I found this bike push-trailer.

enter image description here

Because the trailer project is open-source I discovered that the used hub motor is MUXUS XF40 (specs here)

In many places on the internet a learned that if such motors doesn't runs at their optimal RPM (usually high) they could get overheated.

Because the trailer could work in 2 modes:

  • 6 km/h engine support in Hand-Car-Mode
  • 25 km/h at 250W in Pedelec-Mode

I wondering how is possible to manage the "hand-car-mode" where the hub-motor runs very low RPM (max. 6km/h) without overheating or other problems. Or the "low rpm overheating" isn't real issue and it is possible easily a safely regulate the BLDC motor to run in any LOW rpm?


adding to Chris H 2018-04-30 post... I'm going to take a stab at some basic math...

BLDC motors can put out their rated power at high RPM partly due to air cooling enhanced by the high RPM (like computer chips, if you add active cooling, they can go even faster... it's all about how much heat they can dissipate).

At lower RPM the motor gets less air cooling as a by-product of the RPM, so it dissipates less heat, even though it might be delivering the same amount of power (pulling more weight).

However, if you're pulling the same weight at 6 km/h as at 25 km/h, you're going to be drawing a lot less power out of the motor for mechanical work, so it'll be generating less heat and probably not need to dissipate heat from RPM-based air-cooling, to keep the motor windings from melting their insulation or the wires themselves from fusing.

I'm new to BLDC controllers, but if there's one that allows you to set an RPM-biased torque limit, or RPM-based power limit on the throttle, you could protect your motor from thermal overload. You could chart a few operating ranges or give it a function with a curve to limit torque or power according to RPM. At 100% rated RPM, it could use 100% power. At 75% RPM, 75% power. At 50% RPM, maybe 40% power, and at 10% RPM, maybe 5% power.

A simple thermal cut-out might be a bummer if it happened while toting a heavy load up a hill in hand-cart mode. If the controller just limited motor power, you might be able to keep going.

If you've got a motor rated 1000 W at 1500 RPM, it might be rated 250 W at 100 RPM. At the higher speed, it's doing 1000/1500/60 Watt-hours of work per revolution, or 1/100 of a Watt per revolution. With 250 Watts at 100 RPM, it would be doing 250/100/60, or 0.042 Wh of work per revolution, so it could be moving a load 4x as heavy, and 1/15th the speed. (1/4 power applied to 4x the weight should get 1/16th the speed, I guess?, but I had rounding in my calculations above).

  • Torque is actually the greatest at low RPM's and decreases with speed on a BLDC motor. Power required to operate a motor is directly proportional to the torque not speed. Heat dissipation as Robert pointed out is a problem at low RPM's because there is less cooling. I'm thinking there is a built- in thermal switch that signals the controller to shut down or limit current.
    – P. Barney
    Dec 15 '18 at 0:59

When you're using a motor like that at low speed, it's often at low power, which limits the heat generation. Uphill is obviously an exception, which is why e-bike instructions usually tell you that you must pedal uphill even if there's a throttle control. For short low-speed, high-power bursts, such as standing starts, the heat capacity of the motor is enough to prevent overheating.

In this sort of application the motor is quite exposed, allowing more cooling than deep inside a machine that may have other heat sources.

In many systems a thermal cutout is used to prevent the windings overheating. This might be a good idea here, because an electric hand cart would be quite useful on steep slopes.

  • Also the low expected sustained speed is 1/4 of the maximum speed, which isn't all that slow. The maximum speed could even be a little higher than the optimum for efficiency (for some motor types anyway)
    – Chris H
    Apr 30 '18 at 20:51

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