# Commutator Bar Count?



## major (Apr 4, 2008)

Frank said:


> What impact does the bar count have in the real world? The 6.7" motor I've been using has (IIRC) 57 bars. I'm pretty sure there are other variants with fewer bars (31?) I think the latter is the 8-brush motor which has more brush area. I'd appreciate thoughts, opinions, etc.
> 
> thanks


The relationships between the angular velocity and generated voltage (some would call it BEMF/RPM or Kv), and the developed torque and armature current (some would call it torque per amp or Kt), both depend on the number of series connected turns in the armature. The more turns; the higher the generated voltage at a given RPM and the higher the turns; the higher the developed torque for a given armature current. 

The particular construction used for the EV traction motors commonly applied here is called a hairpin bar wound armature. Further, besides the point, they are almost always single turn simplex retrogressive wave wound 4 pole. The important point here is that the number of series turns in the armature is proportional to the commutator segment count. The more bars; the more coils. Basically one coil per bar.

Higher bar counts typically mean slower motors for a given voltage, OR higher voltage for a given RPM. Higher bar counts typically mean higher torque for a given armature current, OR less current for a given torque.

The downside to higher bar counts is that the increased number of coils (or conductor bars) in the armature necessitates the use of smaller cross sectional area in those conductors. This means lower current capacity (ampacity) and higher resistance. 

An upside to increased bar count is that the voltage between adjacent segments on the commutator is lower for a particular applied armature voltage. This can reduce flashover.

Generally speaking, motors of this type use higher bar counts for higher voltage application or slower speed application. The higher bar count often necessitates more narrow brushes. Also, it can be viewed as devoting more motor volume to inactive material (insulation) thereby decreasing efficiency and/or torque density. And high bar counts typically increase motor cost.

What was the question  Just kidding. It is just a motor design parameter. Used to get the best motor for your application. Not good: Not bad. Just there


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## Frank (Dec 6, 2008)

Thanks for the discourse . It is very interestingly actually.

Other than the implications of reduced brush area it almost seems the (potentially) higher voltage allowed by this approach might be wasted if motor speed is never high enough to "take advantage" of this effect. That is, if the differential voltage between bars can be ex. 18V instead of 14V, if the motor can't spin fast enough to require this than maybe a smaller bar count motor would work better. ?? I'm talking drag race application only here.

I'm looking at the X91-4001 or L91-4003 motors as possible replacements. I haven't verified details yet but if they are the 8-brush versions they offer 50% greater brush area. I'm hung up on this detail, hope I'm right, lol.


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## major (Apr 4, 2008)

Core size for torque (armature steel diameter and stack length).

Brush size for current (contact area on the comm surface).

Two motor attributes I'd look for in a drag motor


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