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Old 08-19-2005, 12:07 PM   #1
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Default Using Mulitiple ESCs and Larger Motors

I am getting back into building my cutsom rig, and I have a few questions. I want to use a large gear motor, but it is not a "hobby" grade motor. I am not worried about weight issues, or driveline strength, or center of gravity and stuff like that. What I am not too sure about is the electronics aspect of it. I will give the specs of the motor sop you know what I am working with:

162 RPM @ 12 VDC
1.5 Amps - No Load Rating

I know a bit about electronics, but I am not too good with conversions and equations. Is it possible to use a "hobby" esc for this motor? Also, if I wanted to use two motors, or maybe 3, could I use a seperate ESC for each and tie them all into one RX? I would be using a seperate battery pack for each motor/ESC combo.

Also, would a standard battery pack be able to run one of these motors? I realise this motor is rated for 12V, but I may be able to run it at a lower voltage, with lower rpms. I am just wondering if it can put out enough "juice" to run the motor.

I know this is a far-fetched plan, but it is something I have been wanting to try for a while now, just for fun. Any input is welcome, thanks.

Last edited by Mad Scientist; 08-19-2005 at 12:13 PM.
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Old 08-19-2005, 02:04 PM   #2
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I fit really pulls 1.5 amps, you should have no problem. But I would try to find the stall amps and make sure they don't exceed the limits of your chosen esc. But you should be fine, some motors we use pull 50 amps.
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Old 08-19-2005, 04:10 PM   #3
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Yeah, I'll try and find a spec sheet for more details, all it says is that the no-load rating is 1.5 amps. I wasn't sure what kinda of draw "typical" rc motors have, so I just wanted some input before I go ahead and buy everything.

What about using multiple ESCs on one RX?
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Old 08-19-2005, 08:04 PM   #4
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Assuming you set them up the same, I wouldn't see a problem with it.
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Old 08-19-2005, 08:46 PM   #5
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Moved to Electronics.
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Old 08-20-2005, 02:26 AM   #6
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with 162 rpm at 12 volts will be really really slow.probably be way better to use a tipical hobby motor with a gear reduction.with such low rpm's can't imagine having much torque
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Old 08-20-2005, 09:52 AM   #7
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I have an 18v motor and gearbox from a Milwaukee cordless drill in my newest rig. I run it with a standard 7.2v pack and a Tamya 302bk $50 esc. Never overheated the esc and tons of torqe and 2 speeds to boot. Top speed in high is a little slower than I walk.
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Old 08-21-2005, 11:33 PM   #8
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Quote:
Originally Posted by SPEED_RACER
with 162 rpm at 12 volts will be really really slow.probably be way better to use a tipical hobby motor with a gear reduction.with such low rpm's can't imagine having much torque
rpm's have nothing to do with torque in electric motors. the torque curve is a flat line, meaning they put out the same amount of torque at 1 rpm as they do at 200 or 5000.

the only thing i would be worried about is how damn slow you want this crawler to be. with typical reduction gears, you are looking at 4-10 rpm at the wheels. that is around 6.5-16 feet per minute.

of course you could maybe gear it higher with a total reduction of 2:1 or so, but then you have to wonder if your motor is strong enough for that kind of gearing, which i just now figured out is what SPEED_RACER was probably talking about.

and if you run a 7.2 volt battery pack, that will equate to less power also.

the only thing i can think of this working is that this is a very large motor that draws a ton of amps under load.

Quote:
Originally Posted by egdinger
I fit really pulls 1.5 amps, you should have no problem. But I would try to find the stall amps and make sure they don't exceed the limits of your chosen esc. But you should be fine, some motors we use pull 50 amps.
50 amps? what are you running that in? i've fried 10 gauge wire with 50 amps in school. (demonstration on proper wire selection) i have vacuum cleaner motors that barely draw that much.
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Old 08-22-2005, 06:07 AM   #9
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Quote:
Originally Posted by Kur
Rpm's have nothing to do with torque in electric motors. the torque curve is a flat line, meaning they put out the same amount of torque at 1 rpm as they do at 200 or 5000.
Wrong. Electric motors make their peak torque at their lowest RPM. They have almost zero torque at no load high RPM.

Quote:
Originally Posted by Kur
50 amps? what are you running that in? i've fried 10 gauge wire with 50 amps in school. (demonstration on proper wire selection) i have vacuum cleaner motors that barely draw that much.
What voltage? I ran Dewalt motors in my E-maxx and was drawing over 100A. Running 10ga wire was not a problem (if you get high quality). Vacuum motors don't need high A because they have high V.
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Old 08-22-2005, 04:47 PM   #10
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Quote:
Originally Posted by MKX10
Wrong. Electric motors make their peak torque at their lowest RPM. They have almost zero torque at no load high RPM.
don't want to sound rude, but you are wrong. an elecrtric motors torque is created by magnetic force which is a constant. applying more voltage only increases speed and therefore horsepower (up to a certain rpm) while the torque stays constant. you are getting your HP and torque values mixed up.

also, everything has almost no torque at no load because a load is required to measure torque.


Quote:
Originally Posted by MKX10
What voltage? I ran Dewalt motors in my E-maxx and was drawing over 100A. Running 10ga wire was not a problem (if you get high quality). Vacuum motors don't need high A because they have high V.
some starter motors in 1:1 cars barely draw 100 amps. again i don't want to sound rude but it is becoming obvious that you do not know how electricity works. a vacuum motor is under a heavy load for it's size and combine that with 115-120 volts running through it and you are creating a huge amount of amperage draw. amperage and voltage are directly related, there is even a formula for it AxR=V.

A = amperage, R = resistance, V = voltage

in an electric motor at room temperature, the resistance is very low, leading you to belive that it would draw high amperage from 14.4 volts. and it will, for a split second. as soon as that high amperage runs through the motor and is acted apon by the magnets, the motor wire heats up creating a higher resistance. so the motor goes from less than .1 ohm to .3-1.5 ohms or more in less than a second. the smaller the motor is, the less heat it will generate but also the less heat it can withstand before it frys. and high amperage is what creates heat.

a vacuum motor while running has a pretty constant resistance of around 2.5 ohms. if we use our formula, AxR=V we come up with Ax2.5=115 so A=46.

if we use the same formula on a typical hobby type motor in an e-maxx and we assume it is a high quality one whose resistance never gets over 1 ohm, Ax1=14.4 that means A=14.4 and in an e-maxx that is 2 motors so a total draw of 28.8 amps. and usually the resistance would be higher so the amp draw would be lower.

now you say that you use cordless drill motors in your e-maxx? well the resistance in those motors would be even higher than in a typical hobby motor so if you are still using 14.4v then you are actually drawing less amperage.

also, another way to figure this out is that if you are using 3000mah batteries, you can only draw a total of 6amps for an hour. that also comes out to 12amps for 30 minutes, 24amps for around 15 minutes (<--my e-maxx average run time) 50 amps for less than 7 minutes (by now electronics are frying and high resistance created by heat causes more draw so less run time, at 50 amps most of your circuits are toast.) if you run 100 amps, your run time would be less than 3 minutes but in actuallity you wouldn't even get that because your esc and batteries would boil out of the truck.

i do know what i am talking about because i went to school at UTI, and aced all of the electronics classes.

it's not voltage that frys electronics, it is amperage. it's just that increasing voltage also increases amperage.
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Old 08-22-2005, 05:58 PM   #11
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Kt = T / I or T = Kt*I

Motors draw their highest current at stall (initial start-up). So using the formula above, when is peak torque? You are correct in saying they have constant torque IF the current is constant. In RC there is hardly ever constant current.


Where did I say that the drill motors ran at 100A for 3 minutes straight? They had peak draw of around 120A. (That was documented by onboard data collection). They usually ran around 20A during race conditions.


You're not the only person that went to school
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Old 08-23-2005, 03:35 AM   #12
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Quote:
Originally Posted by MKX10
Kt = T / I or T = Kt*I

Motors draw their highest current at stall (initial start-up). So using the formula above, when is peak torque? You are correct in saying they have constant torque IF the current is constant. In RC there is hardly ever constant current.


Where did I say that the drill motors ran at 100A for 3 minutes straight? They had peak draw of around 120A. (That was documented by onboard data collection). They usually ran around 20A during race conditions.


You're not the only person that went to school

ok, you didn't say that you were drawing 100 amps for 3 minutes but you also did not say that you were drawing 100 amps peak at start up. what you said was "I ran Dewalt motors in my E-maxx and was drawing over 100A." that sentence lead me to misunderstand you. i still don't believe you were drawing that much though even for start up, unless for some reason during that particular start, your truck was under unusually high load. but this initial "high" draw is what i was talking about in my last post.

you are also correct in pointing out that torque is constant at a constant current, and this is where my inexperience with rock-crawling comes through. i am used to drag-racing and oval track racing where the current is actually constant (full throttle the whole time) i didn't even think of varying current... my bad.

and finally, i made a typo in a previous post that might have lead to some confusion. i said:

Quote:
Originally Posted by Kur
applying more voltage only increases speed and therefore horsepower (up to a certain rpm) while the torque stays constant.
what i meant to say was:

"applying more voltage only increases speed and therefore horsepower (up to a certain rpm) while the torque curve stays constant."

and by that i meant that the rate at which torque rises or falls is constant in relation to the current, creating a line instead of a curve on a graph.

um...
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Old 08-23-2005, 05:54 AM   #13
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We're all still learning. Your previous experience with oval and drag is a bit different than offroad racing or rockcrawling. I can see where you're coming from now.

The ol' E did draw over 100A. Not for extended periods of time, but it did. Multiple trips of a 100A circuit breaker and the onboard data collection proves it. You're right though it did happen at start-up (read full throttle wheelies) or any kind of major direction change (it's tough getting 13lbs worth of stuff moving in another direction other than what's going). It was definitely unusual.

Quote:
and by that i meant that the rate at which torque rises or falls is constant in relation to the current, creating a line instead of a curve on a graph.
That's something I can agree with!
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Old 08-23-2005, 12:04 PM   #14
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how does a vacuum motor draw 46 amps when most household breakers are 15-20 amps?
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Old 08-23-2005, 06:19 PM   #15
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Quote:
Originally Posted by chafey
how does a vacuum motor draw 46 amps when most household breakers are 15-20 amps?
well i was using a particularly powerful motor under high load as an example just to show how difficult it is to reach 100 amps.

i would say that the average variety vacuum motor would run at 18-25 amps. a lot of people do in fact trip their circuit breakers when vacuuming. (my grandmother does this a lot)

i just checked and all of the breakers in my house are 25 and 30 amp, except for the laundry room and stove/oven breaker. my house was built in 1967 so it's not like all the wiring is new or anything either.

i remember before we put in the circuit breakers we had an old fuse box and the 15 amp fuse would bust every time i vacuumed.
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Old 08-23-2005, 07:13 PM   #16
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Quote:
Originally Posted by MKX10
Wrong. Electric motors make their peak torque at their lowest RPM. They have almost zero torque at no load high RPM.

...at zero RPM.

http://www.gizmology.net/motors.htm

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Old 08-24-2005, 03:15 AM   #17
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Quote:
Originally Posted by JasonInAugusta

yes, zero would be the lowest possible rpm wouldn't it?
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Old 08-24-2005, 04:28 AM   #18
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Yup...it's called stall torque.
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