JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech II ELECTRICAL MACHINES – II Previous Question Paper (September/October - 2023 - R16)

Why an Induction motor is called as rotating transformer?

Derive the expression for rotor frequency in terms of stator frequency.

What is the purpose of no-load and blocked-rotor tests on induction motors?

Write the effects of crawling and cogging on the performance of induction motor.

What are the sources of harmonics in the generated EMF in case of alternator? How to minimize them?

Define pitch factor and distribution factor?

What do you understand by infinite bus?

Mention the need for starter in synchronous motors.

Mention the applications of shaded pole motor.

Write any two applications of single-phase induction motor.

Describe the principle of operation of three phase induction motor. Explain why the rotor is forced to rotate in the direction of rotating magnetic field.

A 4-pole, 3-phase, 50 Hz induction motor has a star-connected rotor. The rotor has a resistance of 0.1Ω per phase and standstill reactance of 2Ω per phase. The induced e.m.f. between the slip rings is 100 V. If the full-load speed is 1460 r.p.m., calculate (i) The slip (ii) The e.m.f. induced in the rotor in each phase (iii) The rotor reactance per phase (iv) The Rotor current and (v) Rotor power factor. Assume slip rings are short-circuited.

Using the relevant diagrams, explain how the rotating magnetic field is produced in the three-phase induction motor.

The induced emf between the slip-ring terminals of an induction motor at standstill is 100 V. The rotor windings are stator connected and has a resistance of 0.4 W per phase and standstill reactance of 2.25 W per phase. Calculate, the rotor current when the slip-ring terminals are short-circuited and the rotor is rotating at a slip of 4 percent.

Explain the power flow diagram in a three-phase induction motor and deduce the relation between mechanical power output and electrical input power to a three-phase induction motor.

A three-phase induction motor has a starting torque of 100% and a maximum torque of 200% of the full-load torque. Determine: i) slip at which maximum torque occurs ii) full-load torque and iii) rotor current at starting in per unit of full-load rotor current.

How the equivalent circuit parameters are obtained for an induction motor.

Explain the principle of speed control of a 3-phase induction motor by V/f method and draw the corresponding torque-speed characteristics and discuss the applications and limitations of these methods.

Discuss and derive E.M.F equation for an alternator.

Define voltage regulation of an alternator. Explain the various factors which may affect the regulation of an alternator.

Explain the MMF method of determining the voltage regulation of alternator.

Describe a method of determining direct and quadrature-axis reactance of salient pole alternator.

Discuss and state the conditions necessary for paralleling alternators.

Explain in detail about the concept of hunting and how to overcome it in a synchronous motor.

What are the various methods of synchronizing alternators?

A 2000V, 3-phase, 4-pole, Y- connected synchronous motor runs at 1500rpm.The excitation is constant and corresponds to an open circuit voltage of 2000V.The resistance is negligible as compared to a reactance of 3Ω per phase. Determine the power input, power factor and torque developed for an armature current of 200A.

Using double revolving field theory, explain why a single-phase induction motor is not self-starting.

Explain the construction and working of a shaded pole motor.

Explain the constructional details and principle of operation of a split phase induction motor.

Discuss why single-phase induction motors do not have a starting torque.