Question

In retardation test _____________

(a) Motor switch is made ON and various speed readings are taken

(b) At rated speed various speed readings are taken out at different times

(c) Motor switch is made OFF at rated speed and various speed readings are taken

(d) Some readings are taken while speed is building up and some readings while speed is lowering down

This question was posed to me in examination.

My doubt stems from Swinburne’s Test in chapter Efficiency and Testing of DC Machines

Answer 1

The correct option is (c) Motor switch is made OFF at rated speed and various speed readings are taken

For explanation I would say: The motor is run to rated speed (or any high speed) and the supply is switched-off. As the motor decelerates (retards), several speed-time readings are taken, by a speedometer and watch with seconds hand. Initial readings are taken at small time intervals and the time interval is increased as the motor slows down.