Question

While carrying out retardation test, if t is equal to time constant then _____________

(a) Speed increases to 36.8% of its initial value

(b) Speed reduces to 36.8% of its initial value

(c) Speed reduces to 26.8% of its initial value

(d) Speed reduces to 46.8% of its initial value

I got this question in an online interview.

My doubt is from Swinburne’s Test topic in division Efficiency and Testing of DC Machines

Answer 1

Right option is (b) Speed reduces to 36.8% of its initial value

Easy explanation: While carrying out retardation test, speed of the motor decreases. At time t equal to time constant then speed reduced is given by 36.8% of its initial value. Retardation test result is used to determine initial slope of ω(t).