Motor Torque Requirements (Hub Motor)

[elsikre]

Hi !
I really hope that someone can help me with this one. It should actually be straight forward, but somehow these calculations really just … Well, perhaps I am just making things way too complicated so I really need a fresh view on things, so I really like to hear some of your inputs, thx.

The task is actually a homebuilding project. A small trailer with 2 wheels in the front and a freewheel at the rear. Each front wheel has an mounted hub motor (not yet chosen). From a standstill the trailer will accelerate, drive 100 meter at designated speed, then stop, turn around and drive back. I need to estimate how much torque each hub motor can give, able to do this travel. The travel will first be in a parking lot and afterwards repeated on a grass field. Boiled down to this:

1) To know how much torque needed in each motor (both during acceleration and during continuous driving)
2) To estimate how many amps each of the hub motor will consume during this travel (assuming 24V)

Data:
m(total) = 50 kg
r(front.wheel) = 0.1 m
v(trailer) = 3 m/s
a(trailer) = 1 m/s^2
n(motor) = 0.6
V(motor) = 24 V
Thanks

 

[Navin A S]

in advance. To calculate the torque needed for each motor, you will need to first calculate the acceleration force. This can be done by finding the total mass of the trailer and then multiplying it by the acceleration (F=ma). F = 50kg * 1 m/s2 = 50 N The torque needed to accelerate the trailer is then equal to the force times the radius of the wheel (T = F x r). T = 50N * 0.1m = 5Nm Once the trailer is up to speed and you want to maintain the speed of 3m/s, you need to calculate the drag force of the trailer. This can be done by using the equation F = Cd x A x V^2/2, where Cd is the drag coefficient, A is the frontal area of the trailer and V is the velocity. Assuming a drag coefficient of 0.5 and a frontal area of 1m2, the drag force is: F = 0.5 x 1m2 x (3m/s)2/2 = 4.5N The torque needed to maintain the speed is then equal to the drag force times the radius of the wheel (T = F x r). T = 4.5N * 0.1m = 0.45Nm To calculate the current draw for each motor at 24V, the torque needed can be divided by the motor's efficiency (I = T/n). I = 5Nm/0.6 = 8.33A I = 0.45Nm/0.6 = 0.75A So the hub motors would need to provide 8.33A during acceleration and 0.75A during continuous driving.