EV hub motor torque for pulling a roller

[bal_jop]

TL;DR

ev hub motor torque, for pulling roller

I am working on a 500 kg roller, which I want to pull using electric motor. I am considering hub motor, since it is easier to frame a basic ev with it. Now I need to know the capacity of the hub motor, I need to choose.

Roller weight = 500 kg
Frame weight + driver weight = 150 kg
speed required = 10 km/hr
Surface on which the roller is to run is predominantly flat.

Being new to such design projects, I may have missed details. Please ask me for them. Help me with the calculation.

 

[anorlunda]

You did not mention two key factors. How fast must it accelerate? How much friction is in the system?

 

[bal_jop]

@anorlunda, it need not go beyond 10km/hr. Could you explain the friction part?

 

[anorlunda]

@anorlunda, it need not go beyond 10km/hr. Could you explain the friction part?

Sure. If there was no friction a spinning roller would spin forever, and it would take zero energy to keep it rolling, and the answer to your original question would be "zero capacity." But that is not reality. Friction may be small but it is always nonzero.

Axles have friction. We also have rolling friction that depends on the material of the roller and of the surface and on the weight of the load carried. Rolling friction helps bring your bicycle to a halt when you stop pedaling.

Most often, friction is difficult or impossible to calculate, we must determine it by experiment. But you may be able to find numbers derived from other people's experiments; perhaps a rubber tire on asphalt, or steel on steel. Google "coefficient of rolling friction" to search for sources of such numbers. But beware; none of those numbers may match your situation.

 

[bal_jop]

@anorlunda, Thank you for taking your time to explain.

The closest coefficient of friction, I am able to find is;

0.04 - 0.08

car tire on solid sand, gravel loose worn, soil medium hard

I took this from;
https://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html
Fr = c * mass * acc. due to gravity
Fr=0.08 * 650 * 9.81
= 510.12 N

But we are already talking in terms of weight, so are we already done with our m.g calculation, so is my 650 kg already W, so Fr = 0.08 * 650 ?
If so, Fr = 52 N

The surface I am going to roll the roller on is a play ground with grass grown on a field level hardened soil.

 

[anorlunda]

That's a good start. Now we know more than before. Let's calculate the power needed, for steady speed.

52 N * 10 km/h = 520 n km/h or about 144 watts. Given some margin, a 200 watt motor should do the job.

What do you mean by a hub motor?

Starting from a complete stop might be difficult. Not all motors are strong a zero speed. So tell us more about the type of motor you are considering.

By the way, is this a school project? If yes, please show us the complete statement of the project requirements.

 

[bal_jop]

Thank you so much for staying on.
Hub motors are electric BLDC motors, with the stator providing the shaft to mount the payload, and the rotor being used as the wheel.
Please check
https://gogoa1.com/t/hub-motors
Now, What would be the initial torque and power required to move the roller, from 0 speed.

Like above, give me the pointer, I'll try to do the math.

This is my personal project. I want to avoid the manual labor involved in pulling the roller to level my field.

 

[anorlunda]

OK, those motors in your link to look to be a good match (except that the prices are sky high. You should also check alibaba.)

I think we need help from others to calculate the breakaway starting torque. Then you can compare with the published specs for the BLDC motor.

By the way, it sounds like a fun project. If good answers don't come, I'll move it to the DIY forum where different eyes can look at the question.

 

[JBA]

If the motor torque is sufficient to run the roller it would appear, assuming your plan to walk/guide the roller while in operation and the above resistance force is accurate, then a slight operator applied push might be all is that is required to overcome any added inertial resistance to start the roller in motion.

 

[bal_jop]

@anorlunda, Apologies for my late response. In many of the 250 watt motors, I see the torque is in the range of 30-40 N. Please go ahead and post it to DIY forum, you could point me there so that I can continue following up.
@JBA, I would prefer the user to stop and start the roller at their convenience. A manual push every time would not be ideal, since, the user has to unmount, and apply force from awkward angle.

 

[anorlunda]

The question may be unaswerable without more information.

A hard roller on a hard surface will have very little breakaway force other than inertia. But if the surface or roller are soft, the roller sinks in and very large forces are needed to break out. Think of a car stuck in mud or snow.

If you don't have more detailed info, I suggest taking a chance and ignore the breakaway problem. That's what @JBA was saying.

 

[bal_jop]

At last...
Got it working. It is not as heavy a weight, it is only a 300 kg total gross weight, but a working model.


Thank you so much @anorlunda and @JBA for giving me some of the design insight.

 

[JBA]

Glad we could help,thanks for the video, Looks really nice, Great work!