Electric Motor Specification

[bertie991]

I am designing an electric all-terrain powered wheel chair, i have no experience what so ever in terms of selecting an adequate motor. Can anyone advise me on how i should determine what size of motor I would need. In fact it will probably be all wheel drive so 4 motors will be needed.
I have done some calculations based on different driving conditions, i.e hill gradient, accelerating/decelerating and have came up with values for required torque. Is it right to assume that if i have a 100 Watt motor driving with a torque of 2n/m it will have an angular velocity of 50 rad/s however if the torque is stepped up to 50n/m it will have an angular velocity of 2rad/s or is there a limit to how much torque a motor can handle. I realise 100 Watt is far to small a motor for my application but it was chosen for convenience.

Thanks

 

[waross]

Check the data sheets on some real motors rather than asking for "convenient" generic data.

Bill
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"Why not the best?"
Jimmy Carter

 

[jraef]

...is there a limit to how much torque a motor can handle.

When a motor is given a rating, it is a shorthand expression of torque at a given speed. For example,

HP = Torque (ft. labs.) x RPM / 5250
or
kW (output) = Torque (Nm) x RPM / 9550

In addition, there is a power consumption rating of an electrical device like a motor, and if expressed in kW it would be higher than the output mechanical kW because of losses in the motor. For motors however, in order to avoid confusion the kW rating on the nameplate is (almost) always referring to the output mechanical kW. The electrical power that it requires to provide this output is called the "absorbed power" of the motor.

So in answer to this part of your question, absolutely there is a limit to the torque a motor can develop, but "handle" is a somewhat nebulous term. A motor is capable of delivering a specific amount of torque. If the load demands more torque than the motor can deliver, the motor is overloaded and fails.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[ScottyUK]

Also consider that some motors, notably series-wound DC machines, can develop stall torque which can easily reach or exceed the mechanical limits of the shaft or gearbox. In geared applications where speed is reduced and torque is multiplied this is often a problem.


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If we learn from our mistakes I'm getting a great education!

 

[PHovnanian]

What Bill said.

You'll be better off starting with an existing motor spec. You not only get a better idea of the trade offs available in torque, efficiency, speed and other parameters. But you'll pick parts that are available over the counter rather than spec'ing something oddball. In the end, your application may require oddball. But that's going to cost you extra money. You certainly don't want to design yourself into that corner from the outset.

 

[waross]

Is this a class project?

Bill
--------------------
"Why not the best?"
Jimmy Carter