Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech II Year ELECTRICAL TECHNOLOGY Previous Question Paper (September - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
Explain the principle of operation of dc motor and derive its torque equation.
Answer
A DC motor operates on the principle of Lorentz force: when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. The direction of this force is determined by Fleming's Left-Hand Rule, which states that if the thumb, forefinger, and middle finger of the left hand are held mutually perpendicular, the forefinger points in the direction of the magnetic field, the middle finger in the direction of the current, and the thumb indicates the direction of the force experienced by the conductor.
In a DC motor, the field windings produce a stationary magnetic flux, and the armature windings carry current supplied through the commutator and brushes. The interaction between the armature current and the magnetic field creates a torque that causes the armature to rotate.
To derive the torque equation, consider the following parameters: P is the number of poles, Φ is the flux per pole in Webers, Z is the total number of armature conductors, A is the number of parallel paths, and Ia is the total armature current.
The force on a single conductor is given by:
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 generated by one conductor is the product of force and the radius r of the armature:
The total torque Ta produced by all Z conductors is:
Substituting the flux density B = Φ / (Area per pole) = Φ / (2πrl / P), the expression simplifies to:
Since P, Z, and A are constants for a given machine, the torque is directly proportional to the product of flux and armature current:
where Ka = (P Z) / (2π A) is the armature constant.
Explain the losses occur in a DC machine.
Answer
Losses in a DC machine are categorized into electrical, magnetic, and mechanical losses:
- Copper Losses: These are I2R losses occurring in the armature winding, field windings, and brush contact resistance. Armature copper loss is variable, depending on the load current.
- Magnetic (Iron/Core) Losses: These occur in the armature core due to the rotation of the armature in the magnetic field. They consist of Hysteresis loss (due to magnetic domain reversal) and Eddy current loss (due to circulating currents in the core). These are considered constant for a fixed speed and flux.
- Mechanical Losses: These include friction losses at bearings and the commutator, and windage loss due to air resistance against the rotating armature. These are constant at a given speed.
- Stray Load Losses: Small additional losses caused by distortion of the main flux due to armature reaction.
Explain magnetization and load characteristics of dc generator.
Answer
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What is the principal of operation of transformer and Derive an emf equation of a single phase transformer?
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Explain the losses present in the transformer.
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Write short note on the open circuit test on transformer.
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Explain the working principle of 3-phase induction motor.
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Distinguish the difference between squirrel cage and slip ring induction motor.
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Explain different starting methods of 3-Ø Induction motor.
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Explain how the regulation is predetermined using synchronous impedance method.
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Explain the construction and operation of stepper motors.
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How the shaded pole motor works explain in detail?
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