Testing of Machines Questions and Answers

Answer: A

Correct answer: turn-to-turn faults by comparing the responses of two phases

A steep-fronted surge is applied to two nominally identical windings and their waveforms superimposed. Identical windings give identical traces; a single shorted turn changes the inductance enough to separate them visibly. This is one of the few tests that finds a turn-to-turn fault, which a resistance or insulation measurement will miss entirely.

Answer: B

A low direct voltage applied between winding and frame, with the current traced along the winding, narrows the fault down to a coil or a slot. Insulation resistance measurement establishes that a fault exists but not where.

Answer: C

Correct answer: the amplitude and frequency of shaft and bearing movement

A vibration test records how much the machine moves and at what frequencies, and the frequency content is the diagnostic part: unbalance appears at running speed, misalignment usually at twice it, bearing defects at their own characteristic frequencies, and electrical faults at twice supply frequency.

Answer: D

Correct answer: once per revolution

An eccentric mass produces a rotating force vector fixed to the rotor, so a stationary sensor sees it once per revolution - at one times running speed. That signature is what distinguishes unbalance from misalignment, which appears predominantly at twice running speed, and it is why a vibration spectrum is diagnostic rather than merely a magnitude.

Answer: A

Correct answer: reduce the vibration caused by mass eccentricity

Balancing adds or removes small masses at chosen radii until the centre of mass lies on the axis of rotation. Dynamic balancing does this in two planes at once, so that the rotor is balanced both for net force and for the couple that a long rotor can develop even when statically balanced.

Answer: B

Correct answer: a stated percentage above rated speed for a short period

The overspeed test - commonly 120 per cent of rated speed for two minutes - proves the mechanical integrity of the rotor against centrifugal stress with a margin over any speed it could reach in service. Since centrifugal force goes as the square of speed, 120 per cent applies 44 per cent more stress than rated running.

Answer: C

Correct answer: the subtransient and transient reactances and their time constants

Applying a short circuit to an unloaded machine and recording the current envelope reveals the whole decay: the very large initial current governed by the subtransient reactance, its decay to the transient value as the damper currents die away, and finally the steady state. These parameters are what the power system’s fault studies and protection settings depend on.

Answer: D

Correct answer: reveals mechanical faults and confirms the direction of rotation

The no-load run is cheap and catches a great deal: bearing noise, rubbing, unbalance, a wrong connection, an incorrect direction of rotation. It proves nothing about the rated performance, which is why it sits alongside the electrical routine tests rather than replacing them.

Answer: A

About four per cent per ten kelvin means a hot and a cold reading differ by a fifth or more, so comparisons are meaningless without correction. Standards specify seventy-five degrees as the usual reference for copper windings.

Answer: B

Correct answer: of limited value, since its agreement with the limit cannot be judged

A test exists to establish whether the machine meets a specification, and that judgement requires knowing how well the measurement is known. A temperature rise of 78 degrees against an 80 degree limit passes if the uncertainty is one degree and is undecided if it is five. Calibration alone does not supply that figure.