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Synchronous Generator

Synchronous Generator

Synchronous Generators are that converts mechanical energy into electrical energy as every generator does, but in its case, the output is synchronized with the input. In simple words, the frequency of the generated voltage will be synchronized with the mechanical rotation. The faster you turn the rotor, the higher the frequency you get. We will discuss this in detail.

Parts of Synchronous Generator:

Winding:

There are two types of winding in a Synchronous Generator.

Armature Winding:

The winding in which the voltage is induced is termed as Armature Winding. The armature winding of the synchronous generator is located at the stator. This is a three-phase winding in which three phase voltages are induced. The load is then connected with the terminals of this winding.

Field Winding:

This is the winding which produces the main magnetic flux. It is located at the rotor. To create the magnetic field a DC voltage is applied to the Filed Winding.

Production of Magnetic Field:

The magnetic field is produced by applying DC voltage on the field winding which is placed on the rotor. There are two methods to apply DC at Field Winding.

  1. External DC source and Slip Rings
  2. Brushless Exciter

External DC source and Slip Rings:

The simplest method is that an external DC source is connected with the Filed Winding with the help of Slip Rings.

Slip Rings:

Slips rings are a pair of conducting rings placed on the rotor. These rings are insulated from the rotor shaft. The rings collect the current via carbon brushes. One terminal of the DC source is connected to one slip ring through a carbon brush and the other terminal is connected with the other slip ring in the same manner. Further, these slip rings are connected with the terminals of the Field Winding. The slip rings are to ensure the supply to Filed Winding when the rotor is moving.

 

Brushless Exciter:

Another way to give excitation is by mounting a generator inside the Synchronous Generator. For this purpose, a small AC generator is placed having its Armature winding on the rotor and field winding on the stator. This small generator is known as Brushless Exciter. The Armature winding is placed directly on the rotor as the voltage is induced in this winding and we have to excite the Field winding of the synchronous motor. The field winding of this generator is placed on the stator to which we connect DC supply.

The working of Brushless Exciter is simple, we give DC to its field winding which is present on the stator. The field winding produces the magnetic flux. As the Armature winding is placed on the rotor, it will move with the rotor and then there will be a change of flux produced due to the movement. Due to the rate of change of flux the voltage is induced in the armature winding of the brushless exciter. This induced voltage is three-phase voltages, but we have to provide DC to the Field winding of the Synchronous generator. Therefore, a three-phase rectifier is also mounted on the stator which converts the output of the brushless exciter to DC and the output of the rectifier is then supplied to the field winding of the Synchronous Generator.

We need to provide DC supply on the field winding of the exciter and by controlling the current in this winding we can control the current in the field winding of the Synchronous Generator.

Pilot Exciter:

There is one more component that is used to eliminate the supply provided to the field winding of the exciter. This component is known as Pilot Exciter. This is also an AC generator, but it has permanent magnets to produce magnetic flux. The permanent is mounted on the rotor shaft and the Armature winding is placed on the stator of the synchronous generator. Due to the movement of magnets with the rotor, a changing magnetic flux is produced, and the voltage is induced in the armature of the pilot exciter. The output of the pilot exciter is AC which rectified and then supplied to the field winding of the Brushless exciter which is also present on the stator. Due to the pilot exciter, we do not need any external source for the excitation purpose.

To summarise we can say permanent magnets of the pilot exciter is at the rotor. Due to the movement of the rotor, the voltage is induced in the armature of the pilot exciter which is then rectified and supplied to the field winding of the brushless exciter. The voltage is then induced in the armature winding of the brushless exciter which is rectified and supplied to the field winding of the synchronous generator.

Why Brushless Exciter is preferred over slip rings for large generators?

The slip ring method is easier, but it increases the losses. The reason is the carbon brushes are in mechanical contact with the slip rings which is the cause of wear and tear. This requires more maintenance. There occurs a voltage drop at brushes as well. But still, this method is used for smaller generators as it is cost effective.

The Brushless Exciter needs less maintenance as compared to slip rings and brushes, but its cost is much higher. We can include both the techniques in the generator to deal with emergency cases.

Stator:

Now we can define the stator of the synchronous generator. It is the static part which consists of,

a)      In case of Slip Rings and Brushes only
  • Armature Winding of Synchronous generator
b)     In case of Brushless Exciter
  • Armature Winding of Synchronous generator
  • Field Winding of Brushless Exciter
  • Armature Winding of Pilot Exciter

Rotor:

The rotor is the rotating parts which consist of,

a)      In case of Slip Rings and Brushes only
  • Field Winding of Synchronous generator
  • Slip Rings
b)     In case of Brushless Exciter
  • Field Winding of Synchronous generator
  • Armature Winding of Brushless Exciter
  • Permanent magnets of Pilot Exciter

Working:

The working of a generator is simple, when we apply excitation on the rotor it produces the magnetic flux. We have already discussed the methods to provide excitation. When the rotor is rotated the flux is changed in the Armature. Due to this rate of change of flux voltage is induced in the Armature Winding. This induced voltage is three-phase and the terminals are placed in outside the stator. We can now connect the load with this generator.

The internally generated EMF can be given as,

Where,

EA = Internally generated emf

Ф = Flux produced in the machine

ω = Speed of Rotor (rad/sec)

K = Machine constant

The internally generated emf is directly proportional to the flux, and the flux is directly proportional to the amount of current. It means if we want to increase the generated voltage we need to increase the flux, and the flux is increased by increasing the filed current. Therefore, by controlling the filed current we can control the generated voltage.

Armature Reaction in Synchronous Generator:

When the load is connected to the stator, current flows in the Armature coil. The flow of current produces a magnetic field. This stator magnetic field disturbs the magnetic field produced by the field winding. This effect of the armature coil is known as Armature Reaction. The net magnetic field of both the coils will be responsible for the voltage appearing at the terminal.

Due to the magnetic field of the armature coil, a voltage is produced in it which is known as Stator Voltage. Hence the voltage at the terminal will be equal to the sum of Internally generated emf and the Stator voltage.

Where,

Vф = Voltage of single phase

EA = Internally generated voltage

ES = Stator Voltage

Equivalent Circuit of Synchronous Generator:

Field winding can be represented by a resistor and an inductor. The field resistance is changed to change the excitation.

Where,

VF = Voltage of Field Source

IF = Field Current

LF = Field Inductance

RF = Field Resistance

 

There is voltage induced in the armature, therefore, there will be a voltage source in the Armature circuit to represent it. Further, there will be armature resistance as well. Due to Armature reaction, there will be stator voltage, this stator voltage lags 90˚ behind the armature current and it depends on the Armature current. Therefore, it can be represented as inductor having reactance(X) as,

By putting the value in eq (1), we will get,

The resistance and self-inductance will present in the Armature coil. Then the equation will be,

To make it compact we can represent both the reactance as their sum, i.e. XS = X + XA

The equivalent circuit will be,

Hence the complete equivalent circuit can be made as,

Star Connection:

The Star (Wye) connection can be made as,

Delta Connection:

 

Relationship between Speed, Frequency and Poles:

The frequency of the generated output can be given as,

Where,

f = Frequency of the output voltage (Hz)

n = Speed of Rotor (rev/min)

P = Number of Poles

In the synchronous generator, the frequency depends directly in the speed of rotation of the rotor. If you want to increase the frequency you can increase the speed of the rotor. The poles also have an effect on the frequency.

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