Testing of Machines Questions and Answers

Answer: A

Correct answer: the plate is what a purchaser and an installer rely on

Everything downstream is sized from the nameplate: cables, protection settings, starter ratings, the driven load. An error on the plate propagates into every one of those decisions, and since nobody dismantles a machine to check its rating, the plate is verified against the test results before dispatch.

Answer: B

Without the instruments and the ambient conditions the readings cannot be interpreted or repeated, and a dispute cannot be resolved. Accredited laboratories treat the report as part of the test rather than as an afterthought.

Answer: C

Insulation resistance, polarisation index and partial discharge all mean far more as a trend than as a number, since the acceptable range is wide. Keeping the original works readings for comparison is therefore worth the filing space.

Answer: B

After a motor is rewound, the windings must be further insulated with varnish. The coils are dipped or impregnated in insulating varnish and baked, to bind the turns, to fill the air spaces and to give protection against moisture and mechanical stress.

Rubber, paper and oil are not used in this way on the finished winding. Paper is used as slot insulation before the coils are put in, and oil is used in transformers. So the second option is the answer.

Answer: A

A short circuit test gives Copper losses at full load.

In the short circuit test the low-voltage winding is shorted and a reduced voltage, typically 5 to 10% of rated, is applied to the other winding until full-load current flows. Because the voltage is so low, the flux and hence the iron loss are negligible, and the wattmeter reads the full-load copper loss. The test also gives the equivalent resistance and impedance.

It is done at rated current, not half load, and iron loss is measured by the open circuit test. Since iron loss is negligible here, the reading is not the sum of the two.

Answer: A

The open circuit test yields Core losses.

Rated voltage is applied to one winding, usually the low-voltage side, with the other open. The flux is at its normal value, so the full core (iron) loss occurs, while the no-load current is small and its copper loss negligible. The wattmeter therefore reads the core loss. The test also gives the no-load current and the shunt branch parameters.

Copper loss is measured by the short circuit test, so the open circuit reading is neither copper loss nor the sum of both. Leakage reactance is found from the short circuit test as well.

Answer: B

The test that is not a routine test is the Radio interference test.

Routine tests are made on every transformer before dispatch to confirm it is built correctly: winding resistance, ratio and polarity, impedance voltage and load loss, no-load loss and current, and insulation tests. Measurement of radio interference voltage is a special test, carried out only when agreed between maker and purchaser.

Polarity and impedance tests are standard routine checks, and the paper treats the core insulation voltage test as one too, which leaves radio interference as the odd one out.

Answer: A

The efficiency of two identical transformers on load is found by the Back to back test.

In the back to back (Sumpner's) test the primaries of the two transformers are connected in parallel across the supply and the secondaries in series opposition. One wattmeter reads the core loss of both at rated voltage, and a second supply circulates full-load current to give the copper loss of both, so the transformers run at full load while drawing only their losses.

The open circuit and short circuit tests each give one kind of loss, and neither loads the transformer under actual working conditions, so 'Any of the above' is also wrong.