4, 2008)

Pete (gottdi) sent me a message saying that he didn't understand _Flux _as used in my equations. So I responded with this:



> Magnetic flux, represented by the Greek letter Φ (phi), is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field. The SIunit of magnetic flux is the weber (in derived units: volt-seconds), and the unit of magnetic field is the weber per square meter, or tesla.


This is from http://en.wikipedia.org/wiki/Magnetic_flux 

Do a search on _magnetic flux_. It is a fairly tough concept, but what all electric motors and generators need to convert electricity to mechanical power, or vice versa. 

If you can look back at some of my posts about SepEx, you'll see where I talk about measuring the sat curve. This is somewhat the _flux_. Calculating the flux is quite difficult. Now-a-days, they seem to use magnetic finite element analysis software. But even then, you need a model of the magnetic circuit which you're unlikely to get unless you saw the motor in half 

If you're comparing two motors of the same size and by the same maker, as Pete was, then the actual value of flux is likely to be nearly the same for similar conditions, ie load. 

Hope that helps,

major


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## Guest (Sep 22, 2009)

Major,

I had to ask! Wow that is a chunk to process. Ok, so now for the next question! If I use a motor with 67 com bars will it perform better than the one with only 37? Given that both are 9" motors and series motors and the same brand. 

Pete 

PS. I think some of that flux stuff is a bit to process for now but I will read through it all over then next week.


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## major (Apr 4, 2008)

gottdi said:


> If I use a motor with 67 com bars will it perform better than the one with only 37?


Hi Pete,

Better? I'd say different. One design may in fact be a better choice for a given application. But there is no "better" for all applications. That's why there are different motor designs.

Generally speaking, the higher turns (comm bars) are used for higher voltage applications. And conversely, low turns for low voltage. But the high turn motors support lower current, and conversely low turn motors higher current. So, if you're looking strictly for power rating, same size motors at the same speeds will have similar power ratings. Just different volt/amp ratios.

Then you throw into the mix the system components and vehicle dynamics. Like controller current limit and gear ratio(s) and mass and the like. The appropriate motor choice is not as simple as the commutator bar count. But I think the more you understand about such things, the better decisions you can make. I try hard to avoid making such decisions for others. I just try to explain a little about how motors work.

Regards,

major


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## Guest (Sep 22, 2009)

That helps. Thanks. I have two motor with the noted com bars. I will see how each works with my current application. I will post when I have the motor installed and tested. It will be an interesting test for me and the results should be good for others. 

Thanks
Pete


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## Amberwolf (May 29, 2009)

I think that my question may be slightly related to this topic, but possibly not. I'm still learning about all this stuff. 

If I have a 4-pole PMDC motor, is it possible to run it using only two of the poles? 

Meaning, if I apply power only to one set of brushes, (which are wired in pairs 180? apart), should the motor operate "normally", except for consuming only part of the current, and producing only part of the speed and torque? Potentially with cogging issues at lower speeds since it is a PM motor?

I have tested with only one set of brushes installed and it does function, reducing (at 12V) the no-load current from about 4A to about 3.5A (using my gloved hand to grip the shaft as hard as I can, which slows it down a little, gives a load of about 6A with 4 brushes, and about 5A with 2 brushes). 

The speed (loaded or unloaded) is significantly less with only one set of brushes, though I don't know by how much--I'd presume 1/2 speed; it sounds like it. 

I am thinking of doing this because most of the time I will not need all of the torque or speed that this motor can produce, and so do not want to "waste" the power the second set of windings would consume unless I am actively using them. 

So if I setup a pair of relays (contactors) at the motor housing near the input contacts, and wire the brushes independently, so that I can cut off the power to one of the two brush pairs after I have gotten moving from a stop or finished going up a hill (the only times I should need the extra torque), is it likely to have any serious side effects?

Before I actually destroy anything by doing this, I thought I should ask the opinion of the experts.  If I have some idea beforehand of what might happen, I may be able to design protection against it before I experiment with it on the actual vehicle.
________
The Legend Condo Pattaya


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## major (Apr 4, 2008)

Amberwolf said:


> If I have a 4-pole PMDC motor, is it possible to run it using only two of the poles?
> 
> I am thinking of doing this because most of the time I will not need all of the torque or speed that this motor can produce, and so do not want to "waste" the power the second set of windings would consume unless I am actively using them.


Hi Amber,

First off, your premise is flawed. Running the motor at lower loads does not waste power. The power out of the motor is reduced, but so is the power into the motor. Running motors at partial loads increases motor efficiency most times.

Second flaw. Using 2 of the four brushes does not change the pole count. It will effectively change the armature resistance and the brush voltage drop, in a bad way. So you just end up making the motor less efficient.

You might as well switch an external resistor in series with the motor. Which is a power waster as well.

Regards,

major


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## Guest (Sep 22, 2009)

I think you can run your motor like that because my VW 12 volt Generator only has one pair of brushes and when powered up it runs just fine. You might be risking faster brush wear. I think the amperage would be the same but just being pushed through half the system overloading the brushes and causing undue damage to the commutator. I'd just limit the amperage to the motor for economy cruising. 

Pete 

Some here with more knowledge on this subject may have a better answer.


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## Amberwolf (May 29, 2009)

major said:


> First off, your premise is flawed.


Which doesnt' surprise me. Every time I think I understand something about motors, I usually get it turned upside down (or right side up, depending on your POV) by helpful people like you.  (No, that's not sarcasm, I *mean* that.)




> Running the motor at lower loads does not waste power. The power out of the motor is reduced, but so is the power into the motor. Running motors at partial loads increases motor efficiency most times.


Well, I didn't exactly mean that running it at a partial load was wasting the power, only that the current input into it is higher with 4 brushes rather than two (though not all that much, with the very small battery capacity I can carry, it would make a difference to my range).




> Second flaw. Using 2 of the four brushes does not change the pole count. It will effectively change the armature resistance and the brush voltage drop, in a bad way. So you just end up making the motor less efficient.


Oh. Why didn't I see that before? Sometimes I feel really dumb when the obvious is pointed out to me, when I should have been able to see it myself.  

So the current drop I'm seeing is only because the resistance into the motor (via the brushes) is increased, which is why the motor drops in speed.... That makes perfect sense. 




> You might as well switch an external resistor in series with the motor. Which is a power waster as well.


Yeah, I see your point.  

I guess if it would really have worked, there'd be switchable-brush-count motors on the market!

Apologies for the interruption...back to your regularly scheduled intelligent thread. 
________
How To Roll A Blunt


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## major (Apr 4, 2008)

Amberwolf said:


> I guess if it would really have worked, there'd be switchable-brush-count motors on the market!


 
Well there kinda is, or was. Not sure they still use them or not. But windshield wiper motors used to be 2 pole PM motors with 3 brushes. The 2 brushes 180 degrees apart would be for low speed. That third brush would then be used with one of the others for high speed. It was offset from the 180 degree opposed position enough that it would alter the armature coil excitation enough to get a field weakening effect. A lot of wiper motors were made that way.

Regards,

major


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