How much current does the motor draw to produce the required torque?

AI Thread Summary
To determine the current drawn by the motor to produce the required torque, the torque equations for both the wheel and the coils are set equal. The torque on the wheel is calculated using the friction force and wheel radius, while the torque on the coils involves magnetic field strength, current, and coil area. The discussion also touches on calculating back emf, with the relationship between voltage, current, and resistance highlighted, although resistance and inductance values are missing. The user expresses confusion about the relevance of additional information provided and questions whether it is necessary for calculating motor efficiency. Overall, the focus is on applying fundamental physics equations to solve for current and back emf in the context of the electric car's motor.
xSnoopy
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Homework Statement


A small electric car overcomes a 269N friction force when traveling 28.6km/hr. The electric motor is powered by ten 12.0V batteries connected in series and is coupled directly to the wheels whose diameters are 50.6cm. The 295 armature coils are rectangular, 10.0cm by 14.8cm, and rotate in a 0.590T magnetic field. How much current does the motor draw to produce the required torque?


What is the back emf?

How much power is dissipated in the coils?


Homework Equations


Not too sure...

The Attempt at a Solution


This is kind of weird as my homework goes from Waves and EM to this question :S regardless...

I'm not really sure where to start on this, I was guessing that the armature coil would be producing a current I? So:

B=μIn, where B=0.590T and n = 295/(0.148m) but I'm guessing that's not it at all...

Any help?
 
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Okay I seem to got the first part

Torque on Wheel = Torque on coils in motor

TorqueWheel = ForceFriction*RadiusWheel

TorqueCoil = B*I*(AreaCoil)

Where I can solve for I or current. :)

But now I'm stuck on calculating the back emf...
I guess I know...

V(battery) - IR = emf or -L(dI/dt) = emf but I don't have either R or L... Any help?
 
Is it easier than you think? You know the force and velocity so you know the power required. You know the battery voltage so presumably you can calculate the current from P=IV.

Is the rest of the info unnecessary? I don't think there is enough info to calculate the motor efficiency?
 
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