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Assume that a 12V permanent magnet DC motor is to be used for moving the robot legs.

It has a stall current of 2 ampere and a no load speed of 3000 r.p.m.

Assumer that the friction at no load spped was so little that it can be neglected .

When the suply is at 11V, the motor is moving the legs at its rated speed of 2000 rom due to firction, what current would it draw?

I try to answer:

Vmax = 12V

Imax = 12A

ωmax = 3000 rpm <<<ω=omoega, i.e. rotational speed

V1 = 11V

I1 = unknown

ω1 = 2000 rpm

back e.m.f proportional to ω

ω1/ ωmax = back e.m.f 1 / back e.m.f max

and back e.m.f = V- Ir

ω1/ ωmax = [ V1 - I1 ( r ) ] / [ Vmax - I max (r)]

However, I find that I do not have the resistance, how can I calculate? thanks.