AC Machines — Summer 2024

Derive the maximum starting torque condition for three phase induction motor.

Explain production of rotating magnetic field in poly-phase induction motor.

Ano-load test conducted on a 30 hp, 835 r/min, 440 V, 3-phase, 60 Hz squirrel- cage induction motor yielded the following results: No-load voltage (line-to- line): 440 V No-load current: 14 A No-load power: 1470 W Resistance measured between two terminals: 0.5  The locked-rotor test, conducted at reduced voltage, gave the following results: Locked-rotor voltage (line-to-line): 163 V Locked-rotor power: 7200 W Locked-rotor current: 60 A.

Explain working principle of Isolated Induction Generator.

Briefly explain double cage induction motor.

Explain the torque slip characteristics of a three-phase induction motor and describe the significance of stable and unstable region.

List the types of starters for a 3 Phase Induction Motor. Explain any two starters in brief.

Derive the emf equation of an alternator.

Define voltage regulation and explain any one method in case of an alternator.

Find the mechanical power output at a slip of 0.05 of the 185 W, 4 pole, 110 v, 60 Hz single phase motor. Whose constants are as given below: Resistance of the stator main winding – 1.86-ohm Reactance of the stator main winding – 2.56- ohm Magnetizing reactance of the stator main winding – 53.5 ohm Resistance of rotor at standstill – 3.56 ohm Reactance of rotor at standstill – 2.56 ohm

Explain with reason why synchronous motor is not self-starting.

Describe the effect of armature reaction in case of a synchronous generator.

A 250 W, 230 v, 50 Hz capacitor start motor has the following constant for the main and auxiliary windings: Main winding impedance = (4.5+ j 3.7) ohm Aux. winding impedance = (9.5 + j 3.5) ohm Determine the value of the starting capacitor that will place the main and auxiliary winding currents in quadrature at starting

What is hunting? How to minimize it?

Explain double field revolving theory and its application to single phase AC motors.

A 1000 kva, 3300 V, 3 phase, star connected alternator delivers full load current at rated voltage at 0.8 p.f lagging. The resistance and synchronous reactance of the machine per phase are 0.5 ohm and 5 ohms respectively. Estimate the terminal voltage for the same excitation and same load current at 0.8 p.f leading.1

Define the pitch factor and distribution factor in case of synchronous motor.

Briefly explain V-curves of synchronous motor.

From the following test results, determine the voltage regulation of a 2000 V, 1 phase alternator delivering a current of 100 Aat (1) unity p.f, (2) 0.8 leading p.f and (3) 0.7 lagging p.f. Test results: Full load current of 100 Ais produced on short circuit by a field excitation of 2.5 A. An e.m.f of 500 Vis produced on open circuit by the same excitation. The armature resistance is 0.8 ohm.

Justify the statement “Commutator acts as a frequency converter for the Schrage motor”.

Explain the construction and working of universal motor.

Draw the schematic diagram and explain the construction and working of shaded pole single phase motor.

Explain the construction and working of Repulsion motor.

Define slip. Write the equation of slip. What is the range of full load slip of induction motors?

Explain concept of linear induction motor. Also give relative advantage and disadvantage.