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KidsWheels
Need new motors? Grind a gearbox? Adding teeth to a pinion?
User avatar
By LeeHene
#146451
Hey guys, I need some input.
My son is going through Banebot 18v 775 motors every few months. I'm using all heavy wiring, Gruber 18aH batteries and a Kelly KDS 200 amp controller. Accel and decel curves are set as slow as possible, and output to the motors is restricted to under 18v. No traction bands or anything on the wheels. Total combined weight of the buggy and my son is about 140lbs (70lbs each).

Is there a better type of motor that will hold up? He's about to outgrow it. =(
User avatar
By toycrusher
#146453
LeeHene wrote:Hey guys, I need some input.
My son is going through Banebot 18v 775 motors every few months. I'm using all heavy wiring, Gruber 18aH batteries and a Kelly KDS 200 amp controller. Accel and decel curves are set as slow as possible, and output to the motors is restricted to under 18v. No traction bands or anything on the wheels. Total combined weight of the buggy and my son is about 140lbs (70lbs each).

Is there a better type of motor that will hold up? He's about to outgrow it. =(


What vehicle is it installed in? What kind of failures are you experiencing? Is it getting so hot that it melts the fan on the end of the can?
User avatar
By Hammer-fm
#146459
I agree that a better description of the failure mode would be helpful. Do the brushes wear out? Does it overheat (wiring insulator melted/fan damaged/etc.).

I haven't used the controller you mentioned, but the configuration program demo shows a number of settings. You mentioned that you'd set the accel/decel ramp to its lowest setting (this is the "throttle Up/Down Rate" in the configuration program?). Is the 18V limit done via the "Motor Top Speed" setting?

Have you set a limit on the Max Armature Current? The soft start would reduce how of ten a high current draw is seen since it will be ramping up the PWM relatively slowly, but in a high-drag situation (uphill, in grass), you could still end up seeing 150+ amps, and that much current dumps a lot of heat into the motor.

Also note that the worst case conditions for heat generation is 'just enough throttle to keep the motor stalled' -- in this scenario there's no cooling. Do you see this happening quite a bit or does your son stomp on it and start it rolling pretty immediately?
User avatar
By LeeHene
#146462
This is the current setup. (Copied from my build thread)

-Banebot 775 18v motors
-Axial 16t 32p pinions
-Kelly 200amp 24-36v drive
-Two Gruber Power 18ah batteries wired in series. (6 AWG wire)
-40amp main OL
-60amp main contactor
-Dual 30 amp DPDT reverse and brake contactors (one set for each motor)
-Two control relays for brake/throttle interrupt
-When braking, throttle input to the drive is switched to 0v before engaging the brake.
-Shifter high/low/rev is fully functional. Top speed for low and rev is calibrated by sending the throttle signal through a resistor chain via the orignal switches in the shifter.

It's been working well for over a month.
Battery life is great (2+ hours WOT off-road) and there is no visible wear on gears!
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User avatar
By LeeHene
#146463
On the previous motor failure, things got so hot that it melted the brush bar. This last failure, the armature got hot enough the discolor it some and it lost continuity, resulting in the opposite gearbox to shred teeth off the second and third gears.
User avatar
By LeeHene
#146464
Its a Power Wheels Dune Racer. The controller is set to limit output voltage to 60%. In the real world it tests at 17.5VDC when batteries are at 26VDC.
Output current is limited to 50% of max, 200/2=100amps, shared between two motors, each with a stall current of 120amps.
None of the wiring is stock. 6AWG main to the drive, then splits to 10AWG for each motor group. All connectors were soldered to the wires at 800° F.

The compartment housing the motors and controller has fans forcing air in and out. The only damage to the motors has been in the area where the brushes contact the commutator.

My son is 6, so he never drivers slow (which is why I restricted drive output so heavily). Our land is really hilly, so I tried to set this thing up for torque, not speed.

More info: My pictures didn't transfer over with the copied text in my last post. Here's my build thread: viewtopic.php?f=9&t=18982
User avatar
By Hammer-fm
#146469
I looked at the build thread and it looks like you've definitely covered the bases well. The primary comment would be that all of these types of motors are "intermittent" rated. If you are running them below their maximum efficiency point very much they will likely wear out fairly quickly even with good ventilation. That said, It looks like (from your other post) that these are only lasting 2-3 months? (Feb->Apr->now), and that does seem pretty short. I haven't taken mine out to inspect them since I've had them in (~4 months, 50A total limit, 22.5V max, no additional cooling), but hadn't seen anything from any other users that reported such short liftetimes. Mine get run almost exclusively on hilly pavement, and most of the time with "slicks", so overall they get pretty easy treatment. (That said, the same treatment at similar speeds torched the stock motors).

One question:

Do you have any datalogging or indication of how often it's pulling at or near the 100A limit while already moving (eg. hillclimb/etc.)? If it's often hitting that limiter then that would suggest that lower gearing may improve the situation. The thermal strain is related to the current being drawn, not the applied voltage, which varies because of back-EMF in the motor. You could reduce the gearing and raise the max voltage to make up for the speed loss. An 8514 motor should have ~140mohms of resistance, so 50A is only ~7V plus whatever back EMF the motor is already making. The motor is most efficient at 20A according the the datasheet, so even if you needed to up the max voltage you'd probably get better life overall.

Unfortunately the only stock lower gearing is an older Hurricane gearbox combined with 14T pinions (a 12% reduction). You could combine this with a correspondingly lower current limit (44A) and higher voltage (20.5V) and get the exact same max torque and max speed that you have now -- but with 20-25% lower thermal loss under high-load condition and some amount of accelerated wear due to higher RPM under lighter loads. Power loss in the windings is I^2*R, so reducing the current by 12% will reduce the thermal loss by about 25%. This would reduce brush temperatures as well as the overall current density, which I'd think this would reduce brush wear substantially if it's often running 40-50A today for any extended period of time (>5 second duration). It's just a really small motor to be sinking 300+ watts into on for any extended period of time (300W is how much it burns in the windings at 50A).

Reducing the current by itself (without changing the gearing) likely won't improve the situation as it will just sit on the limiter that much more often (running slower, for longer duration).

I'm not sure whether that would end up being a good tradeoff or not without having some sort of datalogging of the current being used during regular driving now.


You probably won't have room to do what i did on my son's F150 truck -- which is to install two 775-series motors in each gearbox (posted under "quad damage") to help spread the thermal load.

I haven't see anyone install a CIM motor in the stock gearbox -- that would be an interesting build. That's an example of a motor with substantially more heatsinking (in the form of mass, mostly) -- shows up very clearly in the extended power tests. I'll post it here since they also test the Banebots 18V / 8514 motor. Vex motor comparison
User avatar
By LeeHene
#146479
Thank you for such an in-depth response!
One quick point, I'm running my main power through a 40amp overload that is not tripping. (Of course that doesn't rule out burst current)
Unfortunately, no data logging.

One test I did before starting installation of the drive and related gear was to test how much current the motors pulled going up a major incline on grass with my son. At 13VDC, current of both motors combined held at 33 amps, tops. The first set of motors ran mainly on 12VDC for a little under a year. Oddly, both went out at the same time, a couple months after incorporating the drive.

This last time, my son had just figured out he could use his brake pedal to skid. Not 45 minutes after warning him to quit, a gearbox was toast.
I made him save up (a month or two) for the anticipated repairs (metal gears). To my surprise, I found a dead motor on one wheel and a stripped gearbox (second and third gears) on the other side when I finally opened it up.

Each motor has a set of two 1 Ohm 10watt resistors in parallel to dissipate power from dynamic braking.

Do you think its possible that current from BRAKING could be what is taking out the motors?

...I knew I should've gone brushless!
User avatar
By taz11
#146485
Glorydays wrote:WOW! This post really make me feel stuppid. I shudda staid in scool :(


LOL! :lol: :lol: :lol: :lol:

I ran those motors for years. The failures that I did have were usually related to a full throttle stall condition. Not sure that helps at all.......you've documented it way more than I ever did.
User avatar
By Hammer-fm
#146505
I wouldn't expect the braking to be a big problem here, although it's obviously more stressful than having a real mechanical brake. Two 1-ohm resistors in parallel should never be pulling more than 35A out of each motor -- and that would only happen if it was at full speed (going directly from throttle to brake). Realistically it's going to be quite a bit less than that, and that's going to be a short transient. Relative to the amount of heat/power used for acceleration I think it's probably only a very small contribution.

With 33A @ 12V, It sounds like we'd expect up to 50-60A at the full 18V for both motors, depending on how much the grass/terrain drag coefficient goes up as a function of speed. The motor wiring loss for 30A (one motor's current) is going to be somewhere around 110W in the 8514 motor (130mohm resistance, i^2*R = 900 * 0.13). At 25A this would be ~80W. That's great, except that the problem is that I have no idea whether that's too much with the cooling that you have, or if it should be fine -- I don't have a good way of estimating the rate of heat transfer out of the motor. We can look at the rate of heat rise (it's around 0.8 C/second @ 110W input power, with NO cooling) -- but without knowing how much is being dissipated it's hard to tell what the steady-state temperature is.

BTW the 12V corresponding # (33A total) is much lower for power loss in windings, due to the square function (16.5A => 35W). Given that they lasted for a long time @ 12V, you may indeed be running into just an overtemp case due to the higher speed and related power use. It would be nice if there was a way to have a "short term" current limit of 100A and a long-term limit of something like 30-35A -- the limit would mean slower speeds going up hills (similar to how it was at 12V) but you could keep the higher speed you get from 18V when going on flat terrain. It's just that you can't get the thing moving from a stop with only 35A if you're in any sort of high-drag terrain.

Hopefully there's someone else on the board that has more engineering background in motors. It would be good to have some commentary from someone that does more instrumented testing.
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