RC motors, rated current and power?

Why do RC motors have power ratings quoted? What does a power rating even mean, since 1 kW at 10V draws much more current and produces much more heat than 1 kW at 100V? Is the quoted power just a made up marketing term?

Take a Turnigy SK3 6374 192 Kv for example, specs quoted by HobbyKing:

Turns: 14T
Voltage: 10~12S Lipoly
RPM/V: 192KV
Internal resistance: 0.016 Ohm
Max Loading: 80A
Max Power: 4032W

This one is particularly funny, because 80A * 12cells * 3.7V/cell = 3.5kW, which is less than the quoted max power. Should I really take this to say dont run me above 12s, and dont run more than 80A of current?

Good question. I'm sure it is mostly for marketing hype and not even remotely accurate. More like a peak rating, much like the power ratings on stereo systems.

You are totally correct about the current being the only relevant spec. 80A is 80A regardless of the pack voltage, though it will be hard to get 80A flowing through the motor if the voltage is too low. And if you run it at 80A continuously, it will probably melt in about a minute.

80A * 12cells * 3.7V/cell = 3.5kW
but
80A * 12cells * 4.2V/cell = 4032 W

Those are maximum ratings. When you have your battery fully charged, it's 4.2 for a short time

Don't forget that those are airplane motors they expect strong air flow through them from the propeller.

Yes, that's exactly how you should interpret it. Don't run it above 12S, don't use more than 80A AND provide super good airflow for cooling (impossible for ebike usage) OR use those 80A only for a short time (depending on the cooling and motor mass, something between 20 s to a few minutes) (useless for ebike usage).

If you want to use RC motor, go way bigger (bigger, heavier, more mass, good cooling) than you want to use. Compare it to hub motor (size and weight) to get some real world expectations. These are not a magical motors, that can provide 4 times more power than a hub motor while being 1/4 in size and weight. They have some good use cases (friction drive), but not for kW range

I use these motors in both rc airplanes and one as a friction drive, they can be good, but you must set your expectations lower I have 2 kW rated motor that I use for 350 W continuous, it gets hot, especially in summer, but still safe temperatures.

Beware when using an RC motor for an electric bike. The posted ratings are dubious at best, and for an electric bike, they are WAY off. In their design usage, they accelerate easily in a very short amount of time. On an ebike, they accelerate at a much longer amount of time, resulting in the motor getting VERY hot in a short amount of time.

Of course they can be used for ebikes, but they are not designed or rated for that application. A temperature sensor being used during experimentation is your friend...

This is for a skateboard application, but the motor principles should be the same. I'm very curious why skateboards seem to use much smaller motors than bike guys, I assume its because bike guys go faster in general. Boosted seems to achieve quite reasonable performance with a pair of (what appear to be) 5045 airplane motors, which typically top out at around 32A (unreliable, inflated specs from the same vendors), and even with slightly lower voltage 12s LiFePo cells as well. If that's the case, and given that 1x 80A > 2x 32A, I hope it should be enough?

I also ran the numbers, assuming I gear to 25mph, and ignoring drive train losses, rolling resistance, air resistance etc, 80A should be enough to accelerate me from 5 to 15 mph on a 20% grade in 3.5s. That seems reasonable.

5-15 mph time (s) 3.5
Mass (kg) 97
Wheel Diameter (mm) 76
Gearing 3
Kv (rpm/V) 192
Grade 20%

delta Speed (m/s) 4.5
Acceleration (m/s^2) 1.3
Net Force (N) 123.9
Gravity (N) 186.4
Motor Force (N) 310.3
Wheel Torque (N*m) 11.8
Motor Torque (N*m) 3.9
Kt (N*m/A) 0.0497
Motor Amps (A) 79.0

Max Speed @12s, 0% grade (mph) 25.3



What are the typical top speeds that e-bike guys are gearing to? It seems like a single 6374 should be enough honestly. I guess the bike platform is so much smoother and more stable, y'all can comfortably control MUCH more powerful builds?

I dont think there is a such thing as typical for ebike guys. I think the motor size difference between bikes and skateboards is related to wheel size, packaging, and instantaneous loading.

Wheel size/gearing:
The smaller the motor gets the higher rpm it needs to spin for a given amount of power. It becomes mechanically complex to gear down 6k rpm to 300-500 at a bike wheeel. For the same road speed a tiny skate wheel doesnt require much reduction.

Packaging:
Its hard to get a large dia motor under a board but 80 or 100mm outrunners dont take up much room on a bike.

Instantaneous loading/current:
On a board you are top heavy compared to a bike. It would be hard to launch a 10kw skateboard without it flying out from under you. A rider on a bike which is a heavier overall package to begin with, can lean forward, grip the bars, brace against the pedals, and launch really hard. The peak load on the drive system in that brief moment can be way higher than what would be possible on a board. The drive system on the bike needs to be capable of surviving those peaks so everything must be larger to buffer the temp spikes.

Motor ratings are a bit silly but I think they give you a general idea of the way the manufacturer intended the motor to be used. For instance if a motor is offered in multiple kv with different max battery voltage recommendations they usually come out to similar rpm. Thats the rpm Im going to consider the bare minimum for maximum power output.

Gotcha, makes sense. Thanks guys!

Bump,

For anyone interested in more development regarding using RC motors on bikes.

Looks like a 80 mm or 100 mm or larger RC motor is really the size to use when designing for a bicycle.

Low KV as well.