Can someone tell me how armature current is found if you know:

Vt = 240
Ra = 0.25
pout = 30k
efficiency = 92%

 

Follow below;

Ia = Et - Ecemf / Ra
where
Ia = armature current
Et = terminal voltage
ECEMF = counter EMF
Ra = armature resistance

 

You might need a bit more thought on that.

You know the mechanical output is Pout=30kW, with an overall efficiency of 92% [η=0.92]. Also given Ra=0.25 with a DC input Vd=240V.

Steps required

1. Calculate the electrical input power to the air gap emf, i.e. Pe. [Pe=Pout/η]
2. Set up the equation for the armature circuit. [Vd=Ia*Ra+Pe/Ia]
3. Solve the quadratic equation from step 2 above to find Ia. Only one of the quadratic roots will give the correct value for Ia.