Electrical Machines 1

Permeability in a magnetic circuit corresponds to ____ in an electric circuit.

The area of his hysteresis loss is a measure of

The unit of reluctance is

The direction of rotation of a d.c. series motor can be reversed

Match the type of machine in List-I with characteristic/application in List-II and tick the correct answer from the code given below : List-I A. Dc shunt generator B. Dc series motor C. Level compounded dc generator D. Dc series generator

List-II

1. Electric traction
2. Has good voltage regulation
3. Must have residual flux
4. Used as booster

Code :

The armature resistance of a 6-pole lap wound d.c. machine is $0.05\text{ ohm}$. If the armature is rewound as a wave-winding, what is the armature resistance?

The use of higher flux density in the transformer design

Cores of large power transformer are made from which one of the following?

It is advised to use auto-transformer if the transformation ratio is

Scott connections are used in

Explain how the primary current increases as the current on the secondary side of the transformer is increased.

Define reluctance and inductance.

Explain why open-circuit and short-circuit tests should be performed on LV and HV sides respectively.

Transformer is a constant-flux machine. Justify with the help of equivalent circuit.

Derive a condition for maximum efficiency of a transformer.

What is meant by commutation in d.c. machine? Differentiate between good commutation and bad commutation. Enumerate the mechanical and electrical conditions leading to poor commutation in d.c. machines.

Explain the advantages of four-point starter over the three-point starter.

Describe and compare the various methods of speed control of d.c. motors.

Explain the terms 'base speed' and 'speed regulation' of d.c. motor.

A $250\text{ V}$ d.c. series motor has armature and series field resistance of $0.25\ \Omega$ and $0.15\ \Omega$ respectively.

A $5\text{ kVA}$, single-phase transformer has a core loss of $40\text{ watts}$ and full load ohmic loss of $100\text{ watts}$. The daily variation of load on the transformer is as follows: 7 am to 1 pm : $3\text{ kW}$ at p.f. 0.6 1 pm to 6 pm : $2\text{ kW}$ at p.f. 0.8 6 pm to 1 am : $6\text{ kW}$ at p.f. 0.9 1 am to 7 am : No load. Determine the all-day efficiency of the transformer.

For a commutator machine with 6 poles and 40 coils, determine for a simplex lap winding— (i) the number of commutator segments; (ii) back pitch and front pitch; (iii) commutator pitch.

The following test data were taken on a $30\text{ kVA}$, $2400/240\text{ V}$, $50\text{Hz}$, single-phase transformer: Open circuit test : $V = 2400\text{ V}$, $I = 0.3\text{ A}$, $P = 230\text{ W}$ Short circuit test : $V = 70\text{ V}$, $I = 18.8\text{ A}$, $P = 1050\text{ W}$ Determine the primary terminal voltage, real and reactive power input and efficiency, when a current of $12.5\text{ A}$ at $240\text{ V}$ is drawn from the low voltage side by a load of 0.8 p.f. lagging.

Explain why the e.m.f. generated in the armature of a d.c. motor is called 'back e.m.f.'.

Discuss how the parallel operation of two single-phase transformers is affected by— (i) unequal voltage ratio; (ii) unequal per unit leakage impedances but same $x_e / r_e$ ratio.

Explain the Biot-Savart law.