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PEC UNIVERSITY OF TECHNOLOGY

End-Term Examination

Programme: B.E (Electrical) Year/Semester: 14151

Course Name: Electric Drives Course Code: EE331

Maximum Marks: 100 Time allowed: 3 Hrs

Course Instructor: Prof. Dhiraj Bharat Date & Time:

Notes :

All questions are compulsory.

Unless stated otherwise, the symbols have their usual meanings in context with subject. Assume suitably and state, additional data required, if any. Unless stated, the machine may be assumed to be operating in linear region.

The candidates, before starting to write the solutions, should please check the question paper for any discrepancy, and also ensure that they have been delivered the question paper of right course code.

Q.No.

Q1.(A)

Q1.(B)

Q2.(A)

Q2.(B)

A DC shunt motor is fed with a 250 V supply. The shunt field winding resistance is 100 and armature resistance is 0.25 . The motor draws 75 A of armature current at a speed of 1000 rpm. It is desired to run the machine at 700 rpm with torque requirement of one-half of that at 1000 rpm. This

is effected by putting a resistance in series with the supply. Determine

(a) The value of resistance to be connected for the above condition. (b) The current in the field and armature for the new condition.

A DC shunt motor is fed with a 240 V supply. The shunt field winding resistance is 100 and armature resistance is 0.04 . The machine draws a current of 200 A from the supply and runs at 1200 rpm. Determine

(a) The value of external resistance to be inserted in the field circuit to increase the speed to 1500 rpm with torque requirement being unchanged.

(b) With the above inserted resistance in place, calculate the new speed of the machine if the load torque is reduced to half.

A DC series lift motor is fed with a 240 V supply. The machine has total resistance of 0.2 and takes a current of 40 A from the supply at a speed of 1800 rpm. Determine

(a) The resistance to be inserted in series to increase the speed to 3600 rpm when the current drawn is 10A, assuming the magnetic circuit of the machine to be linear till 40 A.

(b) The resistance to be inserted to make the speed 900rpm when the supply current is 60 A, given that the flux at 60 A is 18 % greater than that at 40 A.

(c) The speed of the motor when connected directly to the mains (without inserting external resistance) and the supply current being 60 A.

A 460 V, 60 Hz, 8 pole , star connected three phase induction machine has the following approximate

equivalent circuit parameters: RC=50 ; Xm=6 ; R1=0.076 ; R2=0.06 ; Xe=0.3893 . The machine was operating in the motoring mode at a slip of 0.05 when plugging operation was initiated.

Determine

(a) Braking torque at the moment of plugging. (b) Braking torque when the machine reaches zero speed. (c) Explain the cause of less braking torque at the moment of plugging compared to the

motoring torque, even when the rotor current is many fold at the moment of plugging.

Marks

[5]

[5]

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[3]

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Q3.

Q4.(A)

Q4.(B)

Q5.(A)

Q5.(B)

A first quadrant chopper is used to feed the armature of a separately excited DC machine required to

be run under constant speed variable torque condition. The supply voltage of the chopper circuit is

220 V. The parameters of the DC machine are : Ra=3 , La=3 mH. The chopping frequency of the chopper is 500 Hz. At D=0.7, the motor runs at 1500 rpm with a back emf of 80 V. Determine

(a) Average torque produced by the machine when running at 1500 rpm. (b) Calculate the minimum average torque that machine can provide at a speed of 1500 rpm

with chopper operating in continuous conduction mode. What will be the duty ratio of the

chopper for this condition?

(c) Calculate the armature current ripple in part (a) and (b) above. (d) Discuss the various methods to reduce the torque ripple in chopper fed DC drives. What will

be the current ripple in part (a) if the chopping frequency of the chopper is doubled?

The armature circuit of a separately excited DC motor is connected to a single phase full wave

fully controlled thyristorized converter fed from = 220 2 sin(200) .The DC motor draws constant rippled DC current from the converter and has a ripple free back emf Eb=140.106 V.

Neglecting the armature resistance Ra and taking the armature inductance to be 40mH, find the

current ripple in the armature current of the DC machine. Assume the firing angle of the converter to

be 30 degrees.

Derive the expression for the AC side source current THD of a single phase semi controlled converter

feeding a DC machine. Assume that the machine is drawing continuous ripple free current from the

DC side of the converter. Also draw the waveforms of the source current. Discuss the advantages and

disadvantages of using single phase fully controlled converter in place of semi converter.

Discuss in detail the advantages of using the d-q model of the induction machine over abc model for

the purpose of vector control. Also derive the voltage equations of the induction machine in the d-q

model in stationary reference frame.

Explain the closed loop control of self controlled synchronous motor drive. What are the advantages

of the operation in the self control mode?

[2]

[5]

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[10]

[10]

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