Armature turns

[WAM]

Okay, who can school me on why fewer turns on an armature = higher rpm? I know that's true, but intuitively I would think the more turns the faster the motor.

Related question: Does a 20T armature give twice the kv as a 40T? If not, why not?

My last question is trickier -- I'll hold off. Don't want to distract you.

[Colgout]

More turn the arm, the thinner and longer the wire hence, higher resistance and lower amp drawn. Therefore lower EMF procedure and slower.

Dont think so (20T vs 40T), because beside the armature static resistance, there is a thing related to the comm and brushes connection, I think its called "contact resistance", the slower it runs, the lower the contact resistance is. I think this is related to brush bounce and comm condition...etc. That's why electric motor produces highest torque at start, because its at it lowest resistance form (closed to stationary and minimum contact resistance only static resistance), therefore highest amp drawn and highest EMF produced.

There maybe other factors affecting the Kv of the motor. I hope I remember them correctly, its been almost 18 years....

[EddieO]

Think of the wire like a hose. If I have ten foot hose thats the size of softball and you have a 20 foot garden hose, which one gets the water there faster in a bigger amount. Arms work basically the same way....this is a real simple way of explaining it.

There is no formula for KV or easy math to say a 20t will produce X and a 40t will produce half of X.

Most parts of a motor can effect KV....arm, magnets, wire fill, brushes, springs, you name it.

Here is a fun set

Motor A 1590 KV
Motor B 1290 KV

Everything the same.....except the magnets. Was even the same endbell and armature literally.

Or...

Motor E 1940 KV
Motor F 1790 KV
Motor G 1660 KV
Motor H 1570 KV


Difference? the can thickness. Everything else was the same, even used the same arm and endbell from the above two.

KV is easy to alter but does not remotely tell everything about a motor.

Later EddieO

[EeePee]

Holmes Hobbies had a decent reference of numbers for motor turns and their RPM on his website a while back. No way can it be used as de facto anymore with all the differences in motors these days, but it's still nice to see.



10t - 6526
15t - 4317
17t - 3753
20t - 3216
21t - 3082
23t - 2790
25t - 2588
35t - 1565
45t - 1210
55t - 987
65t - 837

[WAM]

Okay, I get the wire length/diameter thing. But there's only so much room for wire, and you guys fill it. So it should be about the same amount of copper when all's said and done. Aren't there variations in wiring where instead of one long wire, you have multiple short wires in parallel? Or do you count the turns differently if some are in parallel?

And here's the last question. POWER in a rotating motor is the ability to make TORQUE at RPM. You can get that straight out of a physics text. So a high turn motor makes more torque and can pull a larger pinion. A low turn, high RPM motor has less torque and cannot pull as big a pinion. So doesn't this make a zero sum game for wheelspeed? Whether you get your speed from motor rpm or from higher gearing, does it perhaps come out the same in the end?

[Charlie-III]

Originally Posted by EeePee

Holmes Hobbies had a decent reference of numbers for motor turns and their RPM on his website a while back. No way can it be used as de facto anymore with all the differences in motors these days, but it's still nice to see.



10t - 6526
15t - 4317
17t - 3753
20t - 3216
21t - 3082
23t - 2790
25t - 2588
35t - 1565
45t - 1210
55t - 987
65t - 837

Nice list EeePee.

Thanks JRH for the original info.

But, like any "dyno numbers", "you can make numbers say whatever you want". Basically, dynos are great for back to back testing on a given day.
Change the dyno (even if same make/model) or the day, things change.

The above list is a nice guideline, it's not "carved in stone" based on many uncontrolled factors.
This is NOT a slight on anyone, it's a truth.

But, it's a decent guideline.

[EddieO]

You need to ignore the fill and think about the path. A four lane 5 mile long highway may use same amount of asphalt as 10 mile Lon country road, but which one regardless of speed limit am I gonna reach the first.

Higher turns use much smaller wire... U can wind a 20t all day with 19 awg but 40t arms use 21.5 at best... Look at the difference on awg chart at the size difference.

Power is torque x rpm...

Typically when you go down in turns you gain more rpm than torque you give up, so you see a power gain overall. Don't be fooled though, lower turn motors mak gobs of torque it's just in a different spot in the powerband.

Originally Posted by WAM

Okay, I get the wire length/diameter thing. But there's only so much room for wire, and you guys fill it. So it should be about the same amount of copper when all's said and done. Aren't there variations in wiring where instead of one long wire, you have multiple short wires in parallel? Or do you count the turns differently if some are in parallel?

And here's the last question. POWER in a rotating motor is the ability to make TORQUE at RPM. You can get that straight out of a physics text. So a high turn motor makes more torque and can pull a larger pinion. A low turn, high RPM motor has less torque and cannot pull as big a pinion. So doesn't this make a zero sum game for wheelspeed? Whether you get your speed from motor rpm or from higher gearing, does it perhaps come out the same in the end?

Don't compare KV ratings to dyno stuff. While there can be variables to a degree it's a pretty simple test, a much different animal than a dyno pull. Using like equipment in a KV test would show little variance.

Originally Posted by Charlie-III

Nice list EeePee.

Thanks JRH for the original info.

But, like any "dyno numbers", "you can make numbers say whatever you want". Basically, dynos are great for back to back testing on a given day.
Change the dyno (even if same make/model) or the day, things change.

The above list is a nice guideline, it's not "carved in stone" based on many uncontrolled factors.
This is NOT a slight on anyone, it's a truth.

But, it's a decent guideline.

[Charlie-III]

Originally Posted by EddieO

Don't compare KV ratings to dyno stuff. While there can be variables to a degree it's a pretty simple test, a much different animal than a dyno pull. Using like equipment in a KV test would show little variance.

I have looked at (and tried to make a chart of) KV to brushed turns chart.

You can "get close", but it seems it's in the realm of +/-25%, thus not really useful.

Maybe I could have used better terms, but have not found a good "relevant" chart for comparison.
Part is likely differences in test methods....thus my dyno reference.

[EddieO]

It's not really the test methods or tools.... It's the variables in the arm construction if all other variables are the same....wire size, pattern, comm size, armature size, etc all play a roll so no chart will ever work.

I am working on pretty nutty complex charts for the website....trying to take all variables into account.

Later EddieO

[JohnRobHolmes]

As a general rule you can trust that doubling the wind would half the speed. The basic rules of motor constants kt and kv rely on the relationship. Brushed motors don't hold perfectly true because of brush losses as the amperage climbs, a formula to map the relationship would just have more factors. Look up maxwells equations and lorentz force law if you want nitty gritty on how a coil makes force.

And of course you can always change the motor in a bazillion other ways, but thats a different subject all together. Then there is variance within a batch. But for a given motor build the kv at specified amperages and voltages can be predicted accurately with a large enough sample size like we used on our chart.

[WAM]

Okay, when I started this thread I was already aware of the 20 ft garden hose vs 10 ft fire hose analogy. And sure enough, that's what you guys came back with.

But after thinking about how that makes sense, that shorter fatter wires will offer less resistance, thus higher amperage through the armature and a higher rush of current speeds the motor up....how does that explain why the garden hose motor makes more torque? Less current = more torque? If we cut current all the way back to near zero, will we have massive torque?

Now the garden hose thing isn't working for me.

[JohnRobHolmes]

Motors make torque with amps, the volts and back EMF dictate the amperage when combined with the ohmic resistance. Read this for some knowledge on how these all come together for predictive modeling. http://web.mit.edu/drela/Public/web/...or1_theory.pdf

A motor that is twice the KV also needs twice the amperage for a torque output as defined by the kv kt (kq in the MIT writeup) relationship.

A twice as fast motor also has twice as thick wire and half the coil resistance, all else equal.

Half the coil resistance allows the motor to pull twice the current so it can make the same torque as the slower model.


Power comes from amperage (torque) and voltage (rpm) combined. If you have two systems with the same motor speed, they are basically the same system with different winds and voltages. In other words, don't overthink things too much on this.



In practice we find limitations to the driving electronics, and with brushed motors the commutator interface. Most ESCs are designed to run 3s or 4s. Using this voltage we run into copper 540 brush saturation around the 2100kv mark, or about 27 turns. Any faster and we start to lose stall torque as the brushes become a proportionally larger factor in the overall motor terminal resistance. Much slower than 30 or 35 turns and the coil resistance starts to become high enough that we can't make as much stall torque without increasing voltage.