How to determine amp rating ?

[ta_man]

Originally Posted by Tat2

OK; I've been asked a question by a friend of mine that I don't know the answer to, so I'm coming to you guys, who know more about these things than I do, with the hope that someone here can answer these questions.
Here goes...
My buddy is looking for a brushless motor for his TRX Rustler, and he's apparently been looking at some motors that don't list an amp rating (continuous or peak) in the motor specs. What he/we want to know is if you know some of the motor's basic physical specs (length and diameter of the can, Kv rating, # of poles, armature size, motor wire gauge) is there a way to determine, or a formula to calculate the amp rating of the motor?

To specifically answer that question I would say: Yes
But I would qualify that "Yes" by saying that a person for whom the "Yes" would be applicable would be someone who knows as much about electrical engineering and motor power systems as Patrick DelCastillo. In other words, the "Yes" answer applies to the President of Castle Creations. He could probably figure out the amp rating from physical parameters.

When I see people asking those kinds of questions, especially people who do not know that the 200A rating of a VLX ESC is a bogus number, I think they are trying get an answer to a question that cannot be answered using the information they seek. They are asking those questions because they do not know the right question[s] to ask. (Not dissing your friend: most all of us were in that boat at some time or other. I certainly was.)

Your friend wants a motor for his rustler. He doesn't need to calculate amp rating from motor parameters to pick out a motor. The reason I say your friend doesn't need that information (doesn't need - he still might want it, but that's not the same thing) is because he's not going to run any motor appropriate for a Rustler at max continuous rating.

I've been doing RC for 10 years and I have never once based a motor choice on amp rating. Maybe if I was trying for RC speed records I might need to know that (but not for a Rustler). But for anything a Rustler is useful for, you don't need that infiormation, [Opinion alert!:]nor will it help you with a decision.

I'm not dissing Rustlers either. I raced one regularly for 7 years and still have more than one. But I know their limitations, and most brushless motors (other than the Novak Spec motors) designed to fit them exceed those limitations. That is why I say your friend doesn't need to know the amp rating to choose a motor for his Rustler.

Originally Posted by Tat2

Sorry if that's the wrong question to ask, or if it's worded poorly, but I wasn't given much to go on either. While I know that an EagleTree or similar system would be the sure fire way to determine the answer to these questions, what I/we want to know is if you can determine a motor's amp rating, before actually buying the motor, just based on it's physical specs. Or is that impossible to do with such little info to go on? How does an OEM determine the amp rating range of a given motor of theirs? Is there a formula they use, or we can use, to calculate these things?

Here are the questions that were asked of me, in their original form:
"I have a few brushless systems; vxl, tekin and castle, but for the purposes of our discussion here I'll use the VXL as an example.

The vxl3s esc is rated for 200A continuous and 320A peak and the motor is rated for 65A cont, and 100A peak.

My question is; Is there a way to tell how many amps a motor is rated for based on the Kv or turns, among other physical charteristics like armature size and # of poles, for example? I see a lot of motors on eBay, and many don't list the amperage specs.

If Patrick knew every design parameter of the motor, he might be able to calculate an amp rating. But the amp rating doesn't mean much in the real world because so few of us run motors at the max continuous amp rating.

Originally Posted by Tat2

So, without knowing what the OEM rates a given motor for in terms of it's amp draw, is there a way to tell, or calculate that rating range? ...Burst or Peak?

For you, no.

Originally Posted by Tat2

Also if two motors were both 3500Kv, but one was rated for 80A continuous and the other was rated at 70A continuous, would that just mean that the 70 is just a more efficient motor and would deliver the same performance of the 80?"

The 70A rated motor might be smaller and thus deliver less performance than the 80A rated motor. The 70A motor may have thinner wire, thus more resistance and thus the lower amp rating. Or the 80A motor may be smaller but more efficient and be able to sustain a higher current because of less energy lost to heat.

KV is not an indicator of perfromance, it is how fast the motor turns on a given voltage. It says nothing about whether the motor would have enough torque to move your car at a reasonable speed. You have to look at parameters besides KV (like size and design) to determine that. And motors of the same external size may have internal differences that make comparison based on KV invalid (for example, the 4600KV CM36 vs the 4600KV 1406).

Other people may have a different take on the opinions expressed above, so don't consider them absolutes.