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Higher RPM Motor or More Volts?

Posted: Sun Jul 08, 2018 4:54 pm
by EngineerDad

I have a 12V Kid Trax Xtreme Quad.

I was looking to get some additional speed out of it. It was running 2 x 9000RPM motors on the rear wheels.
Instead of going to 18V or 24V to get the additional speed (knowing 24V may damage some of the electrics), I opted to purchase two new 30,000 rpm motor/gearbox combos's from Ebay.

My question is am I losing torque by going to such a high rpm motor? My kids ride this thing on the grass and the ground is undulating so high torque is a must. Have I increased or decreased the current pull by going to a higher RPM motor?

Would I be better to re-install the 9000rpm motors and up the voltage to 18V? Or should I just go to a 15,000rpm motor instead of the 30k?

Due to the huge increase in RPM rating of the new motors, I also installed one of the these speed controllers ... model.html
however I was disappointed with the ramp control as it took anywhere from 3-5secs before the wheels would start turning? So I removed it and now just have the motors working directly off the foot switch (there where no relays installed originally)

Any advice would be greatly appreciated.

Re: Higher RPM Motor or More Volts?

Posted: Mon Jul 09, 2018 5:46 pm
by Hammer-fm
Without a full spec sheet it's going to be hard to answer your question accurately. However, I suspect this will be problematic.

In almost all cases, a higher no-load RPM motor (given the same nominal voltage) will produce less torque per amp consumed. However, many times the total stall torque will be the same because it also pulls more current. however, this assumes perfect power delivery.

In reality, a higher stall current creates more load on the wiring and batteries, and a typical SLA battery is going to be a limiter. A typical 12Ah 12V battery is going to have an equivalent "CCA" rating of around 80-100A -- that's where it goes all the way down to ~7.2V under load.

The stock Power Wheels motors pull ~60A stall (each), and produce 400mN*m of torque, and 15.5k RPM. One aftermarket motor that I've suggested as a replacement runs 23.5k RPM, pulls 85A stall and produces 440mN*m of torque -- or about 25% less torque per amp. A 775-sized motor can produce 10-15% more torque per amp even at the same no-load RPM.

Is it possible that the 30k motor is paired with a different gear set inside (more reduction)? I'd be a bit surprised to see a 3x increase in speed in an aftermarket gearbox.

Any chance you have specifications on the 9k RPM motors that are in there now?

Re: Higher RPM Motor or More Volts?

Posted: Mon Jul 09, 2018 5:51 pm
by Hammer-fm
Also... most of the ESCs have very low current output capability (30-50A), which means ... much less torque available at startup.

If you can fit a 775-series motor, it may be worth trying to put in a 15k 775-motor, such as this one. I don't know enough about the Kidtrax gearboxes to know how much work that swap is, but it would give you better torque than the 550-motors that are likely in there (or at least a smaller downgrade vs. a standard 15k RPM 550 motor -- in theory this is 60% more RPM).