electric generators - Electrical Engineering Gate

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Friday, October 10, 2014

electric generators

A generator is a device that converts mechanical energy to electrical energy for use in an external circuit. The source of mechanical energy may vary widely from a hand crank to an internal combustion engine. Generators provide nearly all of the power for electric power grids
The reverse conversion of electrical energy into mechanical energy is done by an electric motor, and motors and generators have many similarities. Many motors can be mechanically driven to generate electricity and frequently make acceptable generators

Types of generator
Essentially, there are two basic types of generators
• DC generators 

 AC generators: Asynchronous (Induction) generators and Synchronous generators 

The induction generator is nothing more than an induction motor driven above its synchronous speed by an amount not exceeding the full load slip the unit would have as a motor. Assuming a full load slip of 3%, a motor with a synchronous speed of 1200 rpm would have a full load speed of 1164 rpm. This unit could also be driven by an external prime mover at 1236 rpm for use as an induction generator

The induction generator requires one additional item before it can produce power – it requires a source of leading VAR’s for excitation. The VAR’s may be supplied by capacitors (this requires complex control) or from the utility grid. Induction generators are inexpensive and simple machines, however, they offer little control over their output. The induction generator requires no separate DC excitation, regulator controls, frequency control or governor

Synchronous generators are used because they offer precise control of voltage, frequency, VARs and WATTs. This control is achieved through the use of voltage
regulators and governors. A synchronous machine consists of a stationary armature winding (stator) with many wires connected in series or parallel to obtain the desired terminal voltage
.The armature winding is placed into a slotted laminated steel core

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