Design of AC Machines — Winter 2022

Define ‘Specific electric loading’ and ‘Specific magnetic loading’ in connection with 3-phase induction motor.

Explain the factors affecting the choice of specific Magnetic loading in case of a Synchronous machine.

Derive an output equation for 3-ф induction motor with usual notation.

State the rules for the selection of rotor slots in 3-phase squirrel cage induction motor.

State important design difference between turbo alternators and hydro generators.

Define SCR. Discuss its effects on synchronous machine performance.

Find the main dimensions of a 2500 kVA, 187.5 r.p.m., 50 Hz, 3-phase, 3 kV, salient pole synchronous generator. The generator is to be a vertical, water wheel type. The specific magnetic loading is 0.6 Wb/m2, and the specific electric loading is 34000A/m. use circular poles with ratio of core length to pole pitch = 0.65. specify the type of pole construction used if the run-away speed is about 2 times the normal speed

What is the role of damper winding in (i) synchronous generator and (ii) synchronous motor?

What is dispersion coefficient? What is its effect on max. power factor

Determine the main dimensions of a 15 kW, 3-phase, 400 V, 50 Hz, 2810 r.p.m squirrel cage induction motor having an efficiency of 0.88 and a full load power factor of 0.9. Assume: Specific magnetic loading = 0.5 Wb/m2, specific electric loading = 25000 A/m, winding factor = 0.955. Take the rotor peripheral speed as approximately 20 m/s at synchronous speed.

Explain the terms “critical speed” and “run away speed” with reference to synchronous machine.

Draw and explain briefly the current distribution wave form speeded over one pole pitch in bars and end rings squirrel cage induction motor

A 11kW,3 phase,6 pole 50 Hz,415V star connected induction motor has 48 slots each containing 9 conductors. Calculate the values of bar and end ring currents. The number of rotor bars is 64. The machine efficiency of 0.86 and a power factor of 0.85. The rotor mmf may be assumed of 85 percent of stator mmf. Find area of rotor bar and area of end ring, if current density is 5 A/mm2

Explain direct axis and quadrature axis synchronous reactance in synchronous machine

Calculate the value of capacitance for maximum starting torque in 1-phase induction motor1

The following is the design data available for a 1250 kVA, 3-phase, 50 Hz, 3300 V, star connected, 300 r.p.m. alternator of salient pole type: Stator bore D = 1.9 m, stator core length L = 0.335 m, turns per phase =150, pole arc/pole pitch = 0.66, single layer concentric winding with 5 conductors per slot, short circuit ratio=1.2. assume that the distribution of gap flux is rectangular under the pole arc with zero values in the interpolar region. Calculate:

Specific magnetic loading (c) armature mmf/ pole

Gap density over pole arc (d) air gap length. Mmf required for air gap is 0.88 of no load field mmf and the gap contraction factor is 1.15.

Compare the output equation of both 1-phase and 3-phase IM.

State why a turbo alternator has smaller diameter and large length but hydro alternator has larger diameter and small length

Describe the methods for reducing the effect of harmonics torque in 3-phase squirrel cage induction motor.

Discuss application of FEM technique for design problems.

Derive the equation of MMF of damper winding.

Explain briefly the methods for improving e.m.f. wave form of an alternator.

Discuss the effect of variation of main dimension (D&L) on performance (Rating, losses & efficiency) of 3-ф induction motor.