the "comm" and the "stack"

[rice98w]

hi

why is it, that when i look at my motors, some comms are actually smaller/larger than others? i dont mean the diameter, i mean the actual length.

can this be used to your advantage, are their certain characteristics each measurement of comm has? and if so would anyone care to explain them?


and..

the "stack", this is the actual thing the wire is wound around right? well how can this measurement be used to your advantage? i have heard the term "oh look, its got a 5mm stack" which part is it thats 5mm and how does it make a difference in performance?

thank you for any replies
matt

[nitro-head]

i thought the things that the wire is wrapped around were called 'laminates'

am i wrong?

[marcoski]

They are laminations, laid one on top of another in a stack, hence the name. The origonal question is quite technical, definately one for Mike Haswell's attention!

[MattW]

Comm length, the only thing this does for you is gives you more scope to alter the positioning of the arm within the magnetic field. I actually have a couple of arms that could do with being "lower" in the can - i.e less shimming on the wire end, but it isn't possible as the brushes wouldn't be in full contact with the comm. I think some arm manufactures just specify different lengths, don't think there is much else in it.

Stack thickness, this is the Web thickness. The web is the part of the laminations that the wire is wound round. I believe that the most common sizes are 4 and 5mm. A 4mm web thickness would give more RPM and hence is "generally" prefered for 12 turn touring car motors. 5mm should give more torque, less RPM but more efficiency.

Personally i find that anything in the region of 10 turn and lower is better with a 5mm stack. ALthough as is usually the case with these things, there is more that one way to get the result.

I believe that Trinity/Epic also do a Tapered stack. I think it was used on the RPM versions of the D4, but i'm not totally sure about that.

[PDW]

IIRC, the comm length is variable by small amounts, but it is made to look huge by the problem Matt descibes - its positioning relative to the arm.

The 'stack' is the assembly of laminations around which the wire is wound. There are a number of variations. The length of the stack dictate how much copper you can get in the magnetic field - longer stack, more copper. The thickness of each lamination affects the performance of the electromagnet created when the copper coils are energised.

The web design affects all sorts of things, and three designs were approved by IFMAR over the years. Straight, the web is parallel from the shaft. Tapered, the web tapers thinner from the shaft and - tapered, the web tapers thicker from the shaft. You cannot see which web design you have when the armature has its windings in place.

IME, for every motor builder/tuner/expert, you will get a different answer to the question "How does each of these characteristics affect my motor?" - let's hope Mike tries to help! In theory, if you have 19 turns in a double and 16 turns in a triple, for a given wire size you have the same amount of copper in the magnetic field, and should therefore get the same performance from the motor. It just don't work that way! :) :)