# Shunt DC Machines (Rotational Losses)

#### jegues

Joined Sep 13, 2010
733
See figure attached.

They give us details of the no load conditions for this shunt DC machine and I assume that's how you are supposed to find the rotational losses but I can't seem to figure it out.

How do you obtain the rotational losses using the information provided from the no load conditions?

Thanks again!

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#### t_n_k

Joined Mar 6, 2009
5,455
At no load the motor presumably draws 5A at a set terminal voltage of 230 V.

The shunt field would take 2A, so the armature has the remaining 3A. The armature resistance drop is 0.3V so the armature emf = 229.7V. The no-load power input is therefore found from the 3A current and 229.7V armature emf - which would correspond to the rotational loss.

#### jegues

Joined Sep 13, 2010
733
At no load the motor presumably draws 5A at a set terminal voltage of 230 V.

The shunt field would take 2A, so the armature has the remaining 3A. The armature resistance drop is 0.3V so the armature emf = 229.7V. The no-load power input is therefore found from the 3A current and 229.7V armature emf - which would correspond to the rotational loss.
Thank you very much.

It wasn't clear to me from the question that the no load condition was indeed at a terminal voltage of 230V.