Top Electrical Engineering Interview Questions and Answers (Part 3)

DC Motors

A DC motor converts electrical energy into rotational mechanical energy. The rotation is produced by the interaction of magnetic fields generated by the motor's windings and the current flowing through them. DC motors are often used in applications requiring precise speed control.

Applications of Electric Motors

Electric motors are used in a wide range of applications, including:

  • Industrial machinery (fans, pumps, conveyors).
  • Machine tools.
  • Household appliances.
  • Automotive systems.
  • Robotics.

Universal Motors

A universal motor is a type of motor that can operate on both AC and DC power supplies. This is because the current in both the field and armature windings reverses simultaneously.

Advantages and Disadvantages of AC Motors

Advantages Disadvantages
Relatively low cost; long lifespan; high efficiency; readily available in various sizes and ratings. Requires variable frequency drives for speed control; high starting current.

Advantages and Disadvantages of DC Motors

Advantages Disadvantages
High efficiency; precise speed control; high torque at low speeds. Higher initial cost; require a controller; generally more expensive to maintain.

Slip Speed in AC Motors

Slip speed is the difference between the synchronous speed (the speed of the rotating magnetic field) and the actual rotor speed in an induction motor. It's expressed as a percentage.

Slip Speed Formula

Slip (s) = (Ns - Nr) / Ns

Advantages of Skewing Rotor Conductors

Skewing (slightly offsetting) rotor conductors in induction motors helps reduce noise and torque pulsations. It also prevents cogging (magnetic locking between rotor and stator teeth).

Transformer Losses

Transformers experience two main types of losses:

  • Iron Losses (Core Losses): Hysteresis losses and eddy current losses in the transformer's core.
  • Copper Losses (I²R Losses): Losses due to resistance in the windings.

Iron Losses (Hysteresis and Eddy Currents)

Hysteresis loss is due to the energy required to repeatedly magnetize and demagnetize the core. Eddy current loss is due to circulating currents induced in the core by the changing magnetic field.

Copper Losses

Copper losses are caused by the resistance of the transformer windings. They are proportional to the square of the current flowing through the windings.

Dielectric Losses

Dielectric losses are caused by energy dissipation in the insulating materials (transformer oil, solid insulation) within a transformer due to the changing electric fields.

Open-Circuit and Short-Circuit Tests

Open-circuit and short-circuit tests are performed on transformers to determine their parameters (like core losses, copper losses, and equivalent circuit components) without applying a full load to the transformer.

Additional Interview Resources

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