A dc shunt motor runs at a no-load speed of 1140 rpm.
At full load, armature reaction weakens the main flux by 5% whereas the armature circuit voltage drops by 10%.
The motor full-load speed in rpm is
A. 1080
B. 1203
C. 1000
D. 1200
Answer: A
Share your understanding of this question with the correct explanation.
To calculate the full-load speed of the DC shunt motor, we need to take into account the effect of armature reaction and the voltage drop in the armature circuit.
Given:
No-load speed = 1140 rpm
Armature reaction weakens the main flux by 5%
Armature circuit voltage drops by 10%
The effect of armature reaction on the speed is opposite to that of the voltage drop. As armature reaction weakens the main flux, it tends to increase the speed. However, the voltage drop in the armature circuit reduces the speed.
Since the armature reaction weakens the main flux by 5%, the effective flux is 100% - 5% = 95% of the main flux.
The voltage drop in the armature circuit is 10%, which means the effective voltage available for the motor is 100% - 10% = 90% of the rated voltage.
The speed of a DC motor is directly proportional to the product of the flux and the voltage. Therefore, the full-load speed can be calculated as follows:
Full-load speed = No-load speed × (Effective flux) × (Effective voltage)
= 1140 rpm × (95/100) × (90/100)
= 1081.8 rpm
Rounding off to the nearest whole number, the full-load speed of the DC shunt motor is approximately 1080 rpm.
Therefore, the correct answer is A. 1080 rpm.