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JEP – 988

*JEP988*

II Semester M.E. (Power Electronics) Degree Examination, July 2014 (2K8 Scheme) PE-213 : ELECTRIC DRIVES Time : 3 Hours

Max. Marks : 100

Instructions : 1) Answer any five full questions. 2) Any missing data may be assumed suitably. 1. a) With the help of a block diagram, explain the major components of an electric drive.

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b) A d.c. motor with separate excitation with the data : K eλ p = 2 Wb, R a = 5Ω, L a = 0. 1 H is fed through a thyristor (Fig. 1) from a single phase a.c.

source whose voltage is V(t) = 120 2 sin 376.8t. The motor speed is constant at n = 750 rpm. i) Calculate the motor current ia(t) for a delay angle α = + 30 ° .

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ii) How does the motor voltage Va varies with time ?

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iii) How does ia(t) varies with time in the presence of a free wheeling diode in parallel with the motor armature ?

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Consider the thyristor and the diode as ideal switches.

(Fig. 1) A d.c. motor supplied through a thyristor.

P.T.O.

JEP – 988

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*JEP988*

2. a) A d.c. motor with permanent magnet excitation with the data R a = 1Ω, K eλ p = 0 .055 Wb / rpm is fed through a first-quadrant chopper from a 120 V

d.c. supply at a constant (ideal) armature current of 10 A. Determine i) the range of duty cycle K from zero to maximum speed; ii) the range of speed.

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b) With the help of a circuit diagram, explain the principle of operation of a four quadrant chopper drive of a d.c. motor using MOSFETs. 10 c) A d.c. chopper is used in regenerative braking of a DC series motor. Draw the chopper circuit and explain its working with relevant equations. 3. a) From the basic fundamentals derive the closed loop transfer function of a converter fed separately excited D.C. motor drive.

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b) With the help of a schematic diagram explain micro-computer control of dual converter fed dc drive. 10 4. a) With a neat circuit diagram and waveforms explain six step operation of a 3 phase bridge inverter.

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b) Explain sine-triangle comparison method (any one method) used in PWM VSI fed drive.

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5. a) Mention the various methods of speed control of I.M. With a neat circuit diagram explain speed control of a 3 phase induction motor by stator voltage variation using a 3 phase ac voltage controller. 10 b) A 2.7 KW, 415 V, 50 Hz, 4 Pole, 1410 rpm, delta connected squirrel cage induction motor has the following parameters referred to the stator : R s = 2 Ω, X s = X′r = 5Ω Rr′ = 5 Ω, X m = 80 Ω

The motor speed is controlled by stator voltage control. When driving a fan load it runs at rated speed at rated voltage. For a motor speed of 1200 rpm, determine : i) load torque, ii) rotor current Ir, iii) stator supply voltage, iv) the motor input current Ii, v) the motor input power.

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*JEP988*

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JEP – 988

6. a) Explain with relevant diagrams the V/f control of 3 phase induction motor. Compare its performance with stator voltage control.

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b) Explain the field oriented control of a 3 phase induction motor along with relevant diagram and equations. 7. a) With a neat circuit diagram, explain current source inverter fed 3 phase induction motor drive. b) What is a brushless d.c. motor ? How do you control it ? Explain any one method in brief. __________________

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