Electrical Machine-II (EX-501) - Important Questions

Explain the construction and working / operating principle of a (PM) brushless DC motor, including characteristics and applications.

Explain armature reaction in a DC machine, its cross-magnetizing and demagnetizing effects and effect on commutation, and methods for effective sparkless commutation

Explain the construction and working principle (and characteristics) of a PM brushless DC motor.

Explain the construction of DC machines with all its parts and principle of operation.

Explain the construction and working principle of a DC machine with neat diagram.

A shunt generator delivers $450\,A$ at $230\,V$ and the resistance of the shunt field and armature are $50\,\Omega$ and $0.03\,\Omega$ respectively. Calculate the generated EMF.

Derive the EMF equation of a DC generator.

Explain the commutation process in DC machines.

Explain the three-phase three-pulse (half-wave) brushless DC motor with diagram, operating principle, torque-angle characteristics and applications.

Explain armature reaction in a DC machine, its effects, and methods/remedies to reduce armature reaction.

Calculate the generated emf of a DC generator from flux per pole, speed, poles and lap winding details.

Explain demagnetizing and cross-magnetizing effects of armature reaction and derive expressions for demagnetizing and cross-magnetizing ampere-turns with brush shift from geometrical neutral axis.

Explain / describe Swinburne's test with circuit diagram and how efficiency of a DC machine is determined from it

What are the methods of speed control of DC motors?

Advantages of Swinburne's method and why it is convenient and economical for testing DC shunt machines

Describe/compare three-point and four-point starters for DC motors, including drawbacks of three-point starter, with neat sketch.

What are the methods of speed control of DC motors? Explain them.

Explain Hopkinson's test and Swinburne's test on DC machines with diagram and compare them.

Explain the torque equation and operating characteristics of DC motors.

Explain the working of a two-point starter used in a DC motor.

Describe the types of losses in DC motors and methods to reduce the losses.

Classify DC motors and explain/sketch their speed-current, speed-torque and torque-current characteristics.

Two identical DC series motors mechanically coupled and electrically in series: find supply current for given speed and scaled torque.

Explain the drawbacks of a three-point starter and describe a four-point starter with a neat sketch.

Derive the emf equation of an alternator and explain the meaning of distribution factor.

Explain the synchronous impedance (EMF) method for determining voltage regulation of an alternator.

Explain armature reaction and its effect on terminal voltage of an alternator at unity power factor load.

Explain the construction of an alternator and its EMF equation

Explain voltage regulation of an alternator by the Potier-triangle / ZPF method.

Calculate frequency and induced (line) emf of a 3-phase synchronous generator from poles, slots, conductors, flux per pole and speed.

Explain why DC excitation is given to the rotor winding and not to the stator winding of a synchronous alternator.

Calculate the distribution factor for given slots per pole and winding arrangement.

Explain why the MMF method of finding alternator voltage regulation is optimistic.

A 500 kVA, 3-phase, star-connected alternator has a rated line-to-line voltage of 3300 V. The resistance and synchronous reactance per phase are $0.3 \Omega$ and $4 \Omega$ respectively. Calculate the line value of the emf generated at full-load, 0.8 p.f. lagging.

Calculate the voltage regulation of a 3-phase star-connected alternator given per-phase resistance, synchronous reactance, load current, voltage and power factor using the synchronous impedance method.

What is armature reaction? Explain its effect on terminal voltage of an alternator at zero power factor lagging and zero power factor leading loads with phasor diagrams.

Determine how two alternators running in parallel share a given total kW load from their kW ratings and speed regulation from full load to no load.

Draw the phasor diagrams of a salient-pole synchronous generator supplying lagging, leading and unity power factor loads.

Explain Blondel's two-reaction theory of salient-pole synchronous machine / alternator with phasor diagram and equivalent circuit.

Explain synchronizing of an alternator with the infinite busbar and describe the different methods of synchronizing.

Explain Blondel's two-reaction theory of salient-pole synchronous machines with phasor diagram.

Explain synchronization of an alternator with infinite bus bars, including conditions and methods of synchronizing.

Explain two-reaction theory of salient-pole synchronous machine with equivalent circuit model and phasor diagram for lagging power factor load.

Explain why synchronous machines are operated at power angles much less than $90^\circ$ and what happens when the angle exceeds $90^\circ$.

Numerically determine the power angle of a salient-pole alternator delivering rated kVA at upf from given $X_d$, $X_q$ and rated voltage.

Explain why in the three bright lamp method the synchronising switch is closed when the lamps are brightest and how this ensures no phase difference between alternator and busbar.

Explain how the slip test is performed to determine $X_d$ and $X_q$ of a salient-pole synchronous machine, with diagram.

Draw and explain the power-angle characteristics of a salient-pole synchronous machine.

Explain hunting phenomenon in synchronous machines/alternators/motors, its causes, and role of damper winding / damping methods to reduce it

Explain why a synchronous motor is non-self-starting (has no net starting torque) and describe the methods for starting it, including auxiliary motor starting and damper winding / induction motor starting.

Explain construction and working of repulsion motor

Explain V-curves and inverted V-curves of a synchronous machine, including experimental determination with diagram.

Explain V-curves of the synchronous motor/machine, including operation and experimental determination with lab diagram.

Describe the hunting phenomenon in synchronous machines, its causes and methods to reduce it including use of damper winding

Explain transient and sub-transient reactances of a synchronous machine.

Explain the construction and working principle of hysteresis motor.

Explain transient and subtransient reactance in synchronous machines.

Explain construction and working of hysteresis and reluctance motors.

Consider a 3300 V delta connected synchronous motor having a synchronous reactance per phase of $18 \Omega$. It operates at a leading p.f. of 0.707 when drawing 800 kW from mains. Calculate its excitation e.m.f. and load angle.

Why back EMF is greater than supply voltage in an over-excited synchronous motor

Write a short notes on (any two)

What is negative sequence reactance? How negative and zero sequence reactances are measured in the laboratory?