Question

A 440V DC shunt motor takes a current of 3A at no load.The armature resistance including brushes is 0.3 ohms and the field current is 1A.Calculate the output and efficiency when the input current is 20A.​

Answer 1

Answer:

Explanation:

Field current=  400200=2A  

So, no load armature current= 5–2=3A  

So,

ENL=V−IaRa=400–3×0.5=398.5V  

and full load armature current

= 50–2=48A  

So,

EFL=400–48×0.5=376V  

E is the back emf and for a dc motor

E=kφN  

So, E is directly proportional to speed (N) as φ for a dc shunt motor remains constant for constant field current and k is constant.

So,  EFLENL=NFLNNL=376398.5=0.94  

Or, it can be said that full load speed is 94% of no-load speed.