# Current in DC motor (1 Viewer)

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#### farry1024

Question:
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.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

#### ideasrule

Homework Helper
You know that the stall current is 2 amperes, and when the motor is stalled there is no back emf. So the resistance is 12 V/2 A = 6 ohms.

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