# DC Motor geometry and design



## Georgia Tech (Dec 5, 2008)

Its intresting to note that if we had one machine that had a long stack and another machine with a shorter stack but with a larger radius and their magnetics had the same effective net suface area. Question, is it true that the shorter motor will have more torque per amp, less rpm per volt than the longer motor with more rpm per volt. With both having the same number of com bars?


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## major (Apr 4, 2008)

Georgia Tech said:


> Why is one topology of a "long" stack more or less advantageous over a short lam stack with a larger diamete


Hi GT,

This all depends on the design constraints. What you're talking about is often called the aspect ratio. L/D. Or is the D/L? True, you can vary this and end up with the same air gap area. Often D^2*L is used for to compare air gap areas, even though it is technically a volume measurement. Theoretically, equal DsquaredL figures represent equal torque capabilities for the motor regardless of the L/D. Within reason.

Generally, smaller diameter is used for higher speed. Larger diameter for slower machines. The larger L/D has the advantage of large portion of the conductor being active (under the air gap) opposed to end turns. This works well for low pole counts, 2, 4, or 6. For slower machines, usually having high pole counts, 8 or more, the low L/D becomes more attractive. End turns are shorter compared to active.



> It seems to me that moment of inertia would play a bigger role in the motors that have a large Armature circle as opposed to a longer armature.


Sure. Especially for fast servos. Not so much an issue with EV traction motors.



> One example that also comes to mind is the Prestolite MTC-4001 vs the ADC 8 inch motor. From my reading the MTC-4001 had a superior design in its lamation stack up.


I know the MTC motor pretty well. Not much about the ADC8. But have looked at some comparison. Pretty close in HP rating and DsquaredL. MTC being a little less D and a little more L. Kind of what one would expect. The 33 slot MTC lamination was actually designed years and years ago, 1960's maybe, for low voltage (36) pump motors for forklifts. Those old timers did a great job with slot geometry. Result was an excellent armature WRT efficiency. Can't say too much about the guts of the ADC8, never saw the inside of one. I think the ADC8 has a 37 slot armature, so there will be some difference in Torq/amp, along with unknown difference in field turns.

Regards,

major


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## major (Apr 4, 2008)

Georgia Tech said:


> Its intresting to note that if we had one machine that had a long stack and another machine with a shorter stack but with a larger radius and their magnetics had the same effective net suface area. Question, is it true that the shorter motor will have more torque per amp, less rpm per volt than the longer motor with more rpm per volt. With both having the same number of com bars?


No. (I guess I need 10 characters)


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## Georgia Tech (Dec 5, 2008)

major said:


> Hi GT,
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Now sometimes you make reference D/L then sometimes you say L/D. Also when you refer to the 8 pole or greater machines do you mean that the end turns are greater in length compared to the active lengths?


Also what are your experiences with Prestolite motors at one time I found them more fascinating than the other ones, now I am starting to look more into the GE 9inch motor. What are some of you positive and negative experiences with this motor? I have never used any of these motors in a practical application.

thanks for your response


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## major (Apr 4, 2008)

Georgia Tech said:


> Now sometimes you make reference D/L then sometimes you say L/D.


My bad. Just testing you I corrected it.



> Also when you refer to the 8 pole or greater machines do you mean that the end turns are greater in length compared to the active lengths?


No, the more poles the shorter the coil span, so the less the end turn.

I worked on Prestolite motors in a former life. And, at least back then, looked at quite a few GE DC motors. The GE's larger than the golf cart size seemed well designed and built.

Regards,

major


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## Georgia Tech (Dec 5, 2008)

major said:


> My bad. Just testing you I corrected it.
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> 
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> ...


Ahhhhh I do follow you on the coil span. So in your opinion which do you think was a better built motor, the GE motor or the Prestolite motor??

There is an absolute AWSOME looking GE motor on Ebay for $1200.00, been completely rebuilt and refurbished, it has a longer stack lam than the normal GE 5BT1346B50 motors, it look magnifacent. I wish like CARZY I could buy this thing but it is WAY WAY too expensive for me....


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## major (Apr 4, 2008)

Georgia Tech said:


> So in your opinion which do you think was a better built motor, the GE motor or the Prestolite motor??


Hey GT,

You lookin' to get me in trouble?  Both good motors. I'm sure you can find faults with either. You know....was that motor built on a Monday morning type of thing. These things aren't done by robots, people pound the things together. But for the most part, no outstanding differences except black vs gray paint.

Regards,

major


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