ELECTRICAL FUSE

Hello readers, I am here with the topic 'Electrical fuse' from power system. Let's start this, firstly we will discuss its contents:

INTRODUCTION : 

A fuse is a short piece of metal, inserted in the circuit, which melts when excessive current flows through it and thus breaks the circuit. The fuse element is generally made of materials having low melting point, high conductivity and least deterioration due to oxidation e.g., silver, copper etc.  It is inserted in series with the circuit to be protected. Under normal operating conditions, the fuse element is at a temperature below its melting point.  Therefore, it carries the normal current without overheating.  However, when a short-circuit or overload occurs, the current through the fuse increases beyond its rated value.  This raises the temperature and fuse element melts (or blows out), disconnecting the circuit protected by it. 

RELATABLE TERMS :

Current rating of fuse element :- It is the current which the fuse element can normally carry without overheating or melting.  It depends upon the temperature rise of the contacts of the fuse holder, fuse material and the surroundings of the fuse.

Fusing current :-  It is the minimum current at which the fuse element melts and thus disconnects the circuit protected by it.  Obviously, its value will be more than the current rating of the fuse element. For a round wire, the approximate relationship between fusing current I and diameter d of the wire is I = k d^(3/2),
 where k is a constant, called the fuse constant.  Its value depends upon the metal of which the fuse element is made. W.H. Preece found the value of k for different materials as given in the table below :

Cut-off current :- It is the maximum value of fault current actually reached before the fuse melts. 

And it may be mentioned here also that outstanding feature of fuse action is the breaking of circuit before the fault current reaches its first peak. It gives the fuse a great advantage over a circuit breaker.

Pre-arcing time :- It is the time between the commencement of fault and the instant when cut off occurs. When a fault occurs, the fault current rises rapidly and produces heat in the fuse element. When the fault current reaches the cut off value, the fuse element melts and an arc is initiated.  The time from the start of the fault to the instant the arc is initiated is known as pre-arcing time.  That time is generally small : a typical value being 0·001second 

Arcing time :- This is the time between the end of pre-arcing time and the instant when the arc is extinguished. 

Breaking capacity :-  It is the r.m.s. value of a.c. component of maximum prospective current that a fuse can deal with at rated service voltage. 

FUSE ELEMENT MATERIAL :

The function of a fuse is to carry the normal current without overheating but when the current exceeds its normal value, it rapidly heats up to melting point and disconnects the circuit protected by it.  In order that it may perform this function successfully, the material of fuse wire should have the following desirable characteristics : 

• low melting point e.g., tin, lead.
• high conductivity e.g., silver, copper. 
• free from deterioration due to oxidation e.g., silver. 
• low cost e.g., lead, tin, copper. 

The above discussion reveals that no material possesses all the characteristics.  For instance, lead has low melting point but it has high specific resistance and is liable to oxidation.  Similarly, copper has high conductivity and low cost but oxidises rapidly.  Hence, a compromise is made in the selection of material for a fuse. The most commonly used materials for fuse element are lead, tin, copper, zinc and silver.  For small currents upto 10 A, tin or an alloy of lead and tin (lead 37%, tin 63%) is used for making the fuse element. For larger currents, copper or silver is employed.  It is a usual practice to tin the copper to protect it from oxidation.  Zinc (in strip form only) is good if a fuse with considerable time-lag is required i.e., one which does not melt very quickly with a small overload. 

The present trend is to use silver despite its high cost due to the following reasons : 

• It is comparatively free from oxidation. 
• It does not deteriorate when used in dry air.
• The coefficient of expansion of silver is so small that no critical fatigue occurs. Therefore, the fuse element can carry the rated current continuously for a long time. 
• The conductivity of silver is very high.

 TYPES OF FUSES :

Fuses may be categorised as : (i) Low voltages fuses  (ii) High voltage fuses 

(i). Low voltage fuses : It can be also categorised into two as:

• Semi-enclosed rewireable fuse 
• High rupturing capacity (H.R.C.) cartridge fuse. 

Semi-enclosed rewireable fuse:  Rewireable fuse (also known as kit-kat type) is used where low values of fault current are to be interrupted.  It consists of (i) a base and (ii) a fuse carrier.  The base is of porcelain and carries the fixed contacts to which the incoming and outgoing phase wires are connected.  The fuse carrier is also of porcelain and holds the fuse element (tinned copper wire) between its terminals.  The fuse carrier can be inserted in or taken out of the base when desired. When a fault occurs, the fuse element is blown out and the circuit is interrupted.  The fuse carrier is taken out and the blown out fuse element is replaced by the new one.  The fuse carrier is then reinserted in the base to restore the supply.  This type of fuse has two advantages.  Firstly, the detachable fuse carrier permits the replacement of fuse element without any danger of coming in contact with live parts.  Secondly, the cost of replacement is negligible. 

High-Rupturing capacity (H.R.C.) cartridge fuse :  The primary objection of low and uncertain breaking capacity of semi-enclosed rewireable fuses is overcome in H.R.C. cartridge fuse. It consists of a heat resisting ceramic body having metal end-caps to which is welded silver current-carrying element.  The space within the body surrounding the element is completely packed with a filling powder.  The filling material may be chalk, plaster of paris, quartz or marble dust and acts as an arc quenching and cooling medium.

(ii). High voltage fuses : The low-voltage fuses cannot be successfully used on modern high voltage circuits.  Intensive research and supply by engineers has led to the development of high voltage fuses. Some of the high voltage fuses are : 

Cartridge type: Similar in general construction to the low voltage cartridge type except that special design features are incorporated.  Some designs employ fuse elements wound in the form of a helix so as to avoid corona effects at higher voltages. On some designs, there are two fuse elements in parallel ; one of low resistance (silver wire) and the other of high resistance (tungsten wire). Under normal load conditions, the low resistance element carries the normal current.  When a fault occurs, the low-resistance element is blown out and the high resistance element reduces the short-circuit current and finally breaks the circuit. High voltage cartridge fuses are used upto 33 kV with breaking capacity of about 8700 A at that voltage.

Liquid type: These fuses are filled with carbon tetrachloride and have the widest range of application to H.V. systems.  They may be used for circuits upto about 100 A rated current on systems upto 132 kV and may have breaking capacities of the order of 6100 A. 

It consists of a glass tube filled with carbon tetrachloride solution and sealed at both ends with brass caps. The fuse wire is sealed at one end of the tube and the other end of the wire is held by a strong phosphor bronze spiral spring fixed at the other end of the glass tube. When the current exceeds the prescribed limit, the fuse wire is blown out.  As the fuse melts, the spring retracts part of it through a baffle (or liquid director) and draws it well into the liquid.  The small quantity of gas generated at the point of fusion forces some part of liquid into the passage through baffle and there it effectively extinguishes the arc. 

Metal clad fuses:  Metal clad oil-immersed fuses have been developed with the object of providing a substitute for the oil circuit breaker.  Such fuses can be used for very high voltage circuits and operate most satisfactorily under short-circuit conditions approaching their rated capacity

ADVANTAGES : 

• It is the cheapest form of protection available. 
• It requires no maintenance. 
• Its operation is inherently completely automatic unlike a circuit breaker which requires an elaborate equipment for automatic action. 
• It can break heavy short-circuit currents without noise or smoke. 
• The smaller sizes of fuse element impose a current limiting effect under short-circuit conditions. 
• The inverse time-current characteristic of a fuse makes it suitable for overcurrent protection. 
• The minimum time of operation can be made much shorter than with the circuit breakers.

DISADVANTAGES :

• Considerable time is lost in rewiring or replacing a fuse after operation. 
• On heavy short-circuits, discrimination between fuses in series cannot be obtained unless there is sufficient difference in the sizes of the fuses concerned. 
• The current-time characteristic of a fuse cannot always be co-related with that of the protected apparatus

RATINGS :

(i)  Current Ratings :- The current rating printed on a fuselink applies only at temperatures below a particular values. It is obviously undesirable to have a proliferation of current ratings and therefore particular or preferred ratings are specified in standards. There is a general tendency to follow the IEC standards. For example fuses with ratings from 10A to 100A are produced for 10A, 12A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A and 100A.

(ii)  Voltage Ratings :- The rated voltage of a fuse is the nominal voltage for which it was designed. Fuselinks will perform satisfactorily at lower voltages, but at much lower voltages, the reduction in current caused by the resistance of the fuselinks should be considered. In the case of A.C. ratings, the R.M.S. symmetrical value is given, and for D.C. ratings the mean value, including ripple, is given.

(iii) Frequency Ratings :- Fuses are most commonly used in A.C. circuits with frequency 50Hz or 60Hz and a fuse designed for one of these frequencies will generally operate satisfactorily at the order. If the arc extinguishes at current zero, then the maximum arcing time on a symmetrical fault will be 10ms at 50Hz and 8ms at 60Hz.

(iv) Breaking Capacity :- The breaking capacity of a fuse is the current which can be interrupted at the rated voltage. The required breaking capacity will depend upon the position of the fuse in the supply system. For instance, 6kA may be suitable for domestic and commercial applications, but 80kA is necessary at the secondary of a distribution transformer.

TESTS : 

(i) Type Tests :- Before production of a type of fuselink commences, type tests are performed to ensure that preproduction fuselink samples comply with relevant national or international standards. Measurements of power dissipation, time current characteristics, overload withstand capability, breaking capacity and resistance are included in these type tests.

(ii) Production Tests :- Routine testing of many important fuse characteristics is not possible because tests, such as breaking capacity, are destructive.  The quality of fuselinks depends upon the quality of the components supplied to the fuse manufacturer. Other tests are made in the case of specialised fuselinks. For example, the conditions of the elements in a high-voltages fuselink is examined using X-ray photography.

(iii) Site Tests :- Before use, every fuselinks should be checked visually for cracks and tightness of endcaps and the resistance should be checked. It should also be checked that the ratings, especially current, voltages, breaking capacity and time-current characteristics, are correct for the application.

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Difference Between Grounding and Earthing

One of the major difference between the grounding and the earthing is that in grounding, the current carrying part is connected to the ground whereas in earthing the non-current carrying parts is connected to ground. 

Definition of Grounding :-

In grounding, the current carrying parts are directly connected to the ground. The grounding provides the return path for the leakage current and hence protect the power system equipment from damage. 

When the fault occurs in the equipment, the current in all the three phases of the equipment become unbalance.The grounding discharges the fault current to the ground and hence makes the system balance.

The grounding has several advantages like it eliminates the surge voltage and also discharge the over voltage to the ground. The grounding provides the great safety to the equipment and improves the service reliability.

Definition of Earthing :-

The ‘earthing’ means the connection of non-current carrying part of the equipment to the earth. When the fault occurs in the system, then the potential of the non-current part of the equipment raises, and when any human or stray animal touch the body of the equipment, then they may get shocked. 

The earthing discharges the leakage current to the earth and hence avoid the personnel from the electric shock. It also protects the equipment from lighting strokes provides the discharge path for the surge arrester, gap and other devices.

The earthing is achieved by connecting the parts of the installation to the earth by using the earth conductor or earth electrode in intimate contact with the soil placed with some distance below the ground level.

Key Differences Between Grounding and Earthing :-

1. The earthing is defined as the connection of the non-current carrying part like the body of the equipment or enclosure to earth. In grounding the current carrying part like neutral of the transformer is directly connected to the ground.
2. For grounding, the black colour wire is used, and for earthing the green colour, the wire is used.
3. The grounding balanced the unbalanced load whereas the earthing protect the equipment and human from an electrical shock.
4. The grounding wire is placed between the neutral of the equipment and the earth whereas in earthing the earth electrode is placed between the equipment body and the earth pit which is placed under the ground.
5.  In grounding the equipment is not physically connected to the ground, and the current is not zero on the ground, whereas in earthing the system is physically connected to the ground and it is at zero potential.
6. The grounding gives the path to an unwanted current and hence protects the electrical equipment from damage, whereas the earthing decrease the high potential of electrical equipment which is caused by a fault and thus protects the human body from the electrical shock.
7. The grounding is classified into three types. They are the solid grounding, resistance grounding and reactance grounding. Earthing can be done in five ways.The different methods of earthing are the pipe earthing, plate earthing, rod earthing, earthing through tap and strip earthing.

Which is more dangerous to the human body: AC or DC current and voltage?

The effects of both on the human body differ, but one is more hazardous than the other.


No matter what, if either AC or DC comes in contact with the human body, it can be hazardous. The actual effect varies, though, as it depends upon several different factors, including the amount of current administered, duration of which it was in contact with the body, pathway of the current, voltage applied, and impedance of the body itself. 

All of that being said, if it comes down to one or the other, AC can generally be viewed as the more dangerous of the two currents — here’s why:

1) To start off, in order for both currents to have the same effect on the human body, the magnitude of DC flow of constant strength needs to be two to four times great than AC; that is, more DC current is needed to induce the same amount of physical damage as AC current. This is because the effect of the currents on the body is a direct result of the excitatory actions of its magnitude — specifically, the actual making and breaking of the current itself. Such excitatory actions include nerve / muscle stimulation, induction of cardiac atrial or ventricular fibrillation, and more. 

For DC to produce the same effect as AC on the human body, its flow of constant strength must be two to four times that being administered by the AC.

2) When death by electric shock occurs, it’s typically due to ventricular fibrillation, and the likelihood of a human suffering this sort of life-ending injury is much higher when coming in contact with an AC than a DC due to the fact that the human body’s threshold of DC-caused ventricular fibrillation is several times higher than for AC. 

3) Generally speaking, the human body’s impedance is higher for DC, and it only decreases when the frequency increase. As such, the severity of electric shock is less when in contact with DC than it is with AC. 

4) It’s easier to let go / remove contact with “live” parts in the case of DC than AC. This runs counter to the popular belief that because the alternating cycles of an AC current pass through zero, the individual is afforded enough time to pull their limb / body away from the part itself, whereas with the constant flow of the DC current, there is no frequency oscillations that afford the brief moment for the person to pull their body away. The basis for this argument can be sourced in the “let-go” experiment, which was reported in the same aforementioned IEC publication 60479. In it, the lowest level of current that could safely pass through a human body was administered through an electrode held in a test person’s hand; it was enough current to make the person unable to open his hand and drop the electrode. 

Without getting into all of the details of the actual experiment, the conclusion was that the test subjects found it easier to release the electrode when DC was administered rather than AC. 

Now, while it can be surmised that AC is more dangerous than DC, the safest solution is to avoid contact with any and all high-voltage electrical conductors, no matter the type of electrical current. As mentioned at the beginning of the article, any contact with an electrical current can be hazardous. 

Polyphase Induction Motor (Construction, Types and Principle of operation)

OBJECTIVES :

The aim of this chapter is to gather knowledge about construction, types and principle of operation of 3-phase induction motors. Introduction is dealt before, you can also get here.

CONSTRUCTION :

A typical motor consists of two parts namely stator and rotor like other type of motors.
1. An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field,
2. An inside rotor attached to the output shaft that is given a torque by the rotating field.

Stator construction

The stator of an induction motor is laminated iron core with slots similar to a stator of a synchronous machine. Coils are placed in the slots to form a three or single phase winding.

Type of rotors 

Rotor is of two different types.
1. Squirrel cage rotor
2. Wound rotor

Squirrel-Cage Rotor :

In the squirrel-cage rotor, the rotor winding consists of single copper or aluminium bars placed in the slots and short-circuited by end-rings on both sides of the rotor. Most of single phase induction motors have Squirrel-Cage rotor. One or 2 fans are attached to the shaft in the sides of rotor to cool the circuit.

Wound Rotor :

In the wound rotor, an insulated 3-phase winding similar to the stator winding wound for the same number of poles as stator, is placed in the rotor slots. The ends of the star-connected rotor winding are brought to three slip rings on the shaft so that a connection can be made to it for starting or speed control.
● It is usually for large 3 phase induction motors.
● Rotor has a winding the same as stator and the end of each phase is connected to a slip ring.
● Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, so it is not so common in industry applications.

PRINCIPLE OF OPERATION

An AC current is applied in the stator armature which generates a flux in the
stator magnetic circuit.
                                This flux induces an emf in the conducting bars of rotor as they are “cut” by the flux while the magnet is being moved (E = BVL (Faraday’s Law)). A current flows in the rotor circuit due to the induced emf, which in term produces a force, (F = BIL) can be changed to the torque as the output.
             In a 3-phase induction motor, the three-phase currents ia, ib and ic, each of equal magnitude, but differing in phase by 120°.
            Each phase current produces a magnetic flux and there is physical
120° shift between each flux. The total flux in the machine is the sum of the three fluxes. The summation of the three ac fluxes results in a rotating flux, which turns with constant speed and has constant amplitude. Such a magnetic flux produced by balanced three phase currents flowing in thee-phase windings is called a rotating magnetic flux or rotating magnetic field (RMF). RMF rotates with a constant speed (Synchronous Speed).
                           Existence of a RFM is an essential condition for the operation of an induction motor. If stator is energized by an ac current, RMF is generated due to the applied current to the stator winding. This flux produces magnetic field and the field revolves in the air gap between stator and rotor. So, the magnetic field induces a voltage in the short-circuited bars of the rotor. This voltage drives current through the bars. The interaction of the rotating flux and the rotor current generates a force that drives the motor and a torque is developed consequently. The torque is proportional with the flux density and the rotor bar current (F=BLI).
              The motor speed is less than the synchronous speed. The direction of the rotation of the rotor is the same as the direction of the rotation of the revolving magnetic field in the air gap. However, for these currents to be induced, the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced.
                                    If by some chance this happens, the rotor typically slows slightly until a current is re-induced and then the rotor continues as before. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unitless and is the ratio between the relative speed of the magnetic field as seen by the rotor the (slip speed) to the speed of the rotating stator field.
            Due to this an induction motor is
sometimes referred to as an asynchronous machine.

SLIP

The relationship between the supply frequency, f, the number of poles, p, and the synchronous speed (speed of rotating field), Ns is given by 120f/p

The stator magnetic field (rotating magnetic field) rotates at a speed, ns, the synchronous speed. If, Nr= speed of the rotor, the slip, S for an induction motor is defined
as S=(Ns-Nr)/ Ns
At stand still, rotor does not rotate ,
 Nr = 0, so S = 1.

At synchronous speed,
Nr = NS, S= 0
The mechanical speed of the rotor, in terms of slip and synchronous speed is given by,
Nr=(1-S)Ns
 Frequency of Rotor Current and Voltage With the rotor at stand-still, the frequency of the induced voltages and currents is the same as that of the stator (supply) frequency, fe.
 If the rotor rotates at speed of Nr, then the relative speed is the slip speed:
Nslip=Ns-Nr
Nslip is responsible for induction.

SCR [V-I Characteristics]

A detailed study of the characteristics reveal that the thyristor has three basic modes of operation, namely the reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. Which are discussed in details below :

Reverse Blocking Mode of Thyristor

Cathode is made positive with respect to anode. Junctions J1 and J3 are reverse biased whereas the junction J2 is forward biased. The behavior of the thyristor here is similar to that of two diodes are connected in series with reverse voltage applied across them. As a result only a small leakage current of the order of a few μAmps flows.

This is the reverse blocking mode or the off-state, of the thyristor. If the reverse voltage is now increased, then at a particular voltage, known as the critical breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. 

A large current associated with VBR gives rise to more losses in the SCR, which results in heating. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a thyristor does not exceed VBR. When reverse voltage applied across a thyristor is less than VBR, the device offers very high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as open circuit.

Forward Blocking Mode

Now considering the anode is positive with respect to the cathode, with gate kept in open condition. The thyristor is now said to be forward biased as shown the figure below.

As we can see the junctions J1 and J3arenow forward biased but junction J2 goes into reverse biased condition. In this particular mode, a small current, called forward leakage current is allowed to flow initially as shown in the diagram for characteristics of thyristor. Now, if we keep on increasing the forward biased anode to cathode voltage.

In this particular mode, the thyristor conducts currents from anode to cathode with a very small drop of potential across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward break over voltage or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending beyond a certain point, then the reverse biased junction J2 will have an avalanche breakdown at a voltage called forward break over voltage VB0 of the thyristor. But, if we keep the forward voltage less than VBO, we can see from the characteristics of thyristor, that the device offers a high impedance. Thus even here the thyristor operates as an open switch during the forward blocking mode.

Forward Conduction Mode

When the anode to cathode forward voltage is increased, with gate circuit open, the reverse junction J2 will have an avalanche breakdown at forward break over voltage VBO leading to thyristor turn on.
Once the thyristor is on we can see from the diagram for characteristics of thyristor. 

In this mode of operation, the thyristor conducts maximum current with minimum voltage drop, this is known as the forward conduction forward conduction or the turn on mode of the thyristor.

SCR (Silicon Controlled Rectifier) {Introduction, Symbol & why Silicon is used?}

The idea for the thyristor is not new. The idea for the device was first put forward in 1950 by William Shockley, one of the inventors of the transistor.

What is a thyristor?

The thyristor has a p-n-p-n structure with the outer layers with their electrodes referred to as the anode (n-type) and the cathode (p-type). The control terminal of the SCR is named the gate and it is connected to the p-type layer that adjoins the cathode layer.

Why Silicon in a thyristor?

Silicon is the ideal choice because of its overall properties. It is able to handle the voltage and currents required for high power applications. Additionally it has good thermal properties. The second major reason is that silicon technology is well established and it is widely used for a variety of semiconductor electronics components. As a result it is very cheap and easy for semiconductor manufacturers to use.

Thyristor symbols & basics

The thyristor or silicon controlled rectifier, SCR is a device that has a number of unusual characteristics. It has three terminals: Anode, cathode and gate, reflecting thermionic valve / vacuum tube technology. As might be expected the gate is the control terminal while the main current flows between the anode and cathode.

As can be imagined from its circuit symbol shown below, the device is a "one way device" giving rise to the GE name for it the silicon controlled rectifier. Therefore when the device is used with AC, it will only conduct for a maximum of half the cycle.

In operation, the thyristor or SCR will not conduct initially. It requires a certain level of current to flow in the gate to "fire" it. Once fired, the thyristor will remain in conduction until the voltage across the anode and cathode is removed - this obviously happens at the end of the half cycle over which the thyristor conducts. The next half cycle will be blocked as a result of the rectifier action. It will then require current in the gate circuit to fire the SCR again.

The silicon controlled rectifier, SCR or thyristor symbol used for circuit diagrams or circuit seeks to emphasis its rectifier characteristics while also showing the control gate. As a result the thyristor symbol shows the traditional diode symbol with a control gate entering near the junction.

Relays working in Electric Traction

A relay is a sensitive device provided to ensure the proper functioning of an apparatus either in the electrical circuit or in the fluid circuit and to safe guard the apparatus during any abnormalities.

*As per working principle in locomotive relays may be classified :
1. Electrical relay ( which work with electrical supply)
2. Mechanical relay ( which works with air or oil flow or pressure or by heat etc.)

Since relays are switches the terminology applied to the switches is also applied to the energising the coil in relays. A relay will switch one or more poles, each of whose contacts can be thrown by one of the three ways -

Normally Open (NO) :
Contacts connect the circuit when the relay activated, the circuit is disconnected when the relay is inactive. It is also called a Form A contact or  "Make contact".

Normally Close (NC) :
Contacts are disconnect the circuit when the relay activated, the circuit is connected when the relay is inactive. It is also called Form B contact or " Brake contact".

Change Over (CO) or Double throw (DT) contact :
Contacts control two circuits, one normally open contact and one normally closed contact with a common terminals. It is also called Form C contact or brake before make contact. If this type of contact utilizes a make before break functionally, then it is called a Form D contact.

ELECTRICAL RELAYS
An electrical relay is sensitive to electricity and it is of two kinds :
(a). Voltage or Tension relay
(b). Current or Intensity relay

A. Voltage relay or tension relay :

The relay ,which works on voltage, is known as voltage relay. They are two kinds :
(1). No or /low voltage relay and
(2). Over voltage relay.

(1). No or Low voltage relay : This relay checks the voltage of auxiliary transformer TFWA. This relay is connected between the two terminals of the source supply of TFWA. The interlock of this relay is provided in the control circuit (Q-44 branch). In case there is no or low voltage at the source of supply, it de-energised and its interlock opens on the control circuit of Q-44 to open D.J.
USE ON LOCO - Q-30.

(2). Over voltage relay : This relay is connected across the two terminals (+Ve & -Ve of RSI block and its interlock is provided in the control circuit. In case there is over voltage in the traction motor its interlock closes on the control circuit and relay Q-51 energises cause auto regresssion of G.R.
USE ON LOCO - Q-20.

B. Current or intensity relay : 

The  current or intensity relay is that which is sensitive to the flow of current. They are of two kinds :
1. Over current relay
2. Difference of current or current differential relay.

(1). Over current relay : This relay is provided in the feeding and traction power circuit in which the over current is to be checked and the interlock is provided in the control circuit. If there is over current in the concerning circuit the relay energises and the interlock of the relay will open in the control circuit of DJ to open it.
USE ON LOCO - QLM, QRSI-1 and QRSI-2 and also QLAon some locos.

(2). Difference of current or current differential relay : This relay is connected in the traction power circuit with the two groups of traction motors in series in which the difference of current  is to be checked, and the interlock is provided in the control circuit. If there isa difference of current of 160Amps between the groups of motors its interlock will close in the control circuit of Q-48 to cause auto regression of G.R., auto sanding and LSP glowing.
USE ON LOCO - QD1 and QD2.

MECHANICAL RELAYS 
The relays, which work by mechanical means, are called mechanical relays. The controlling device is provided in the circuit in which circulation, pressure, temperature or speed is to be checked and the interlock is provided in the control circuit. These relays are of four types namely :
(a). Circulation relay
(b). Pressure relay
(c). Temperature relay
(d). Speed relay

A. Circulation relay : The controlling device of this type of relay is provided in the fluid (air). Circuit to check air circulation and the interlock is provided in the control circuit.
If there is no circulation or improper circulation of fluid (air) its interlock will open in the control circuit to open D.J.
USE ON LOCO : QVSI-1, QVSI-2, QVSL-1, QVSL-2, QVRH, QVMT1, QVMT-2 (all checks air circulation).

B. Pressure relay/Pressure switch : The controlling device of this type of relay is provided on the pipeline of the fluid (air circulation/oil) and the interlock is provided in the control circuit.
USE ON LOCO : QPH (check pressure of oil), QPDJ (check pressure of air)
N.B. Pressure switches provided on loco are RGCP,RGEB, RGAF, P-1, P-2 etc.
C. Temperature relay : Not provided on Loco
D. Speed relay : Not provided on Loco

OTHER RELAYS PROVIDED ON LOCO 

A. Safety relay: The relays used on the control circuit of DJ and their normally closed, inter locks are provided in MTDJ branch to open DJ immediately, are called the safety relays.
All the safety relays are provided with a red target of connecting relays droops. To reset the red target it is necessary to operate the resetting knob.
USE ON LOCO : QLM,QRSI-1, QRSI-2, QOA, QOA, QOP-1 and QOP-2, on some loco QLA also.

B. Temporised or time lag relay : The relays which, after energizing or de-energising takes some time (A few seconds) to close or open its interlock on the control circuit, is called a temporised or time lag relay.
These types of relays may either be electrically time lag or mechanically time lag.
a). On delay relay - QTD 105, QTD 106, QSVM (in STC loco)
b). Off delay relay - Q-118(Temporised for 5 sec.), Q-44 (Temporised for 0.6 sec.)
C). Signalling Relays - The relay used for indication of different signalling lamps are called signalling relays.
USE ON LOCO : QV-60, QV-61, QV-62, QV-63, QV-64, QVLSOL etc.

Electrical Power

REAL POWER [Actual power]:
In AC circuit  because of phase difference between current and voltage the average value of real power is given by 
                          P=V*I*cos(θ);
 and its measure unit is Watt(W).

REACTIVE POWER [Useless power]:
Power merely absorbed and returned in load due to its reactive properties is referred to as reactive power.The average value of reactive power is given by 
                       Q = V *I*sin(θ);
 and expressed in unit VAR. 

Apparent Power:
Total power in an AC circuit, both dissipated and absorbed/returned is referred to as apparent power.
                    S=V*I; 
and is expressed in unit as VA. 

Apparent Power = √ (Real power^2 + Reactive Power^2);

IMPORTANT NOTES:
- Resistor absorbs the real power and dissipates in the form of heat and light.
- Inductor absorbs the reactive power and dissipates in the form of magnetic field.
- Capacitor absorbs the reactive power and dissipates in the form of electric or electrostatic field.

Phase swinging or Hunting(Synchronous Motor)

Topic is Hunting in synchronous motor.

The phenomenon of oscillation of the rotor about its final equilibrium position is called Hunting.

On the sudden application of load, the rotor search for its new equilibrium position and this process is known as Hunting.
The Hunting process occurs in a synchronous motor as well as in synchronous generators if an abrupt change in load occurs.

The steady state or stable operation of a synchronous motor is a condition of equilibrium. In it, the load torque is equal as well as opposite to the electromagnetic torque. The rotor of the motor runs at synchronous speed in the steady state condition, maintain a constant value of the torque angle δ. The equilibrium gets disturbed if a sudden change occurs in the load torque. Thus, a resulting torque takes place which changes the speed of the motor. It is given by the equation shown below.

Where J is the moment of inertia ,ωM is the angular velocity of the rotor in mechanical units.

The speed of the motor slows down temporarily, and the torque angle δ is sufficiently increased. This is done to restore the torque equilibrium and the synchronous speed when there is a sudden increase if the load torque.

The electromagnetic torque is given by the equation shown below

If the value of δ is increased, the electromagnetic torque is also increased. As a result, the motor is accelerated. As the rotor reaches the synchronous speed, the torque angle δ is larger than the required value. Here the rotor speed continues to increase beyond the synchronous speed.

As the rotor accelerates above synchronous speed, the torque angle δ decreases. The point where the motor torque becomes equal to the load torque, the equilibrium is not restored because now the rotor speed is greater than the synchronous speed. Therefore, the rotor continues to swing backwards and as a result, the torque angle goes on decreasing.

When the load angle δ becomes less than the required value, the mechanical load becomes greater than the developed power. Therefore, the motor starts to slow down. The load angle starts increasing again. Thus, the rotor starts to swing or oscillates around the synchronous speed.

The motor responds to a decreasing load torque by a temporary increase in speed and a reduction of the torque angle δ. Thus, the rotor swings and rotate around the synchronous speed. Thus, this process of rotation of the rotor speed equal or around the synchronous speed is known as Hunting. Since, during the rotor oscillation, the phase of the phasor Ef changes about phasor V. Thus, hunting is known as Phase Swinging.

Causes of hunting-
a. Sudden changes of load.
b. Faults were occurring in the    
    system which the generator
    supplies.
c. Sudden change in the field
    current.
d. Cyclic variations of the load
     torque.

Effects of Hunting-
a. It can lead to loss of synchronism.
b. It can cause variations of the    
    supply voltage producing
    undesirable lamp flicker.
c. The possibility of Resonance
    condition increases. If the
    frequency of the torque
    component becomes equal to that       of the transient oscillations of the
    synchronous machine, resonance       may take place.
d. Large mechanical stresses may  
    develop in the rotor shaft.
e. The machine losses increases and
    the temperature of the machine  
    rises.

Reduction of Hunting-
a. Use of damper windings
b. Uses of flywheels
c. The prime mover is provided with     a large and heavy flywheel. This         increases the inertia of the prime       mover and helps in maintaining         the rotor speed constant.
d. By designing synchronous
     machines with suitable
     synchronising power coefficients.

Closed loop Control system

Close loop Control system ::-

It's an automatic control system in which an operation, process, or mechanism is regulated by feedback

Control Characteristics of the system depends upon the output of the system. It is also termed as feedback control system. The control action is actuated by an error signal 'e', which is the difference between Input signal and output signal. The purpose of feedback is to reduce the error between the reference input and the system output.

Advantages of Closed loop :-

i) Accurate and Reliable 

ii) Reduce effect of parameter variations. 

iii) Reduce the effect of non-linearity.

Disadvantages of closed loop:-

i) System is Complex & Costly. 

ii) Reduce the Gain will negative feedback.