Classification of DC Generator

CLASSIFICATION OF GENERATORS

Self-excited generators are classed according to the type of field connection they use. There are three general types of field connections — SERIES-WOUND, SHUNT-WOUND (parallel), and COMPOUND-WOUND. Compound-wound generators are further classified as cumulative-compound and differential-compound. These last two classifications are not discussed .

Series-Wound Generator

In the series-wound generator, shown in figure 1-15, the field windings are connected in series with the armature. Current that flows in the armature flows through the external circuit and through the field windings. The external circuit connected to the generator is called the load circuit.

                Series-wound generator.

A series-wound generator uses very low resistance field coils, which consist of a few turns of large diameter wire. The voltage output increases as the load circuit starts drawing more current. Under low-load current conditions, the current that flows in the load and through the generator is small. Since small current

means that a small magnetic field is set up by the field poles, only a small voltage is induced in the armature. If the resistance of the load decreases, the load current increases. Under this condition, more current flows through the field. This increases the magnetic field and increases the output voltage. A series-wound dc generator has the characteristic that the output voltage varies with load current. This is undesirable in most applications. For this reason, this type of generator is rarely used in everyday practice. The series-wound generator has provided an easy method to introduce you to the subject of self-excited generators.

Shunt Wound Generator

In a shunt-wound generator, like the one shown in figure 1-16, the field coils consist of many turns of small wire. They are connected in parallel with the load. In other words, they are connected across the output voltage of the armature.

             Shunt-wound generator.

Current in the field windings of a shunt-wound generator is independent of the load current (currents in parallel branches are independent of each other). Since field current, and therefore field strength, is not affected by load current, the output voltage remains more nearly constant than does the output voltage of the series-wound generator. In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current

varies. The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR). In a series-wound generator, output voltage varies directly with load current. In the shunt-wound generator, output voltage varies inversely with load current. A combination of the two types can overcome the disadvantages of both. This combination of windings is called the compound wound dc generator.

Compound-Wound Generators

Compound-wound generators have a series-field winding in addition to a shunt-field winding, as shown in figure  The shunt and series windings are wound on the same pole pieces.In the compound-wound generator when load current increases, the armature voltage decreases just as in the shunt-wound generator. This causes the voltage applied to the shunt-field winding to decrease,

which results in a decrease in the magnetic field. This same increase in load current, since it flows through the series winding, causes an increase in the magnetic field produced by that winding. 

            Compound wound dc generator

By proportioning the two fields so that the decrease in the shunt field is just compensated by the increase in the series field, the output voltage remains constant. This is shown in figure 1-18, which shows the voltage characteristics of the series-, shunt-, and compound-wound generators. As you can see,

by proportioning the effects of the two fields (series and shunt), a compound-wound generator provides a constant output voltage under varying load conditions. Actual curves are seldom, if ever, as perfect as shown.

Network Terminology

Network theorem are applied to analyse the electrical network wild discus these theorems one come across the following terms 

1. Electric network:- A combination of various electric element connected in any manner is called electric network.

2. Electric Circuit:- A electric circuit is a closed conducting path through which an electric current either  flows or intended to flow.

3. Parameters:- The various element in electric circuit are called its parameters such as resistor,  inductor capacitors.

4. Linear Circuit :- An electric circuit that contains parameters of constant value , thats their value did not changed with the voltage and current known Linear circuit.

5. Non Linear Circuit :-  An electric circuit that contains parameter  whose value changed with voltage or current known  Non Linear Circuit.

6. Bilateral Circuit:- An electric circuit that possesses the same properties of characteristics in either direction called bilateral circuit. A transmission line is bilateral because it can be made to perform both side equally well in either direction.

7. Unilateral circuit :- An electric circuit that whose properties or characteristics changed with the direction of flow of electric charge called unilateral circuit  a diode rectifier circuit is unilateral circuit .

8. Unilateral Element:- An element which conduct in any one direction called unilateral Element e.g.semiconductor diode.

9. Bilateral Element:- An element which conduct in both direction similarly  called bilateral element e.g. resistor , diac, triac.

10. Active Network:- An electric network that contains one or more source of emf called active network.

11. Passive Network:- An electric Network that does not contain any source of emf called passive network.

12. Node :- A node is a point in the network where two or more circuit element are joined .

13. Junction:- A junction is a point in the network where there or more circuit element are joined in fact it is a point where current is divided.

14. Branch :- The part of a network  that is lies between two junction point is called branch . 

15. Loop:- The closed path of the network is called loop.

16. Mesh:- The most elementary from of a loop that can not further divided is called Mesh .

static characteristics of a thyristor

Important Points About The V-I Characteristics of SCR

Forward Characteristics
When anode is positive w.r.t. cathode, the curve between V and I is called the forward characteristics.

In fig.1, OABC is the forward characteristics of SCR at IG=0.

If the supply voltage is increased from zero, a point reached (point A) when the SCR starts conducting.

Under this condition,the voltage across SCR suddenly drops as shown by dotted curve AB and most of supply voltage appears across the load resistance RL .

If proper gate current is made to flow, SCR can close at much smaller supply voltage.

Reverse Characteristics
When anode is negative w.r.t. cathode, the curve between V and I is known as reverse characteristics.

The reverse voltage does come across SCR when it is operated with a.c. supply.

If the reverse voltage is gradually increased, at first the anode current remains small (i.e. leakage current) and at some reverse voltage, avalanche breakdown occurs and the SCR starts conducting heavily in the reverse direction as shown by the curve DE.

This maximum reverse voltage at which SCR starts conducting heavily is known as reverse breakdown voltage.

SCR in Normal Operation
In order to operate the SCR in normal operation, the following points are kept in view:

The supply voltage is generally much less than breakover voltage.
The SCR is turned on by passing appropriate amount of gate current ( a few mA) and not by breakover voltage.
When SCR is operated from a.c.  supply, the peak reverse voltage which comes during negative half-cycle should not exceed the reverse breakdown voltage.
When SCR is to be turned OFF from the ON state, anode current should be reduced to holding current.
If gate current is increased above the required value, the SCR will close at much reduced supply voltage.
Important Terms In The V-I Characteristics of SCR
The following terms are much used in the study of SCR :

Breakover voltage
Peak reverse voltage
Holding current
Forward current rating
Circuit fusing rating
1.  Breakover Voltage
It is the minimum forward voltage, gate being open, at which SCR starts conducting heavily i.e. turned on.

Thus, if the breakover voltage of an SCR is 200 V, it means that it can block a forward voltage  (i.e. SCR remains open) as long as the supply voltage is less than 200 V. If the supply voltage is more than this value, then SCR will be turned on.

In practice, the SCR is operated with supply voltage less than breakover voltage and it is then turned on by means of a small voltage applied to the gate.

Commercially available SCRs have breakover voltages from about 50 V to 500 V.

2.  Peak Reverse Voltage (PRV)
It is the maximum reverse voltage  (cathode positive w.r.t. anode) that can be applied to an SCR without conducting in the reverse direction.

PRV is an important consideration while connecting an SCR in an a.c. circuit. During the negative half of a.c. supply, reverse voltage is applied across SCR. If PRV is exceeded, there may be avalanche breakdown and the SCR will be damaged if the external ciruit does not limit the current.

Commercially available SCRS have PRV ratings upto 2.5 kV.

3.  Holding Current
It is the maximum anode current, gate being open, at which SCR is turned OFF from ON condition.

When SCR is in the conducting state, it can not be turned OFF even if gate voltage is removed.

The only way to turn off or open the SCR is to reduce the supply voltage to almost zero at which point the internal transistor comes out of saturation and opens the SCR.

The anode current under this condition is very small (a few mA) and is called holding current.

Thus, if an SCR has a holding current of 5mA, it means that if anode current is made less than 5 mA, then SCR will be turned off.

4.  Forward Current Rating
It is the maximum anode current that an SCR is capable of passing without destruction.

Every SCR has a safe value of forward current which it can conduct. If the value of current exceeds this value, the SCR

SCR Thyristor summary

Silicon Controlled Rectifiers known commonly as Thyristors are three-junction PNPN semiconductor devices which can be regarded as two inter-connected transistors that can be used in the switching of heavy electrical loads. They can be latched-“ON” by a single pulse of positive current applied to their Gate terminal and will remain “ON” indefinitely until the Anode to Cathode current falls below their minimum latching level.

Static Characteristics of a Thyristor SCR

✔Thyristors are semiconductor devices that can operate only in the switching mode.
✔Thyristor are current operated devices, a small Gate current controls a larger Anode current.
✔Conducts current only when forward biased and triggering current applied to the Gate.
✔The thyristor acts like a rectifying diode once it is triggered “ON”.
✔Anode current must be greater than holding current to maintain conduction.
✔Blocks current flow when reverse biased, no matter if Gate current is applied.
✔Once triggered “ON”, will be latched “ON” conducting even when a gate current is no longer applied providing Anode current is above latching current.

What is silica gel and its purpose

Function of Silica Gel Breather

Most of the power generation companies use silica gel breathers fitted to the conservator of oil filled transformers. The purpose of these silica gel breathers is to absorb the moisture in the air sucked in by the transformer during the breathing process.

What is Transformer Breathing?

When load on transformer increases or when the transformer under full load, the insulating oil of the transformer gets heated up, expands and gets expel out in to the conservator tank present at the top of the power transformer and subsequently pushes the dry air out of the conservator tank through the silica gel breather. This process is called breathing out of the transformer.

When the oil cools down, air from the atmosphere is drawn in to the transformer. This is called breathing in of the transformer.

Use of Silica gel breather

During the breathing process, the incoming air may consist of moisture and dirt which should be removed in order to prevent any damage. Hence the air is made to pass through the silica gel breather, which will absorb the moisture in the air and ensures that only dry air enters in to the transformer. Silica gel in the breather will be blue when installed and they turn to pink colour when they absorb moisture which indicates the crystals should be replaced. These breathers also have an oil cup fitted with, so that the dust particles get settled in the cup.

Thus Silica gel breathers provide an economic and efficient means of controlling the level of moisture entering the conservator tank during the breathing process.