Equivalent KV / turns in motors

[cyrusd]

Hey folks,

This is totally a newb question I assume, but I've searched the archives and haven't found "the" answer yet. I have learned a lot, just not what I'm looking for

I am trying to make sense of motors. Right now I have a 20t Holmes Hobby CrawlMaster in my AX10. I love it, great low-speed control, just as promised. I'm geared about 5.5:1. What I've noticed is, I have a lot more wheel speed available than I probably will ever need in this rig (spends all its time on the rocks). If I am sacrificing wheel torque for speed, I'd rather change it up!

I know some of this can be achieved with gearing. But I'm interested in understanding more about the role of the motor.

Looking at the HH site, I see that the 20t CrawlMaster runs at about 1500Kv. The equivalent (20t) TorqueMaster runs at about 3216Kv. So there's quite a difference there! I'm wondering: 1, what does that difference imply about each motor, and 2: Lower Kv generally means higher torque, if I'm not mistaken - but what does this mean really? I have never seen torque ratings in any RC motor description, except the "lower Kv, higher torque" maxim. I know the formula for converting HP @ RPM to torque, and I assume there's a simple watts to HP conversion but rarely do I see motors listed with wattage ratings, and unless my understanding is wrong, torque in electric motors is constant, more or less - so does RPM really have anything to do with it at all?

Yep, I'm confused. Feel free to throw pie or knowledge at me.

[cyrusd]

And then there's this: Mythbusters. Lower KV= more torque? - RC Groups

I'm never gonna understand this stuff!

[EddieO]

Crawlmaster=5 segment Armature
Torquemaster=3 segment armature

You comparing an apple to an orange.

A 5 segment is wound completely different than a 3 segment. Typically a 5 segment is about half the winds (there is no perfect formula) of a 3 segment.

A 20t Crawlmaster should be on par with a 40t or so....

Also keep in mind, KV is only part of the equation in a motor........power is made up by RPMxTorque......KV only tells you RPM, which makes comparing motors not as easy as it looks. KV is really a piss poor rating for rc car/truck use.

Later EddieO

[Olle P]

First:
kV-rating is pretty much only applicable to brushless motors. Brushed motors have more of a "kV at X volts" rating.

Second:
When running crawlers on brushed motors there are two properties to look for; (stall) torque and power.
- The torque is what gives the strength to negotiate tough obstacles at low speed.
- The power is what gives the energy to go fast.

As Eddie said, your motor is five pole whereas the majority of motors are three pole. More poles means more torque at the given number of winds, but I can't say about power.
For a given number of poles the rule of thumb is that with fewer winds you get more power but less torque and power efficiency. A low wind motor thus need a smaller pinion to get sufficient wheel torque, and still it will heat up faster than a high wind motor.

Brushless motors are much more complicated to compare since they differ more in design.

[cyrusd]

Eddie and Olle: Thank you! You're definitely helping to fill in the missing pieces. The explanation of number of armature slots helps. However, the piece I still seem to be missing is how to determine actual torque values for one motor, and compare it to another - without some kind of indication of wattage, or amp consumption how do you really know if motor A is more torquey than motor B? It stands to reason that number of winds (taking into account number of slots) gives a reasonable expectation, but without knowing the actual values how does one reasonably compare (for instance) a 20t 5 slot motor from one vendor to an identically configured motor from another? I feel like I'm still missing something.

[EddieO]

You will have a tough time figuring that out. I am working on a dyno project for my site, but its not designed to compare vendor A vs vendor B, just Armature A vs Armature B....

There are a few ways you can get an idea on torque production without hard numbers.

Each modified assumes other variables are equal.

Here are the main ones

Wire Size
Armature Blank
Magnet Type

Small modifiers are

Brush Type and Style
Spring Rating
Comm Size

So, say all other things equal.....motor A is wound with 21.5 AWG, motor B is wound with 21 AWG. Motor B, while not only producing more power, will make more torque. You can go down the list of them, and as long as all other variables are the same, its easy to tell which will produce more torque.......how much, much tougher without a dyno.

That being said......if all is the same on a motor, at that point you won't see a massive difference in power, rpm or torque....construction quality and such will be the main factor there, which can effect power ratings also.

An easy example of this is compare a machine wound arm to a handwound......all else being equal (wire size, arm, can, etc)....the handwound will produce more, simply because its built better. (better windings, epoxy, comm connections, balance, etc).

Hope that makes sense...

Later EddieO

[cyrusd]

Yep, makes sense - realistically there are only a small number of motor vendors whose products I care about enough, for crawler stuff it's pretty much you and Holmes... And both of you have the reputation and skill to produce quality products. So I don't wonder about construction quality so much as having the right data available to make the right choices. I read the (pretty epic) thread Olle started on a similar topic, and understand where everyone is coming from - and I don't expect the whole industry to change over night (or necessarily at all). That said, you and John are both doing great work to provide interested consumers with better data to make their choices, which is great!

[EddieO]

You might even see me at a comp, as I just live up the rode from ya After a long hockey season with my son, I finally get to go comp this weekend!

Later EddieO

[cyrusd]

Oh hey, no kidding - so you do!

Where do you guys skate? I used to go up to Shoreline every now and then. Haven't been in years though I still have my skates and pads hanging around somewhere

What's the comp scene like around here anyway? I haven't been to any yet, still getting my toes in the water.

[EddieO]

My son plays in the everett program...

warcrc.org is the site you wanna check for the all the comps.......there is a second group that runs, but they are more down south people from my understanding, though there are people that run in both. I can't remember the website for them.

Later EddieO