Complete Details of excitation current in an Electrical Alternator or Generator

Excitation current is crucial for an alternator or generator. It is responsible to generate and maintain a voltage in alternator terminal which is the must requirement for continuous generation of electricity.  

Here we are talking about large scale alternators of generators are used in power plants and other heavy applications. In those system, Sophisticated automation system is utilized for supply and control of excitation current.

What is excitation current of an Alternator or Generator

Already we know that for generation of electricity, at first a voltage is to be generated in the stator. With a voltage generated, electricity shall flow by completing the circuit. Our following discussion will be on the basis that, stator is fixed and rotor is rotating. This type of arrangement is used for most of the alternator and generator. For generation of electrical voltage in a circuit, three things are required-

1. Magnetic flux: Magnetic circuit is placed in the rotor. Required magnetic flux is created in the rotor by supplying DC (Direct Current) to the rotor circuit. This Direct Current (DC) is called excitation current. Which is required to generate magnetic flux in the rotor circuit.

2. Conductor: Conductor is placed in the stator. It is placed as coil comprise of lot of conductor for current carrying capacity concern.

3. Relative motion between the magnetic flux and conductor: Relative motion is availed through rotor coupled with a prime mover which would be engine, turbine etc.
So we can say that the current supplied to the rotor circuit to produce magnetic flux is called the excitation current for an alternator. This overall component is called the Exciter machine.

Should the excitation current be fixed or adjustable in Electrical Alternator or Generator:

An alternator operates in varying load condition and situations. And we know that during operation, the alternator terminal voltage is to be keep constant within the range. It can be fluctuating. The current might be varying, but terminal voltage to be remain same or constant within a permissible range.

The excitation current is the current that generates the terminal voltage. Rating of excitation current controls the terminal voltage level. Thus, to keep the terminal voltage constant or fixed, the excitation current is to be variable and adjustable.

So, excitation current has to be adjustable.

Source of excitation current in Electrical Alternator or Generator:

The excitation current is supplied to the rotor through a device called Automatic Voltage Regulator (AVR). The source of the excitation current varies from manufacturer and from different design. In some cases, the excitation current would be supplied from an external battery or generator.

In some cases, an additional winding is placed in the main winding of stator which generates and supplies the current to the Automatic Voltage Regulator (AVR).
The source of excitation current is not directly Direct current (DC). It is basically Alternating current ( AC). The Alternating current is then converted to Direct current (DC) before fed in to the rotor circuit.

In some cases, additional arrangement is done to cope up transient conditions. These are shunt boost connection with large amount of excitation current boosted.

How the excitation current is controlled in Electrical Alternator or Generator :

The excitation current is supplied from the device called AVR (Automatic Voltage regulator). It regulates the excitation current thus controls the generator terminal voltage.

1. AVR (Automatic Voltage regulator) is fed with Alternating current (AC).
2. The source of Alternating current varies from design to design and by different arrangement.
3. AVR (Automatic Voltage regulator) is sophisticated device that supply excitation current by real time calculation. It takes data input from various source such as terminal voltage, load condition, load demand etc.

How excitation current is supplied to the rotor circuit in Electrical Alternator or Generator :

Now we know that, the excitation current is nothing but the Direct current (DC) supplied to the rotor circuit to generate magnetic flux. And, it is needed to be adjustable. So, excitation current from outside is to be supplied to the rotating rotor. This is done through the two method brush less excitation system and Static excitation system.

Types of excitation system in an Alternator

The two most engineered excitation systems are: Brushless excitation and static excitation.

As we already know, rotor is rotating and stator is fixed. The magnetic field circuit is placed on the rotor which is moving. Direct current ( DC) is to be supplied to this rotor circuit for creating magnetic field. Here lies the main challenge. How to supply this Direct current. There are two types of arrangement available.

1: Brush less Excitation system in Alternator or Generator

In this case, induction method is used. At first AC excitation current is converted to DC by rectifier set. Then this Direct current (DC) is fed in to a magnetic circuit which produce magnetic field. The rotor has a three phase winding which interacts with the magnetic field. Thus induction current is generated in the rotor. Then this current is converted to Direct current (DC) by rectifier arrangement. Afterwards, this Direct current is fed in to the rotor magnetic circuit. In this method there is no physical contact between the external excitation current supply and the rotating rotor.

When a brushless excitation system is used, a rotating machine is coupled to the generator non driven shaft, supplying the excitation current. This machine, called exciter machine, has

1. in the rotor a 3-phase winding and
2. in the stator an excitation (field) winding connected to direct current.

Therefore, an alternating current is induced into the rotor winding.

The induced alternating current at the terminals of the rotating 3-phase winding must be rectified. This happens with a 3-phase diode bridge fixed on the shaft and thus rotating. The resulting direct current is fed directly to the generator’s rotor winding.
The direct current for the stator winding of the exciter is supplied via

1. the generator terminals,
2. through an excitation transformer and
3. a rectifier set.

Advantage of brush less excitation system in Alternator or Generator

A brush less excitation system has a relative long response characteristic (time constant), compared to the static excitation system.

2: Static excitation system in Alternator or Generator

In this case direct current is fed in to the rotor magnetic circuit. There is physical contact between the external circuit to the rotating rotor. The Direct current is transferred through slip ring arrangement. This Direct current ( DC) is usually transformed from Alternating current ( AC). It is done beforehand in the electric panel or other place. But Direct current is only supplied to the Rotor circuit.

The excitation current must be transferred from the excitation system through brushes onto the rotor winding. The brushes transfer the direct current via two isolated slip rings to the rotor winding. The brush gear is mounted at the non-driven end of the generator; the components are covered with an air cooled casing.

Advantage of Static excitation system in Alternator or Generator :

A static excitation system has a quick response time in case of a required sudden change of the exciter current, that compared to the brush less excitation system.