How to determine amp rating ?
 

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? 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.

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?

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?"

:rules: Don't shoot me, I'm just the messenger. Any help, insight or answers that you guys can provide would be greatly appreciated. Thx!

I cant help answer your question but I would like to know where you it says the VXL3s is rated for 200a and 320a peak. Thats more than any 1/8th scale BL esc, maybe even more than the Castle Mamba XL

It comes from the specs that TRX lists in the manual.

"VXL-3s Specifications:
Input voltage: 4.8 - 11.1V (4 - 9 cells NiMH or 2S - 3S LiPo)
Supported motors: Brushed / Brushless / Sensorless brushless
Motor limit: None
Continuous current: 200A
Peak current: 320A
BEC voltage: 6.0V DC
Transistor type: MOSFET
On-resistance: 0.00075 Ω Max (0.006/8 FETs)
PWM frequency: 12,000Hz
Battery connector: Traxxas High-Current Connector
Motor connectors: TRX 3.5mm bullet connectors
Motor/battery wiring: 12-gauge Maxx® Cable
Thermal protection: 2-stage thermal shutdown
Case size (l/w/h): 55mm (2.19")/ 39mm (1.54")/ 33mm (1.3")
Weight: 109g (3.84oz)"

Ok, here I go...

1st lets start with motors. If you have 2 motors, that have the same exterior dimensions and weight (weight is a good way to compare similar motors) they should be able to put out similar wattage. Motors are generally measured in wattage (or horsepower, 1hp = 746 watts).

Keep in mind the internal constuction is very important. You may compare a new castle 4 pole motor with a 2pole feigo or clone (such as the vxl motor) and find them very similar, but the 4 pole motor will be more effecinet and produce more power (watts).

So you do need to know the construction of the motor and the exterior dimensions, and weight to be able to get a feel for what you will get.

Also you need to consider kv. Kv is rpm per volt, so you generally pick a battery to suit your motor (or vice versa) considering your desired speeds, the motors max rpm, and what gearing you can run.

And keep in mind that if you have 2 motors that ideantical (brand, size, weight) and different kv they will be capable of the same total output. For example you have 2 s can size 2 pole motors, one with 2000kv and the other with 4000kv. Say the max rpm is 40k rpm. So you can run a max of 10volts with the 4000kv motor, but you can run 20 volts max with the 2000kv motor. The amp draw will be half as much on the 2000kv motor compared to the 4000kv motor, by virtue of the relationship betweens amps, volts and watts.

This formula - volts x amps = watts is very useful. And by swapping values you can get watts/amps = volts and watts/volts = amps, also very useful

For example, say you have a motor that you would like to use. It can output 1hp (746 watts). Now you have kv options with this motor, lets just say you have a 2000kv and a 4000kv option. And the motor has a 50k rpm max

So with the 4000kv you can run 11.1v - 3s lipo

And with the 2000kv motor you can run 22.2v, 6s lipo

Do the math, for the 4000kv motor, 746/11.1 = 67.2 amps

And the 2000kv motor, 746/22.2 = 33.6amps

So you can see how the amp draw is cut in half by doubling the voltage and slecting a motor with half the kv rating. Most will agree the lower kv setup will draw less amps than the formula shows, as lower amp draw prouces less heat, and heat costs power and lowers effecientcy.

As far as comparing the motors you find, best to do some google searching and try to find some actual user data. Most of the no brand ebay motors are not going to be made very well, and there do not appear to be any "diamonds in the rough" so to speak.

Good luck with your research!

So, if I understand you correctly, to determine a motor's potential amp draw you'd just divide watts by volts? watts/volts = amps? Or am I misunderstanding you, or missing something here? I understand that these are very rough estimations, and there's a lot that goes into what kind of draw a brushless system will place on the batteries during real world use, but is it just that simple?
The guy who asked me about this basically just wants to know how to determine what a motors amp rating is, or the amp range (continuous to peak) that the OEM has determined for a particular motor, where the OEM hasn't included that info, so that he won't purchase a motor which is ill suited to his needs. So it's watts/volts = amps?

Watts / volts = amps
volts x amps = watts

That is correct yeah. I would pay no attention to the listed ratings of the traxxas escs as those figures at total fantasy pretty much; most likely calculated by adding together the rating of each FET used, rather than using the rating of a single FET as the rating for the whoel esc ( A FET being the chip that controls current flow essentially ).

It would probably be easier to just see a link / brand & model# for the motors your buddy is looking at, would be alot easier to give a thumbs up or thumbs down without having to know any of the actual ratings as such as brand, dimensions & kv rating are more useful to know generally speaking to determine what setup it could be used in & with what esc & voltage etc.

First of all, the overall power a system will draw is determined by vehicle weight, geared speed, terrain, drive style, motor efficiency, etc. Then, you use ohm's law to calc current based on the power needs and voltage used.

There is no way to exactly calculate amp draw since there are so many variables, but you can get fairly close. The best way is to use a data logger, but failing that, estimating based on similar setups can get you in the ballpark. The "car/truck setup guide" and similar tools (link in my sig) can give you a starting point.

Also, be careful with amp specs on ESC's and motors. Some may be inflated to make them seem better than they are in reality. And even so, the load on the motor determines the current a motor will pull, and therefore what an ESC has to handle.

:great:I knew I could count on you all. That makes sense now. Thanks for schooling me, and helping my buddy out.

Fairly close is what he was looking for. As I said, I know these are rough estimates, and that there are many variables that, in the real world, influence amp draw, but fairly close should be fine for his purposes.

That was the first place I went to see if I could find an answer to these questions.

I know that's a common practice with some less than scrupulous battery manufacturers, but I was unaware that some companies inflate their ESC specs as well. Thanks for the heads up B. Seems to me that inflating an ESC's specs would just be asking for trouble, but I guess it is what it is. (Gee... Traxxas inflating a claim? I'd have never guessed that.:sarcastic:)

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.

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.

For you, no.:diablo:

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.:whistle:

Like I said, I'm just the messenger.

I asked him further about what he's trying to do and he apparently wants to test the limitations of the VXL-3S before dropping a MMM into his Rusty. Which means that he wants to run a motor that many might consider inappropriate for a Rustler. This is what he e-mailed me: "...thanks. Being able to use this calculation to get in the "ballpark" will at least help to narrow it down a little. I wonder how Traxxas came to the amperage draw of the vxl, since it is used in more than one vehicle, none weighing the same, and all come stock with different gearing? Same with the vxl3s esc, according to Traxxas it's rated for multiple times what the motor draws, so a guy would think that it would never come close to having heat problems using th vxl motor, but I've heard of guys going thermal on the esc using the vxl motor, which should mean that they would have had to come close to the limits of the esc. Lol. What was Traxxas's theory behind having an esc that is overkill for the motor? That's kind of what I want to find out by buying another motor. I would like to see if the vxl3s can really do what it says it can do. I am planning to switch over to a castle mmm powered setup, but I want to explore the "supposed" capabilities of the vxl3s esc before I swap it out."
:neutral:I'm still not entirely sure why he wants to do this, but it's not my place to question what he, or anyone else wants to do with their vehicle(s).

I kind of get the feeling from talking to him that, somewhere down the road, this is going to end up being all about the speed.

All opinions are welcome. Thx for the input. :smile:

If his objective is to test his VXL-3s to destruction, rather than just choosing a motor for his rustler, the questions make more sense.

If that were my objective, I wouldn't try to calculate a motor's amp rating (which only means how much it can take for a certain period of time, not how much it will draw in any given situation - again asking the wrong question), I would just get the biggest (and least efficient!) motor in that would fit, get a data logger, and keep increasing the load on the motor with gearing changes and lots of lead weight. Eventually the ESC will pop, and he will have the info from the data logger.

As said above, this is what you really need to drive home:

Some arbitary amp rating for any given motor is just about useless for the purpose of comparing a motor to another motor, or even providing any info about the motor at all.

That amp rating could be read just before the motor melts, or with it running for an hour at 1/10th load with a 40ton a/c system blowing over it. Without any other info the amp rating is useless....

:lol:Thanks... I'll pass that along.

Good point.

from the sound of it, your friend wants basically a motor that is what many would call "too much".

in that case, the best motor for that situation would be the castle creations sidewinder sct 1410 3800kv motor. it can handle MUCH more than the stock motor and if he wants a better esc along with it, that motor can be bought (at a better price) with the esc. i use that specific combo in a much heavier vehicle than the rustler and it handles it very well.

there are plenty of places to buy this motor/esc combo. this link is just an example of the combo i am talking about.

http://www.castlecreations.com/produ...inder_sct.html