GATE 2019 EE – Question 56
A 220 V DC shunt motor takes 3 A at no-load. It draws 25 A when running at full-load at 1500 rpm. The armature and shunt resistances are $0.5\ \Omega$ and $220\ \Omega$, respectively. The no-load speed in rpm (round off to two decimal places) is ________.
Practise this question in The GATE Grind →
Show answer and explanation
Correct answer: 1580
Explanation
The field current is $\frac{220}{220}=1$ A, so the armature current is $24$ A at full load and $2$ A at no load. The back emfs are $E_{FL}=220-24(0.5)=208$ V and $E_{NL}=220-2(0.5)=219$ V. With constant flux the speed is proportional to $E$, so $N_{NL}=1500\times\frac{219}{208}=1579.3\approx1580$ rpm.