Testing of Machines Questions and Answers

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.

Answer: C

Coupling two identical machines and letting one drive the other as a generator returns the bulk of the power electrically, so the supply provides only the losses. Two identical machines have to be available, which is its limitation.

Answer: D

Correct answer: produce rated temperature rise without rated load

The mixed frequency method superimposes two supplies of different frequency so that the machine draws currents producing the rated losses while delivering no useful output. The heating is then correct without a load to absorb full power, which is what makes the temperature rise of a very large machine testable at all.

Answer: A

A megohmmeter applies perhaps five hundred or a thousand volts and the reading is taken after a stated interval, since the current falls as the dielectric polarises. Temperature has a large effect and the reading is corrected for it.

Answer: B

Correct answer: ten minutes to that at one minute

\[ PI = \frac{R_{10\ \mathrm{min}}}{R_{1\ \mathrm{min}}}. \] Clean dry insulation polarises slowly and its measured resistance rises steadily, giving an index of two or more. Wet or contaminated insulation carries a steady conduction current that does not decay, so the resistance barely rises and the ratio approaches one.

Answer: C

Correct answer: moisture or contamination in the insulation

A low index means the current is not decaying with time, which happens when a conductive path - moisture, dirt, carbon tracking - carries a steady leakage that swamps the polarisation current. The remedy is drying and cleaning, and a machine failing the index should be dried before any high-voltage test is applied.