Electrical Machines - short answer questions from AMIE exams (Winter 2022)

 Answer the following in brief:

Find the angle by which no-load current in a transformer lags the applied voltage.

No-load current is nothing but the vector summation of these two currents. Hence, the no-load current will not lag behind applied voltage by exactly 90° but it lags somewhat less than 90°. It is in practice generally about 75°

At what power factor, transformer maximum efficiency occurs for a constant load current.

Unity power factor.

What are the aims and objectives of parallel operations of DC generators.

  • Increased Power Capacity
  • Improved Reliability and Flexibility
  • Enhanced Efficiency
  • Ease of Maintenance
  • Future Expansion

Explain vector grouping of transformers.

Vector grouping is a method for classifying three-phase transformers based on their winding connections and phase shifts. It is a crucial aspect of transformer design and is particularly important when connecting multiple transformers in parallel.

The vector group designation consists of two or three letters, each representing a specific characteristic of the transformer's winding connections. The first letter indicates the high voltage (HV) winding connection, either 'Y' for star or 'D' for delta. The second letter denotes the low voltage (LV) winding connection, also using 'Y' for star or 'D' for delta.

What are various industrial applications of salient pole synchronous machines.

  • Hydropower Plant
  • Power Factor Correction
  • Industrial Drives
  • Wind Power Generation
  • Emergency Power Generation

Describe the circle diagram of induction motors.

The diagram provides information which is not provided by an ordinary phasor diagram. A phasor diagram gives relation between current and voltage only at a single circuit condition. If the condition changes, we need to draw the phasor diagram again. But a circle diagram may be referred to as a phasor diagram drawn in one plane for more than one circuit conditions. 

On the context of induction motor, which is our main interest, we can get information about its power output, power factor, torque, slip, speed, copper loss, efficiency etc. in a graphical or in a diagrammatic representation.

Derive expression for pull out torque of three phase induction motor.

The pull-out torque of a three-phase induction motor is the maximum torque that the motor can develop before it stalls.

{T_{\max }} = 3V_{th}^2/2{w_{sync}}[{R_{th}} + \sqrt {(R_{th}^2 + {{({X_{th}} + {X_2})}^2}} ]

Compare performance of DC and AC servo motors.

AC servo System

  • Efficiency is low
  • Low power output
  • It requires less maintenance
  • Less stability problems
  • Smooth operation
  • It has non-linear characteristics

DC servo System

  • Efficiency is high
  • High power output
  • It requires frequent maintenance
  • More stability problems
  • Noisy operation
  • It has linear characteristics

What are applications of Series Motor?

  • Cranes and Hoists
  • Elevators
  • Electric Traction
  • Vacuum Cleaners
  • Sewing Machines
  • Power Tools

Discuss Potier Reactance of an alternator.

The Potier triangle determines the voltage regulation of the machines. This method depends on the separation of the leakage reactance of armature and their effects. The graph of the Potier triangle is shown in the figure below. The triangle formed by the vertices a, b, c has shown below in the figure is called Potier Triangle.

 In figure XL is Potier reactance.


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