JNTU
JNTU B.Tech II ELECTRICAL TECHNOLOGY Previous Question Paper (March - 2022 - R16)
Answer any five questions. All questions carry equal marks.
Main
Explain the constructional aspects of a DC Machine with a neat diagram.
Answer
A DC machine consists of two main parts: the Stator (stationary part) and the Rotor (rotating part, also called the armature).
- Yoke: The outer frame made of cast iron or steel that provides mechanical protection and carries the magnetic flux produced by the poles.
- Pole Core and Pole Shoe: The pole core carries the field winding, while the pole shoe spreads the flux in the air gap to reduce reluctance and supports the field coils.
- Field Windings: Coils wound around the pole cores that, when energized, produce the main magnetic field.
- Armature Core: A cylindrical drum made of laminated silicon steel stampings to reduce eddy current losses. It contains slots to house the armature conductors.
- Commutator: A cylindrical structure made of wedge-shaped hard-drawn copper segments insulated by mica. It acts as a mechanical rectifier in a generator and an inverter in a motor.
- Brushes and Bearings: Carbon brushes collect current from the commutator and transfer it to the external circuit. Bearings support the rotating shaft.
Derive the torque equation of a DC Motor.
Answer
The torque developed in a DC motor is derived from the mechanical force exerted on the armature conductors.
Let P be the number of poles, Z the total number of armature conductors, A the number of parallel paths, Φ the flux per pole in Webers, and Ia the total armature current.
The force on a single conductor is given by F = BIl, where B is the flux density, I is the current per conductor (Ia/A), and l is the length of the conductor. The torque T is the product of force and radius r of the armature.
- Total force F = B(Ia/A)lZ
- Torque T = F × r = B(Ia/A)lZr
- Since B = Φ / Area and Area = (πDl)/P, substituting these values leads to the final expression:
This can be simplified as T = KaΦIa, where Ka = (PZ)/(2πA) is a constant for a given machine.
Classify the various types of DC Generators and explain each one of them in brief with an example of each type.
Answer
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Explain the procedure of conducting Swinburne’s test on d.c. motor.
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Derive the emf equation of a Single-phase transformer.
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A single phase 50 Hz core type transformer has core of cross section area of 400 sq. cms. The permissible maximum flux density is 6 Wb / m2. Calculate the number of turns per limb on the high and low voltage sides for a 3000 / 220 V ratio.
Answer
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Draw and explain the equivalent circuit of a Single-phase transformer.
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Calculate the efficiencies at half and full load of a 125 KVA transformer for power factor of (i) unity and (ii) 0.8 lagging. The copper loss is 1500 watt at full load and the iron loss is 800 watts.
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Draw and explain the working of a three Phase induction motor.
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Explain the following starting methods of a three-phase induction motor: i) Direct on – line starter ii) Auto transformer starting
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A 6-pole, 3-phase, 50 Hz Induction motor develops mechanical power of 50 hp and has mechanical loss of 1600 watts at a speed of 960 rpm when connected to 520 volts. The power factor is 0.8 lagging. Determine (a) rotor copper loss (b) total input to stator if stator loss is 1800 watts (c) the line current (d) efficiency.
Answer
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Explain the constructional details of the Alternator with a neat sketch and required labeling.
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A 4-pole, 3-phase, 50-Hz, star-connected alternator has 60 slots, with 80 conductors, find the line voltage induced for a flux per pole of 0.943 Wb distributed sinusoidally in space. All the turns per phase are in series. Assume Kc = 0.95 and Kd = 0.96.
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Explain the working of a Split phase induction motor with a neat diagram.
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Explain the working of AC Servo motor with a neat diagram.
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