Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct Answer : C

The motor has 4 poles. The synchronous speed of a motor depends on the supply frequency and the number of poles.

\[ N_s = \frac{120 f}{P} \]

\[ P = \frac{120 \times 60}{1800} = 4 \]

Eight poles would give 900 rpm, six poles 1200 rpm, and two poles 3600 rpm at 60 Hz, so only four poles gives 1800 rpm. The third option is the answer. As a check, 4 poles on a 60 Hz supply give 120 x 60 / 4 = 1800 rpm, which is the speed in the question.

Your Answer : (Not Answered)

Correct Answer : C

Correct answer: the reversal of current in a coil as it passes the brushes

As a coil moves from one side of a brush to the other its connection to the external circuit reverses, so the current in it must reverse too. Doing that cleanly, in the brief interval the brush short-circuits the coil, is what the interpoles, the brush grade and the brush position all exist to achieve.

Your Answer : (Not Answered)

Correct Answer : C

Correct answer: its visible degree, from none to heavy with burning

Sparking is graded by inspection, from none through pin-point sparking at part of the brush edge, to sparking at the whole edge, to heavy sparking with visible burning and blackening. The first two grades are acceptable in continuous service; the last two are not and call for investigation.

Your Answer : (Not Answered)

Correct Answer : B

Correct answer: abrasive contamination or an unsuitable brush grade

Grooves cut into the copper mean something abrasive is passing between brush and commutator: dust drawn in with the cooling air, or a brush grade harder than the application calls for. The remedy is to clean the air and to change the grade, since resurfacing without doing so simply repeats the damage.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct Answer : B

Correct answer: 1.8 degrees

\[ \frac{360}{200} = 1.8\ \mathrm{degrees}. \] The 200-step motor is the industry standard, and 1.8 degrees is the step angle almost every general purpose stepper offers. Halving it to 0.9 requires 400 steps, achieved by doubling the rotor teeth rather than by anything done in the driver.

Your Answer : (Not Answered)

Correct Answer : C

Correct answer: its efficiency and power factor are better at that rating

A large fan runs continuously at a fixed speed, which is exactly the duty a synchronous machine suits: the efficiency advantage is earned every hour, and the leading power factor supports the rest of the plant. Its poor starting behaviour hardly matters for a machine started once and left running.

Your Answer : (Not Answered)

Correct Answer : D

It behaves as a torsional oscillator whose stiffness is the synchronising coefficient and whose mass is the inertia, so the frequency is the root of their ratio. A load fluctuating near that frequency excites the oscillation strongly.

Your Answer : (Not Answered)

Correct Answer : D

Normal excitation is defined as the field current giving unity power factor at that particular load, so it is not a fixed value but changes as the load does. More excitation than that gives a leading power factor and less gives a lagging one.

Your Answer : (Not Answered)

Correct Answer : D

A zero power factor test at rated current, plotted with the open circuit characteristic, gives the Potier triangle from which the leakage reactance and the armature reaction ampere-turns can be read separately. That separation is what makes it more accurate than either simpler method.

Your Answer : (Not Answered)

Correct Answer : C

Correct answer: gives an optimistic result, understating the regulation

The mmf method works with field ampere-turns read from the saturated open circuit characteristic, which errs in the opposite direction from the emf method and gives a regulation better than the truth. The two methods therefore bracket the real value, and the Potier method was developed to fall between them.

Your Answer : (Not Answered)

Correct Answer : C

The auxiliary winding's current leads the main winding's by perhaps thirty degrees, which is enough to make the field elliptical and give a net starting torque. The displacement is small, so the starting torque is modest.

Your Answer : (Not Answered)

Correct Answer : A

The gap dominates the reluctance of the magnetic circuit, so a smaller gap needs fewer ampere-turns to establish the flux and gives a better power factor. Bearing tolerances and rotor deflection set the practical floor.

Your Answer : (Not Answered)

Correct Answer : C

Only part of the power is transformed magnetically and the rest is conducted directly through the common winding, which saves copper and reduces the losses. The two circuits are no longer electrically isolated, which is its chief drawback.

Your Answer : (Not Answered)

Correct Answer : D

Correct answer: the fall in secondary voltage from no load to full load

Regulation measures how much the secondary voltage sags when load is applied, expressed as a fraction of the no-load value. It is caused by the voltage dropped across the winding resistance and, more importantly, the leakage reactance. A low regulation means a stiff supply, which is why distribution transformers are designed for a few per cent.

Your Answer : (Not Answered)

Correct Answer : A

Correct answer: overheat, because it has no ventilation from rotation

A self-ventilated machine carries its cooling fan on its own shaft, so at standstill there is no airflow at all, while the armature is carrying full current and dissipating full copper loss. The heat has nowhere to go. Stall duty therefore has to be derated heavily, or the machine given separate forced ventilation - the same problem that limits any self-ventilated machine at low speed.

Your Answer : (Not Answered)

Correct Answer : D

Correct answer: short-circuiting the no-volt coil when the current is excessive

The overload coil is an electromagnet carrying the load current; on overload it pulls up an armature that short-circuits the no-volt coil. Deprived of its current, the no-volt coil releases the starter arm, which springs back to the off position and disconnects the motor while reinserting the full starting resistance. The two coils thus work together, one sensing loss of supply and the other excessive current.