JNTU
JNTU B.Tech II ELECTRICAL MACHINES II Previous Question Paper (July/August - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
Describe the constructional features of both slip ring and squirrel cage induction motor. Discuss the merits of one over the other.
Answer
Both types of three-phase induction motors consist of a stationary stator and a rotating rotor. The stator is identical for both, comprising a laminated core with a three-phase winding supplied by an AC source to produce a rotating magnetic field.
The Squirrel Cage Rotor consists of a cylindrical laminated core with slots containing uninsulated copper or aluminum bars short-circuited at both ends by end rings. This construction is robust, simple, and maintenance-free. The Slip Ring (Wound) Rotor features a three-phase winding similar to the stator, with the ends connected to slip rings mounted on the shaft. External resistors can be connected to these rings via brushes to control starting torque and speed.
Merits of Squirrel Cage over Slip Ring: It is cheaper, more rugged, has higher efficiency, and requires less maintenance due to the absence of brushes and slip rings. Conversely, Slip Ring motors are preferred for applications requiring high starting torque and smooth speed control.
In case of an 8-pole induction motor the supply frequency was 50 Hz and the shaft speed was 735 rpm. Compute (i) Synchronous speed (ii) Slip speed per unit slip (iii) Percentage slip.
Answer
Given: Poles (P) = 8, Frequency (f) = 50 Hz, Rotor speed (Nr) = 735 rpm.
- Synchronous speed (Ns):Ns = 120f / P = (120 × 50) / 8 = 750 rpm
- Slip (s):s = (Ns - Nr) / Ns = (750 - 735) / 750 = 15 / 750 = 0.02
Slip speed = Ns - Nr = 750 - 735 = 15 rpm.
- Percentage slip:%s = s × 100 = 0.02 × 100 = 2%
Explain the torque-slip characteristics of an induction motor.
Answer
Sign in to view the complete answer paper.
Derive the torque equation of an induction motor. Derive the condition for maximum torque.
Answer
Sign in to view the complete answer paper.
Derive an expression for e.m.f induced per phase in a 3-phase alternator? Mention how different winding factors affect the induced e.m.f.
Answer
Sign in to view the complete answer paper.
The following test results are obtained from a 3 phase, 6000 kVA, 6600V, star connected, 2 pole, 50 Hz turbo alternator: With a field current of 125A, the open circuit voltage is 8000V at the rated speed; with the same field current and rated speed the short circuit current is 800A. At the rated full load, the resistance drop is 3%. Find the regulation of alternator on full load and at a p.f. of 0.8 lagging.
Answer
Sign in to view the complete answer paper.
Explain the variation of current and power factor of a synchronous motor with excitation.
Answer
Sign in to view the complete answer paper.
A 2.3 kV, 3-phase star connected synchronous motor has Zs=(0.2+j2.2) ohms per phase. The motor is operating at 0.5 power factor leading with a line current of 200 A. Determine the generated e.m.f per phase.
Answer
Sign in to view the complete answer paper.
Describe the construction, working principle, characteristics and applications of a shaded pole motor.
Answer
Sign in to view the complete answer paper.
Explain clearly what is meant by synchronous impedance and synchronous reactance of an alternator and how it can be determined experimentally.
Answer
Sign in to view the complete answer paper.
Discuss the advantages of deep bar rotors in an induction motor.
Answer
Sign in to view the complete answer paper.
What are the causes of harmonics in the voltage waveform of an alternator? How can these be minimized?
Answer
Sign in to view the complete answer paper.
Explain the effect of armature reaction on the performance of an alternator. How it depends on the load power factor? Explain with suitable diagrams.
Answer
Sign in to view the complete answer paper.
What are various methods of making synchronous motors self starting? Briefly explain.
Answer
Sign in to view the complete answer paper.
List out the applications of synchronous motor.
Answer
Sign in to view the complete answer paper.