Your Answer : (Not Answered)

Correct Answer : D

In a DC drive supplied from a full converter, regenerative braking returns the energy of the motor to the supply. The converter must then work as an inverter, which for a full converter means a firing angle above \(90^\circ\), where the average output voltage is negative and so the converter can accept power from the DC side. For firing angles below \(90^\circ\) the converter works as a rectifier and the power flows from the supply to the motor.

The key gives more than \(90^\circ\), option D. The item speaks of the converter in the field circuit, which is loosely worded, and I took the key as the usual condition for inverting operation of a full converter. Zero, below 30 degrees and below 60 degrees are all rectifier conditions.

Your Answer : (Not Answered)

Correct Answer : D

Windings designed for a sinusoidal supply may not withstand the inverter's fast edges, and a self-ventilated machine cools poorly at low speed. Inverter-duty windings, output filters and a torque derating curve are the three remedies.

Your Answer : (Not Answered)

Correct Answer : B

Everything about the machine and converter rating follows from that curve, because it says what has to be delivered and where. The other three are constraints on the equipment rather than statements of what the load demands.

Your Answer : (Not Answered)

Correct Answer : A

Correct answer: A test dynamometer loading an engine

A dynamometer must absorb power from the engine under test, which is generating, and must also motor the engine to start it and to test its friction - in both directions of rotation for some machines. That is the full four quadrants, and few applications need them so plainly.

Your Answer : (Not Answered)

Correct Answer : A

Thermal trips integrate current over time, so a delayed trip is the signature of a genuine overload rather than a fault. Checking the mechanical load and the drive's current limit settings is the sensible starting point.

Your Answer : (Not Answered)

Correct Answer : B

The current is roughly in phase with the voltage but far from sinusoidal, so the displacement factor is good while the distortion factor is not. A phase-controlled rectifier is the reverse case, with a displacement factor that falls with firing angle.

Your Answer : (Not Answered)

Correct Answer : C

The first two follow from the slip relation. The third is the reverse of the truth: a converter slows the machine and therefore reduces the power drawn, while the coupling keeps the machine at full speed and burns the difference.

Your Answer : (Not Answered)

Correct Answer : B

Taking the slave's reference from the master's actual speed keeps the two in step even while the master is changing speed. A common reference would let them diverge during transients, which on a shared web or load would be unacceptable.

Your Answer : (Not Answered)

Correct Answer : D

It is governed by the available torque and the inertia, and by the bandwidth of the control loops. A servo drive with a low-inertia machine and a fast current loop can reverse in milliseconds, where a large process drive may take seconds.

Your Answer : (Not Answered)

Correct Answer : D

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

Both are true and the reason is the whole justification. Every electrical braking method - regeneration, dynamic braking, dc injection, eddy current - depends on relative motion to induce the emf that produces the torque, so all of them vanish at standstill. A suspended load would begin to descend the moment the drive stopped it. A friction brake works by contact and is indifferent to speed, so it is mandatory, and it is arranged fail-safe so that loss of supply applies it.

Your Answer : (Not Answered)

Correct Answer : B

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

Holding requires torque at zero speed, which only a friction brake gives. A long descent releases a large energy at a steady rate, which regeneration recovers rather than wasting. An occasional stop involves a modest energy at long intervals, for which a resistor is cheap and entirely adequate. The three duties are the three cases the whole topic turns on.

Your Answer : (Not Answered)

Correct Answer : D

Correct answer: \( \sin\delta \)

For a cylindrical rotor machine with negligible armature resistance,

where \( E \) is the excitation emf, \( V \) the terminal voltage and \( X_s \) the synchronous reactance. The power therefore rises with \( \delta \) to a maximum at 90 degrees and falls thereafter. A salient pole machine adds a reluctance term in \( \sin 2\delta \), which shifts its maximum below 90 degrees, but the cylindrical rotor machine has only the fundamental term.

Your Answer : (Not Answered)

Correct Answer : A

Correct answer: a short-circuited transformer

At standstill the slip is unity, the rotor is a shorted secondary at full supply frequency, and the only thing limiting the current is the leakage impedance. Five to seven times rated current is the usual result. As the machine accelerates the effective rotor resistance \( R_2/s \) rises, the current falls, and the machine settles at its running point - which is why the high current lasts only for the run-up.

Your Answer : (Not Answered)

Correct Answer : B

Correct answer: generates and returns power to the supply

Driven above \( N_s \) the slip becomes negative, the rotor emf reverses, and the torque opposes the direction of rotation. The machine acts as an induction generator, delivering real power to the supply while still drawing its magnetising reactive power from it. This is the regenerative braking region, used on hoists and on wind turbines with cage generators.

Your Answer : (Not Answered)

Correct Answer : D

Correct answer: the slip times the air gap power

Of the power \( P_g \) crossing the air gap, the fraction \( s \) is dissipated in the rotor resistance and the remaining \( (1-s) \) appears as mechanical power:

The loss is therefore fixed by the slip alone, not by the rotor resistance, and any scheme that runs the machine at a large slip wastes a correspondingly large share of the air gap power. This is the whole objection to rotor resistance and stator voltage control.

Your Answer : (Not Answered)

Correct Answer : D

Correct answer: extra heating without extra useful torque

The mean torque follows the mean current, so the ripple adds nothing useful. But heating follows the root mean square current, which the ripple raises, so the machine runs hotter for the same output and may have to be derated. The ripple also causes torque pulsations and extra commutator sparking. A smoothing inductor, or a higher pulse number, is fitted to reduce it.

Your Answer : (Not Answered)

Correct Answer : A

Correct answer: disconnecting the armature and connecting it across a resistor

In dynamic or rheostatic braking the machine is separated from the supply while its field is maintained, so it continues to generate; the armature is connected across a resistor and the kinetic energy is dissipated there as heat. It is simple and needs no cooperation from the supply, but the energy is wasted. Regenerative braking returns it instead, and plugging reverses the supply connections, which is a third method again.

Your Answer : (Not Answered)

Correct Answer : A

Correct answer: four-quadrant operation

A dual converter is two fully controlled bridges connected back to back across the armature. One conducts for each direction of current, and each can be operated as a rectifier or as an inverter by taking the firing angle beyond 90 degrees. Between them they give both signs of voltage and both signs of current, that is motoring and braking in both directions of rotation - all four quadrants of the torque-speed plane.

Your Answer : (Not Answered)

Correct Answer : D

Correct answer: 4.9 kW

Power may be taken either at the load or at the drum, and the two agree:

or equally \( T\omega = 3924 \times 0.5/0.4 \). The lifting power is what the motor must supply in the steady state, before adding the friction of the mechanism and the torque needed to accelerate the inertia at the start.

Your Answer : (Not Answered)

Correct Answer : D

Correct answer: the square of the speed

Windage is aerodynamic drag on the rotating parts, and drag varies with the square of the velocity, so the torque does too and the power varies as the cube. This is why windage is negligible in a slow machine and dominant in a high-speed one, and why a flywheel or a turbogenerator rotor is often run in a partial vacuum or in hydrogen.