Question

What is excitation as applied to electrical machines?

Answer 1

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When a magnetic field is produced by the means of an electric current or to provide motive power it is called as excitation. It maybe generated by a permanent magnet or by electromagnet.The device which provides excittion is known as exciter.

REQUIREMENTS FOR EXCITATION

The main requirement of an excitation system is

• Reliability under all conditions of service
• Stability and fast transient response.
• A simplicity of control
• Ease of maintenance

FACTORS ON WHICH EXCITATION DEPENDS

The amount of magnetic field excitation required depends on the

• Load current
• Load power factor
• Speed of the machine.

EXCITATION IN ELECTRICAL MACHINES

If the load current is large,more excitation is needed in the system which will make the speed is less, and also the power factor of the system becomes lagging.

If the load current is small, less excitation is needed in the system which will make the speed is more, and also the power factor of the system becomes leading.

For small size machines-DC Excitation

For medium size machines-AC Excitation

For large size machines-Brushless Excitation

For small machines

If machine has field coils in it, then,the load current should flow in these coils to generate the field. If no current flows through it the power factor will not get transfered to rotor.For the rotor of the synchronous machine, DC current is required. Direct current is supplied to the rotor of the machine by a DC generator which is called Exciter (to excite the winding of the machine).

For medium size machines

AC Exciters are used in place of DC Exciter in case of medium size machines. AC Exciters have three phase AC generators. The output is rectified and supplied through the slip rings and brushes to the rotor winding of the machine

For large size machines

Unlike AC Exciters,Brushless Exciter does not contain slip rings & brushes.It is a small direct coupled AC generator with its the armature circuit on the rotor and field circuit on the stator. The output of the three phase AC exciter generator is rectified by rectifiers. Hence,The rectified output is connected directly to the field winding(eliminating the use of brushes and slip rings)

A Brushless excitation system requires less maintenance.The power loss is also reduced. The DC required for the field of the exciter itself is sometimes provided by a pilot exciter.The purposeof which is to provide the field current of the exciter. The exciter supplies the field current of the main machine. Thus, the use of a pilot exciter makes the excitation of the main generator independent of brushes and slip rings .

Hope that helps :)