Hall Ticket No:
Question Paper Code: A3211
VARDHAMAN COLLEGE OF ENGINEERING
(AUTONOMOUS) B. Tech IV Semester Regular/Supplementary Examinations, May 2018
(Regulations: VCE-R15) ELECTRICAL MACHINES-II
(Electrical and Electronics Engineering) Date: 22 May, 2018 FN
Time: 3 hours
Max Marks: 75
Answer ONE question from each Unit
All Questions Carry Equal Marks
Unit I
1.
Draw the exact equivalent circuit of 3-phase induction motor and obtain the relationship between:
i. Rotor copper loss and rotor power input
ii. Mechanical power developed and rotor copper loss
6M
Draw the torque-speed or torque-slip characteristics of a 3-phase induction machine and mark the regions of operation, operating point, starting torque and maximum torque on it.
9M
2.
Mention advantages and disadvantages of three phase slip ring induction motor.
6M
A 6-pole, 50Hz three phase induction motor has a rotor resistance of 0.25Ω per phase and a maximum torque of 10Nm at 875rpm. Calculate:
i. The torque when the slip is
ii. The resistance to be added to the rotor circuit to obtain 60% of the maximum torque at starting. Neglect stator impedance
9M
Unit II
3.
Discuss the necessary tests to be conducted for constructing the circle diagram of a three phase slip ring induction motor.
10M
Find the percentage tapping required on an auto transformer for a squirrel cage motor to start the motor against of full load torque. The short circuit current on normal voltage is four times the full load current and full load slip is 3%.
5M
4.
With the help of a neat circuit diagram, explain the working of DOL method of starting of three phase induction motor.
8M
Mention the different methods of starting of 3-phase induction motor. Explain method of speed control of 3-phase induction motor.
7M
Unit III
5.
Explain with the help of phasor diagram and from OC SC tests, how the percentage regulation can be determined from synchronous impedance method.
5M
Calculate the rms value of the induced emf per phase of a 10 pole, 3-phase 50Hz alternator with 2 slots per pole per phase and four conductors per slot in two layers. The coil span is 1500. The flux per pole has a fundamental component of 0.12wb and a 20% third harmonics component.
10M
6.
With neat sketches of salient pole and non salient pole rotor, bring out the difference between salient pole and non-salient pole rotor alternator.
7M
The data obtained on 100kva, 1100V, three phase alternator is:
DC resistance test: E between line 6V D.C, I in lines 10A D.C
Open circuit test: Field current 12.5A D.C., line voltage 420V A.C
Short circuit test: Field current 12.5A, line current rated value.
Calculate the voltage regulation of alternator at 0.8p.f lagging.
8M
Cont…2
2
Unit IV
7.
What are the advantages of connecting the alternators in parallel? What conditions are required to be fulfilled for the successful parallel operation of alternators?
7M
A 75kW, 400V, 4-pole, 3-phase star connected synchronous motor has a resistance and synchronous reactance per phase of 0.04Ω and 0.4Ω respectively. Compute for full load 0.8p.f lead, the open circuit emf per phase and gross mechanical power developed. Assume an efficiency of 92.5%.
8M
8.
Explain the principle of operation of a synchronous motor.
6M
Two A.C. generators running in parallel supply a lighting load of 2000kw and a motor load of 4000kw at 0.8 p.f lagging. One machine is loaded to 2400kw at 0.95 p.f lagging. What is the output and power factor of the second machine?
9M
Unit V
9.
Draw the circuit diagram, phasor diagram and torque-speed characteristic of a two value capacitor phase split single phase induction motor and mention any two applications of this induction motor.
9M
The main and auxiliary winding currents of a 50Hz, 4-pole, 373W, 100V single-phase induction motor are 6A at power factor of 0.766 lagging and 4A at power factor of 0.9659 lagging. Determine total current, power factor, efficiency and shaft torque during running conditions if the motor is running at a speed of 1450rpm.
6M
10.
Draw the labeled speed-torque characteristics of a single phase induction motor and show that it is not self starting.
8M
Draw the circuit diagram and phasor diagram of an A.C series motor. Also justify the necessity of compensating winding.
7M