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Electrical Machines I (EM 1) - 2012 Important Questions

Anna University
2012 Important Questions
131401: Electrical Machines I
Common To :
Tirunelveli, Madurai, Coimbatore, Tirchy All Over Tamil Nadu engineering colleges

Unit 1
1. Discuss in detail magnetic circuits and electrical analog of magnetic circuits.
2. Explain the operation of a magnetic circuit when AC current is applied to a coil wound on an iron core. Draw the B-H curve and
obtain an expression for hysteresis loss.

3. A Ring is composed of 3 sections. The cross sectional area is 0.001m2 for each section. The mean arc length are la = 0.3m, l =
0.2m and Ici = 0.lm. An air gap length of 0.1mm is cut in the ring µr for sections a, b and c are 5000, 1000 and 10000 respectively.
Flux in the air gap is 7.5 x iO Wb. Find (j) mmf, (ii) exciting current if the coil has 100 turns (iii) reluctance of the sections.

4. Discuss in Detail about eddy current? Explain eddy current loss and Derive an expression for energy density in magnetic field.

Unit II

1. Explain in detail various types of three phase transformer connections.

2. A 3 phase transformer bank consisting of 3 single phase transformers is used to step down the voltage of 3-phase 6600V
transmission line. If the primary line current is lOA, calculate the secondary line voltage and line current and output kVA for (i) Y/Delta
and (ii) Delta/Y connections. The ratio of turns per phase is 12. Neglect no load losses.

3. A 100kVA, 6600 / 330V, 50Hz single phase transformer took lOA and 436W at 100V in a short-circuit test, the figures referred to HV side. Calculate the voltage to be applied to the HV side on full load at 0.8 power factor lagging when the secondary terminal voltage is 330V

4. The following data were obtained on a 2OkVA, 50Hz, 2000/200V distribution transformer. Draw the approximate equivalent circuits referred to HV and LV sides respectively.

Unit III

1. Derive an expression for the mechanical force of field origin in a typical attracted armature relay.

2. Derive an expression for the magnetic force developed in a doubly excited magnetic system.

3. Two windings one mounted on the stator and the other on a rotor have self and mutual inductances of Lii = 4.5H, L22 = 2.5H and Li2 = L2i = 2.8 çpU where O is the angle between the of the windings. The resistances of the windings may be neglected. Winding 2 is short circuited and the current in winding 1 as a function of time is Il = 10 sin ot (A). Derive an expression for the numerical value of the instantaneous torque on the rotor in N-m terms of the

4. Discuss in detail various methods of speed control in DC motors.

5. Derive an expression for the magnetic energy stored in a singly excited electromagnetic relay.

Unit IV

1. Discuss the Ward-Leonard method of speed control of DC Motors.

2. A 3 phase, 50 Hz star connected alternator with two layer winding is running at 600 rpm. It has 12turns/coil ,4solts/pole/phase and a coil pitch of lo slots. If the flux per pole is 0.035 Wb sinsodally distributed, find the phase and line induced. Assume that the total turns/phase are series connected.

3. A 3-phase, 50 kW, 4-pole, 50 Hz induction motor has a winding (ac)designed for delta connection. The winding has 24 conductors per slot arranged in 60 slots. The rms value of the line current is 48A.m Find the fundamental of the mmf wave of phase-A when the current is passing through its maximum value. What is the speed and peak value of the resultant MMF/pole?

4. Discuss in detail various methods of speed control in DC motors

5. Discuss the concept of field energy and co-energy in magnetic system.

Unit V

1. In 110V compound generator, the resistances of the armature, shunt and series field windings are 0.06, 25 and 0.04 ohms respectively. The load consists of 200 lamps each rated at 55W, 110V. Find the total electromotive force and armature current when the machine is connected in long and short shunt.

2. A series motor of resistance 1 ohm between terminals runs at 800 rpm at 200V with a current of 15A. find the speed at which it will run when connected in series with a 5 ohm resistance and taking the same current at the same supply voltage.

3. A 250-V dc shunt motor has ,Resistance = 150 ohms and Ra = 0.6 ohms. The motor operates on no-load with a full field flux at its base speed of 1000 rpm with I = 5 A. If the machine drives a load requiring a torque of 100 Nm, calculate armature current and speed of motor. If the motor is required to develop 10 kW at 1200 rpm what is the required value of the external series resistance in the field circuit? Assume linear magnetization. Neglect saturation and armature reaction

4. Draw the circuit of various type of starter and explain its operations in detail

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