max current draw from 35t motors, 8x8x4 MoA
 

Hi all,

I would greatly appreciate your input regarding the current draw to expect from 35t motors.
I am building a 8x8x4 MoA using bully axles,
http://imageshack.us/a/img651/3600/moashorthalf.jpg
I have two RC4WD 35t motors, one integy 35t and one tekin 35t.


I have a hard time finding current ratings for these guys at 12V (or 3s), only thing so far is from RC4WDs website, citing 4.62 Amps at max efficiency (7.2V, around 10K rpm).

Does anyone happen to know about the (typical) current vs. voltage for these motors?

(a simple calculation would get R=7.2V/4.63A=1.55 Ohms > I=U/R=12V/1.55V/A=7.75Amps)

If I wire them in two packs parallel I'd get 15.5Amps @ 11.1V for each pack of motors > about 170 Watts, or 340 Watts total.

But that certainly is not the stall current, so before frying some MosFets, I'd rather learn about the current to expect.

cheers

braq

Re: max current draw from 35t motors, 8x8x4 MoA
 

I wish I could help but that's way over my head. I just wanted to say, that looks amazing. Do you have a build thread? You MUST post video when it is done. I bet it climbs like a beast.

You could post in Holmes Hobbies tech support thread. He's a electronics genius and all around nice guy. I bet he'd help you out with a spot on answer.

Re: max current draw from 35t motors, 8x8x4 MoA
 

Amp draw is kinda dependant on the load placed on the motor. At 7 volts most 35t will draw less than 10 amps with no load. A truck such as yours because of its weight and complex drivetrain is going to be a serious amount of load on the motor.

Curious, why do you need the amp load of the motor? Are you trying to figure out what ESC or Batteries to buy?

Later EddieO

Re: max current draw from 35t motors, 8x8x4 MoA
 

thanks for the compliments, I have indeed started a buildt thread over on 4x4 scale forums. Don't know if a 2nd thread in this forum's bully section will be considered double-posting.

I am trying to decide which ESC to use and if my batteries can keep up with the power in demand.

At the same time I'd like to save on ESCs, using one ESC for two motors. Doesn't make a lot of sense initially (because the money is already in the drivertrains), but I have 3 ESCs already to choose from (and not 4 at the moment).


As an example, I have buildt my own ESC and used SUD50P for the p-channel FETs (the n-channels ones have higher ratings), which theoretically could deliver 50Amps. But I also bought this cheap ESC (initially with having another purpose for it in mind)
RC Boat EL Water Resistant 110A ESC Electronic Speed Controller
and found it uses IRFZ44N FETs (with a 39 amp max rating the 110 amp claim must be within the milliseconds range :lmao:)

Also have one older ESC (from JR) with 35 cont/50 peak rating.

Ideally would be something like this datasheet (Mabuchi Motor)
Search Results | MABUCHI MOTOR
showing current/torque/rpm for the 35t motors.

cheers

braq

Re: max current draw from 35t motors, 8x8x4 MoA
 

On the mean, a crawler will ask for between 2 and 3 amps with a 35t motor. You can expect 6 to 8 on your larger rig overall. Really depends on terrain and driving style though.


Peak is another issue. A regular handwound 35t will be around 56mOhm with typical 50% copper/ graphite brushes. Machine wound between 65 and 85mOhm depending on web size and brush quality. I'll have a fresh shipment of 35t machine in a day or two and will test it. Add in your wire, ESC, and battery resistance to figure out the stall current. Bursts of between 80 and 120a is a good start on 7.2v, but you would have to bury the throttle and not break parts to get there.


Base recommendation, I would shoot for H bridge Rds(on) of 7mOhm or less for two motors. That boat ESC will get hot and has 1/4 the FETs on the reverse path. Lower R the better of course. More important would be keeping the high side from ringing at lower PWM, but with the inductance of a 35t motor you can get away with switching pretty slow.


If you want to use 3 ESCs, use one on the inner two motors and one on each front and rear. The front and rear see most load when approaching and climbing. The middle mostly just pulls through terrain and rarely gets wedged.

Re: max current draw from 35t motors, 8x8x4 MoA
 

I see the 27t mabuchi 540 calculates out to about 168mOhm, its a stinker for the speed! The last 27t Team Torque I tested was 43mOhm.

Re: max current draw from 35t motors, 8x8x4 MoA
 

great input so far. That resistance info is helpful, looking forward to the "new" readings.

Yes, the ol' 540 is in desperate need of those big cooling openings. It will reach 50% efficiency at best :shock: (and only used it for reference what diagram I was looking for)

If I look at (my) other 8x8 it looks indeed as if the front axle has to deliver the most torque. I will "upgrade" to a verified 50A cont. (using a bank of 4 SUD50P04/IRLR8743PBF for each side of the bridge) ESC for the front axles and use the JR 35A cont. for the rear ones.

will let you know how it worked,

braq

Re: max current draw from 35t motors, 8x8x4 MoA
 

Thats a good choice of high side FETs. If your gate driver can take it and you can source it, this is a good one in the same die http://www.irf.com/product-info/data...rfr7440pbf.pdf


27t TorqueMaster Expert 59mOhm- Rebuildable machine wound on 5mm web
27t TrailMaster Sport 123mOhm- Sealed can cheapie 540


You can extrapolate a good 35t estimate from there, 76mOhm and 160mOhm.