Electrical Machines - short answer questions from AMIE exams (Summer 2021)

Answer the following in brief (2 marks each)

List industrial applications of servomotors.

Servo motor is a rotary actuator or linear actuator . It allows for precise control of angular or linear position, velocity and acceleration.  these are used in the machine tools, packaging, factory automation, material handling, printing converting, assembly lines. In many other demanding applications robotics, CNC machinery or automated manufacturing.

Discuss repulsion motor.

A repulsion motor is a type of single-phase electric motor that operates by the principle of repulsion between two magnetic fields. It was formerly used as a traction motor for electric trains but has been superseded by other types of motors due to its limitations.

The working principle of a repulsion motor is based on the repulsion between two magnetic fields. When an alternating current is supplied to the stator winding, it creates a rotating magnetic field. This rotating magnetic field induces an electromotive force (EMF) in the rotor winding. The EMF in the rotor winding causes a current to flow in the rotor winding. This current creates a magnetic field around the rotor. The magnetic field of the rotor interacts with the rotating magnetic field of the stator, and this interaction causes the rotor to rotate.

What is capacitor motor?

A capacitor motor is a type of single-phase induction motor that uses a capacitor to create a rotating magnetic field. This allows the motor to start and run without the need for a second winding. Capacitor motors are commonly used in household appliances, such as fans, compressors, and pumps.

Write expressions for mechanical output and efficiency of series motor.

η = Pout/(Pout + losses) x 100%

What are various tests of induction motor?

  • No load test
  • Blocked rotor test
  • Vibration test
  • Temperature test
  • Phase sequence test
  • Insulation resistance test

What are various tests of D.C motors?

  • Brake Test
  • Swinburne’s Test
  • Regenerative or Hopkinson’s Test
  • Retardation or Running Down Test
  • Field’s Test for Series Motors

Explain vector grouping in the case of transformers.

The vector group of a transformer refers to the phase relationship between the primary and secondary windings of the transformer. It is used to describe the way in which the transformer behaves under different operating conditions, such as when it is used to transform voltage or current.

A vector diagram of a transformer is a vector diagram that shows the phasor relationships between the primary and secondary voltages and currents of a transformer. It also shows the phase shift and polarity of the transformer windings

Define regulation of synchronous generator.

The voltage regulation of an alternator or synchronous generator is defined as the rise in the terminal voltage when the load is decreased from full-load rated value to zero. The speed and field current of the alternator remain constant.

Explain grounding transformer.

A grounding transformer has another name that is the earthing transformer. The grounding transformer belongs to the auxiliary transformer used in three-phase electric power systems in order to provide a ground path to either an ungrounded wye or a delta-connected system.

Draw the equivalent circuit of single phase transformer and explain it briefly.

Equivalent circuit diagram of a transformer is basically a diagram which can be resolved into an equivalent circuit in which the resistance and leakage reactance of the transformer are imagined to be external to the winding. 

The simplified equivalent circuit of a transformer is drawn by representing all the parameters of the transformer either on the secondary side or on the primary side. The equivalent circuit diagram of the transformer is shown below:


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