Current in DC motor (1 Viewer)

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

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


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.

The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving