A dc motor connected to a 460 V supply has an armature resistance of 0.15 ohms.Calculate

(1) The value of back emf when the armature current is 120A.

(2) The value of armature current when the back emf if 447.4V.

the back emf is given as

E = V - IR

here

V = 460 volts

I =  120 amps

R = 0.15 ohms 

so,

E = 460 - (120 x 0.15)

thus,

E = 442 volts

..

the armature current is given as

I = (V - E) / R

here

E = 447.4 volts

or

I = (460 - 447.4) / 0.15

thus,

I = 84 amps

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