JNTU
JNTU B.Tech II ELECTRICAL MACHINES II Previous Question Paper (August/September-2022 - R16)
Answer any five questions. All questions carry equal marks.
Main
Explain the constructional details of different types of 3-phase induction motor.
Answer
A 3-phase induction motor consists of two main parts: the stator and the rotor. The stator is the stationary part, comprising a laminated steel core with slots to house a 3-phase winding, which is fed from a 3-phase supply to produce a rotating magnetic field. The rotor is the rotating part, classified into two types:
- Squirrel Cage Rotor: Consists of a cylindrical core with semi-closed slots containing uninsulated copper or aluminum bars short-circuited at both ends by heavy end rings. It is robust, simple, and low-maintenance.
- Wound (Slip-ring) Rotor: Contains a 3-phase winding similar to the stator, connected in star. The terminals are brought out to slip rings mounted on the shaft, allowing external resistance to be added to the rotor circuit for speed control or high starting torque.
A 6 pole, 3-ϕ induction motor is supplied from 3-phase, 60 Hz supply. Calculate:
- Speed at which the magnetic field of the stator is rotating
- Speed of the rotor when the slip is 0.05
- Frequency of the rotor currents when slip is 0.04
- Frequency of rotor currents at stand still.
Answer
Given: Poles (P) = 6, Frequency (f) = 60 Hz.
- Synchronous Speed (Ns):Ns = (120 × f) / P = (120 × 60) / 6 = 1200 rpm
- Rotor Speed (Nr) at s = 0.05:Nr = Ns(1 - s) = 1200(1 - 0.05) = 1200 × 0.95 = 1140 rpm
- Rotor Frequency (fr) at s = 0.04:fr = s × f = 0.04 × 60 = 2.4 Hz
- Rotor Frequency at standstill: At standstill, slip (s) = 1. Therefore, the frequency of rotor currents equals the supply frequency:fr = 1 × 60 = 60 Hz
Explain the production of rotating magnetic field in a 3-Phase induction motor.
Answer
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The star connected rotor of a 3-ϕ Induction motor has a standstill impedance of (0.3+j3)Ω per phase. The motor has an induced voltage of 80V between sliprings at standstill when connected to its normal supply voltage. Find rotor current and power factor at standstill when external impedance of (4+j2)Ω per phase is connected in the rotor circuit.
Answer
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A three phase 6 pole, 50Hz star connected induction motor gave the following data (line values):
No Load: 400V, 9A and 1250W
Short circuit test: 200V, 50A and 6930W
Full load current = 32A
Stator loss at stand still = 0.55 times of total copper loss. Calculate the following:
- Starting and full load torques
- Maximum Power factor
- Maximum power output.
Answer
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What is crawling of induction motor? How crawling can be minimized?
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Explain the principle of operation of induction generator.
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Describe the method of finding the voltage regulation of an alternator by synchronous impedance method.
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A 10 MVA, 11 kV, 50 Hz, 3-phase, star-connected synchronous generator is driven at 300 r.p.m. The armature winding is housed in 360 slots with 6 conductors per slot. The coil span is five-sixth of a pole pitch. Calculate the flux per pole required to give 11 kV line voltage on open circuit.
Answer
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Describe the method to determine direct axis and quadrature axis reactances of salient pole alternator. Why salient pole alternator is preferred in hydroelectric power stations?
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What do you mean by synchronizing of alternators? Describe anyone method of synchronizing.
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Explain the principle of operation of 3-phase synchronous motor.
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Explain the construction and working principle of 1-ϕ capacitor start and run Induction motor. Discuss its applications.
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