Discussion
A moving coil ammeter with a range of 0-20 A is connected to read the load current of a DC motor while performing a load test. The rated current of the motor is 10 A. When the motor is loaded up to 50% of the rated load, the top control spring of the ammeter suddenly gets snapped. The reading of the ammeter will be:
Answer: A
The control spring of a moving coil instrument carries the current into and out of the coil, as well as giving the controlling torque. If the top control spring snaps, the circuit through the moving coil is broken, so no current flows in it and there is no deflecting torque, and the pointer rests at zero. The reading is therefore 0 A, option (A), and not the motor's actual current of 5 A at half load or any of the values 20 A, 12 A and 6 A.
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