Testing of Machines Questions and Answers

Answer: A

Correct answer: once on a representative machine of a design

A type test proves the design rather than the individual machine, so it is done once on a sample and covers every machine subsequently built to it. It includes the tests that are expensive or destructive - temperature rise, efficiency determination, overspeed - which could not be justified on every unit.

Answer: B

Correct answer: the routine tests

Routine tests are applied to every machine before dispatch and prove workmanship rather than design: winding resistance, insulation resistance, a high-voltage withstand test, a no-load run and a check of the nameplate data. They are quick and non-destructive, which is what makes them affordable on every unit.

Answer: C

Correct answer: the purchaser requests beyond the standard schedule

Special tests are additional to the standard type and routine schedules, agreed between purchaser and manufacturer for a particular application - noise measurement, a sudden short circuit test, partial discharge measurement, testing at an unusual voltage or frequency. They are priced separately because they are outside the normal contract.

Answer: D

Correct answer: the machine can carry its rated load without exceeding its class limit

The temperature rise test establishes the machine’s thermal rating: run at rated output until temperatures stabilise, the rise must stay within the limit for its insulation class. Since insulation life halves for roughly every ten degrees of excess, this test decides how long the machine will last rather than merely whether it works.

Answer: A

Copper resistance rises with temperature in a known way, so measuring it hot and cold gives the mean winding rise directly. Embedded detectors give the hot spot, and frame or outlet air readings are both well below the winding temperature.

Answer: B

Correct answer: the hot spot temperature rather than the mean

Detectors are embedded at the points expected to run hottest, typically between the coil sides in the slot, so they report the worst place rather than an average. The resistance method, by contrast, gives the mean temperature of the whole winding, which will always be lower - and it is the hot spot that determines insulation life.

Answer: C

Correct answer: the input and output to be measured simultaneously at full load

Direct determination measures both ends and takes the ratio, which requires a means of loading the machine to its rating and absorbing that power. It is straightforward for small machines and impractical for large ones, both because of the power involved and because of the accuracy problem.

Answer: D

Correct answer: the difference of two large quantities is very uncertain

A machine at 98 per cent efficiency has losses of two per cent, so the loss is the small difference between two nearly equal large measurements. A one per cent error in each could give a fifty per cent error in the loss and a wholly unreliable efficiency. Measuring the losses themselves avoids this entirely.

Answer: A

Correct answer: measures each loss separately and adds them together

Loss summation determines the iron loss, the friction and windage, the copper losses and the stray load loss by separate tests, sums them, and computes the efficiency from the sum. Since each loss is measured directly rather than inferred as a difference, a moderate error in any one has only a small effect on the result.

Answer: B

Correct answer: taken as an agreed percentage of rated output

Stray load loss arises from flux harmonics, skin effect and eddy currents in structural parts, and it is very difficult to measure with confidence. The standards therefore prescribe a conventional allowance - a stated fraction of rated output - so that efficiencies quoted by different manufacturers remain comparable.