Class H insulation Below are times to failure on 10 motors with a new Class H insulation at 220°C. Motors were periodically inspected for failure. A failure time is midway between the time when the failure was ' found and the previous inspection time. Rounding to the midpoint affects the data little. The table shows a failure or running time for each cause (turn, phase, or ground, each a separate part of the insulation). Each failed part was isolated electrically and could not fail again, and most motorsC stayed on test to a second or third failure cause. Most other data contain only one failure cause per unit. In use, the first failure from any cause determines the motor life.
(a) Use the first failure on each motor. Do hazard calculations and plot the failures on lognormal hazard paper. This estimates motor life with all modes acting.
(b) Do the hazard calculations for the turn failures. Plot the data on lognormal hazard paper
(c) Do (b) for phase failures.
(d) Do (b) for ground failures.
(e) Which failures should be eliminated to improve life most?
(f) Combine the hazard calculations for separate modes to get an estimate of the cumulative hazard function when all modes act. Plot each failure against its cumulative hazard on lognormal hazard paper.
(G) How do the estimates from (a) and (f) compare?
(h) Why do (a) and (f) differ in the upper tail? Which is a better