DC Motors

A 220 V DC series motor draws full-load line current of $38\ \text{A}$ at the rated speed of 600 rpm. The motor has armature resistance of $0.4\ \Omega$ and the series field resistance is $0.2\ \Omega$. The brush voltage drop irrespective of load is 3.0 volts, find:

a) The speed of the motor when the load current drops to $19\ \text{A}$.

b) The speed on removal of load when the motor takes only $1\ \text{A}$ from the supply.

The armature and field resistances of a 200 V DC shunt motor are $0.12\ \Omega$ and $250\ \Omega$ respectively. The rotational loss of the motor is $250\ \text{W}$. The full load line current is $10\ \text{A}$ and motor rpm is 1500. Determine;

(i) The mechanical power developed.

(ii) The power output

(iii) The load torque

(iv) The full load efficiency

Explain the need of starter in a DC motor and describe the working principle of 4 point starter with a neat sketch.