Testing of Machines Questions and Answers

Answer: C

Correct answer: the result is close to the acceptance limit

Where a result sits comfortably inside or outside the limit, a modest instrument error changes nothing. Near the boundary it decides the outcome, and an uncalibrated instrument may pass a machine that should fail or reject one that is sound. This is why calibration certificates are looked at hardest for marginal results.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 give the distinction correctly: type tests prove the design on a sample, routine tests prove each machine as built. Statement 3 misplaces the temperature rise test - it takes many hours and requires full load to be absorbed, so it belongs to the type tests and is not applied to every machine.

Answer: A

Correct answer: 1-Q, 2-R, 3-P

Three tests for three quite different faults. The growler detects a short-circuited coil by the circulating current the applied field induces in it. The bar-to-bar test measures the resistance between adjacent commutator segments and finds an open or high-resistance coil as an anomalous reading. And the polarisation index reveals moisture and contamination in the insulation.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. At 98 per cent efficiency the loss is the difference between two nearly equal large quantities, so a small relative error in either becomes a large error in the loss and in the efficiency derived from it. Measuring each loss separately and adding them avoids the subtraction altogether.

Answer: C

Correct answer: False, since the test itself can damage insulation that has aged

A high-voltage withstand test stresses the insulation and does a little damage each time, which sound new insulation absorbs without consequence. Aged insulation with voids and tracking may not, and repeated testing can precipitate the failure the test was meant to guard against. Standards therefore specify reduced voltages for repeat and maintenance testing.

Answer: D

Correct answer: prove it will perform as specified in service

Testing establishes conformity; it does not improve the machine. A test can reveal that a machine is below specification, but nothing done in the test bay makes it more efficient, less lossy or more highly rated than it was built to be.

Answer: A

Correct answer: a low direct voltage and a sensitive instrument

Winding resistances are fractions of an ohm, so they are measured with direct current - to exclude inductive effects - and a bridge or a four-terminal micro-ohmmeter that eliminates lead and contact resistance. The measurement must be recorded with the winding temperature, since copper resistance changes by about 0.4 per cent per degree.

Answer: B

Conduction through the insulation increases with temperature, roughly halving the resistance for every ten kelvin, so readings are corrected to a reference. Comparing an uncorrected hot reading with a cold one is meaningless.

Answer: C

Correct answer: dried out before any high-voltage test is applied

Absorbed moisture lowers the insulation resistance and provides paths for surface tracking, so applying test voltage to a damp winding may puncture insulation that would have been perfectly sound once dry. Drying and re-testing the insulation resistance and polarisation index comes first; the withstand test comes only after both are acceptable.

Answer: D

Correct answer: keep the windings above the dew point when it is idle

A machine standing still cools to ambient, and when the air is humid the windings condense moisture on their surfaces. Space heaters hold the winding a few degrees above ambient so the dew point is never reached, and they are interlocked to switch on whenever the machine stops.