ELCB AND RCCB

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ELCB

         An ELCB is a specialised type of latching relay that has a building's incoming mains power connected through its switching contacts so that the ELCB disconnects the power 

in an earth leakage (unsafe) condition.The ELCB detects fault currents passing from live (hot) to the earth (ground) wire within the installation it protects. If sufficient voltage appears across the ELCB's sense coil, it will switch off the power, and remain off until manually reset. An ELCB however, does not sense fault currentspassing from live to any other earthed body.

There are two types of ELCBs:

1. Voltage Earth Leakage Circuit Breaker (voltage-ELCB)
2. Current Earth Leakage Current Earth Leakage Circuit Breaker (Current-ELCB).

Voltage-ELCBs were first introduced about sixty years ago and Current-ELCB was first introduced about forty years ago. For many years, the voltage operated ELCB and the differential current operated ELCB were both referred to as ELCBs because it was a simpler name to remember. But the use of a common name for two different devices gave rise to considerable confusion in the electrical industry.

If the wrong type was used on an installation, the level of protection given could be substantially less than that intended.

To ignore this confusion, IEC decided to apply the term Residual Current Device (RCD) to differential current operated ELCBs. Residual current refers to any current over and above the load current.

Voltage Base ELCB

•  Voltage-ELCB is a voltage operated circuit breaker. The device will function when the Current passes through the ELCB. Voltage-ELCB contains relay Coil which it being connected to the metallic load body at one end and it is connected to ground wire at the other end.
  .
• If the voltage of the Equipment body is rise (by touching phase to metal part or failure ofinsulation of equipment) which could cause the difference between earth and load body voltage, the danger of electric shock will occur. This voltage difference will produce an electric current from the load metallic body passes the relay loop and to earth. When voltage on the equipment metallic body rose to the danger level which exceed to 50Volt, the flowing current through relay loop could move the relay contact by disconnecting the supply current to avoid from any danger electric shock.
  .
• The ELCB detects fault currents from live to the earth (ground) wire within the installation it protects. If sufficient voltage appears across the ELCB’s sense coil, it will switch off the power, and remain off until manually reset. A voltage-sensing ELCB does not sense fault currents from live to any other earthed body.
• These ELCBs monitored the voltage on the earth wire, and disconnected the supply if the earth wire voltage was over 50 volts.
  .
• These devices are no longer used due to its drawbacks like if the fault is between live and a circuit earth, they will disconnect the supply. However, if the fault is between live and some other earth (such as a person or a metal water pipe), they will NOT disconnect, as the voltage on the circuit earth will not change. Even if the fault is between live and a circuit earth, parallel earth paths created via gas or water pipes can result in the ELCB being bypassed. Most of the fault current will flow via the gas or water pipes, since a single earth stake will inevitably have a much higher impedance than hundreds of meters of metal service pipes buried in the ground.
• The way to identify an ELCB is by looking for green or green and yellow earth wires entering the device. They rely on voltage returning to the trip via the earth wire during a fault and afford only limited protection to the installation and no personal protection at all. You should use plug in 30mA RCD’s for any appliances and extension leads that may be used outside as a minimum.

Current-operated ELCB (RCB)

• Current-operated ELCBs are generally known as Residual-current devices (RCD). These also protect against earth leakage. Both circuit conductors (supply and return) are run through a sensing coil; any imbalance of the currents means the magnetic field does not perfectly cancel. The device detects the imbalance and trips the contact.
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• When the term ELCB is used it usually means a voltage-operated device. Similar devices that are current operated are called residual-current devices. However, some companies use the term ELCB to distinguish high sensitivity current operated 3 phase devices that trip in the milliamp range from traditional 3 phase ground fault devices that operate at much higher currents.
• The supply coil, the neutral coil and the search coil all wound on a common transformer core.
  .
• On a healthy circuit the same current passes through the phase coil, the load and return back through the neutral coil. Both the phase and the neutral coils are wound in such a way that they will produce an opposing magnetic flux. With the same current passing through both coils, their magnetic effect will cancel out under a healthy circuit condition.
  .
• In a situation when there is fault or a leakage to earth in the load circuit, or anywhere between the load circuit and the output connection of the RCB circuit, the current returning through the neutral coil has been reduced. Then the magnetic flux inside the transformer core is not balanced anymore. The total sum of the opposing magnetic flux is no longer zero. This net remaining flux is what we call a residual flux.
  .
• The periodically changing residual flux inside the transformer core crosses path with the winding of the search coil. This action produces an electromotive force (e.m.f.) across the search coil. An electromotive force is actually an alternating voltage. The induced voltage across the search coil produces a current inside the wiring of the trip circuit. It is this current that operates the trip coil of the circuit breaker. Since the trip current is driven by the residual magnetic flux (the resulting flux, the net effect between both fluxes) between the phase and the neutral coils, it is called the residual current devise.
  .
• With a circuit breaker incorporated as part of the circuit, the assembled system is called residual current circuit breaker (RCCB) or residual current devise (RCD). The incoming current has to pass through the circuit breaker first before going to the phase coil. The return neutral path passes through the second circuit breaker pole. During tripping when a fault is detected, both the phase and neutral connection is isolated.

RCCB

A residual current device (RCD), or residual current circuit breaker (RCCB), is an electrical wiring device that disconnects a circuit whenever it detects that the electriccurrent is not balanced between the phase ("hot") conductor  and the neutral conductor. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions; RCDs are designed to disconnect quickly enough to mitigate the harm caused by such shocks.

• Phase (line) and Neutral both wires connected through RCD.
• It trips the circuit when there is earth fault current.
• The amount of current flows through the phase (line) should return through neutral .
• It detects by RCD. any mismatch between two currents flowing through phase and neutral detect by RCD and trip the circuit within 30Miliseconed.
• If a house has an earth system connected to an earth rod and not the main incoming cable, then it must have all circuits protected by an RCD (because u mite not be able to get enough fault current to trip a MCB)
• The most widely used are 30 mA (milliamp) and 100 mA devices. A current flow of 30 mA (or 0.03 amps) is sufficiently small that it makes it very difficult to receive a dangerous shock. Even 100 mA is a relatively small figure when compared to the current that may flow in an earth fault without such protection (hundred of amps)
• A 300/500 mA RCCB may be used where only fire protection is required. eg., on lighting circuits, where the risk of electric shock is small
• RCDs are an extremely effective form of shock protection

D.C. GENERATOR

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INTRODUCTION

To convert energy from one form to another a machine is required .In the D.C Machine the energy conservation is based on the production of dynamical induced emf .The machine which converts mechanical energy into electrical energy is called a Generator. The machine which converts mechanical energy into electrical energy is called a D.C Generator and when electrical energy is converted into mechanical energy , it is called D.C motor. All these conversions take placed through the medium of magnetism. As  such magnetism plays an important role in the energy conversation and so also its laws. Since the conversation is bilateral from electrical to mechanical and vice –versa , the machine which operates as a generator can also be operated as motor and as such the construction features are same . The basic law which is applicable in the D.C Machine is Faraday’sLaws of Electro-magnetic induction for which the essential requirements are

•                     Conductor
•                     Flux
•                     Conductor process cut the flux
• 
  

PRINCIPLE OF OPERATION

The principle of operation is based on the Faraday’s Laws of electromagneticinduction which states that 

   

•    Whenever a conductor cuts the magnetic field an emf is induced in the conductor
•   The magnitude of induced emf is proportional to the rate of change of flux linkages
• ·   Such induced e.m.f will last only as long as the flux linkage exists

                 Mathematically the law can be expressed as

                       Let N = No of turns of the coil

                      ᶲ1 = Initial flux through the coil (Wb)

                       ᶲ2 = Final  flux through the coil (Wb)

                       t = time in seconds during which the flux changes from ᶲ1 to ᶲ2 Wb

Since flux linkage is the product of flux and number of turns

Initial flux linkage  =  Nᶲ1

Initial flux linkage  =  Nᶲ2

Change of flux linkage  N(ᶲ2-ᶲ1)

Therefore, e.m.f induced in the coil is propotional to the rate of change of flux linkages 

THEVENIN THEOREM

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Thevenin's Theorem states that it is possible to simplify any linear circuit, no matter how complex, to an equivalent circuit with just a single voltage source and series resistance connected to a load. The qualification of “linear” is identical to that found in the Superposition Theorem, where all the underlying equations must be linear (no exponents or roots). If we're dealing with passive components (such as resistors, and later, inductors and capacitors), this is true. However, there are some components (especially certain gas-discharge and semiconductor components) which are non linear: that is, their opposition to current changes with voltage and/or current. As such, we would call circuits containing these types of components, non linear circuits.

Thevenin's Theorem is especially useful in analyzing power systems and other circuits where one particular resistor in the circuit (called the “load” resistor) is subject to change, and re-calculation of the circuit is necessary with each trial value of load resistance, to determine voltage across it and current through it

Thevenin's Theorem makes this easy by temporarily removing the load resistance from the original circuit and reducing what's left to an equivalent circuit composed of a single voltage source and series resistance. The load resistance can then be re-connected to this “Thevenin equivalent circuit

This theorem is very conceptual. If we think deeply about an electrical circuit, we can visualize the statements made in Thevenin theorem. Suppose we have to calculate the current through any particular branch in a circuit. This branch is connected with rest of the circuits at its two terminal. Due to active sources in the circuit, there is one electric potential difference between the points where the said branch is connected. The current through the said branch is caused by this electric potential difference that appears across the terminals. So rest of the circuit can be considered as a single voltage source, that's voltage is nothing but the open circuit voltage between the terminals where the said branch is connected and the internal resistance of the source is nothing but the equivalent resistance of the circuit looking back into the terminals where, the branch is connected.

Kirchoff's voltage law

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Kirchoff's voltage law can be stated in words as the sum of all voltage drops and rises in a closed loop equals zero. As the image below demonstrates, loop 1 and loop 2 are both closed loops within the circuit. The sum of all voltage drops and rises around loop 1 equals zero, and the sum of all voltage drops and rises in loop 2 must also equal zero. A closed loop can be defined as any path in which the originating point in the loop is also the ending point for the loop. No matter how the loop is defined or drawn, the sum of the voltages in the loop must be zero.

The sum of all voltages or potential differences in an electrical circuit loop is 0.

 

KVL example

V_S = 12V, V_R1 = -4V, V_R2 = -3V

V_R3 = ?

Solution:

∑V_k = V_S + V_R1 + V_R2 + V_R3 = 0

V_R3 = -V_S  - V_R1 - V_R2 = -12V+4V+3V = -5V

The voltage sign (+/-) is the direction of the potential difference.

Kirchhoff's Current Law

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In an electrical circuit, the current flows rationally as electrical quantity. As the flow of current is considered as flow of quantity, at any point in the circuit the total current enters, is exactly equal to the total current leaves the point. The point may be considered anywhere in the circuit Suppose the point is on the conductor through which the current is flowing, then the same current crosses the point which can alternatively said that the current enters at the point and same will leave the point. As we said the point may be anywhere on the circuit, so it can also be a junction point in the circuit. So total quantity of current enters at the junction point must be exactly equal to total quantity of current that leaves the junction. This is the very basic thing about flowing of current and fortunately Kirchhoff Current law says the same. The law is also known as Kirchhoff First Law and this law stated that, at any junction point in the electrical circuit, the summation of all the branch currents is zero. If we consider all the currents enter in the junction are considered as positive current, then convention of all the branch currents leaving the junction are negative. Now if we add all these positive and negative signed currents, obviously we will get result of zero

 "The algebraic sum of all currents entering and exiting a node must equal zero"

SLIP RING INDUCTION MOTOR

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 The slip ring induction motor has two distinctly separate parts, one is the stator and other is the rotor. The stator circuit is rated as same in the squirrel cage motor, but the rotor is rated in frame voltage or short circuit current.    A slip ring (in electrical engineering terms) is a method of making an electrical connection through a rotating assembly. Slip rings, also called rotary electrical interfaces, rotating electrical connectors, collectors, swivels or electrical rotary joints, are commonly found in electrical generators for AC systems and alternators and in packaging machinery, cable reels, and wind turbines.  A slip ring consists of a conductive circle or band mounted on a shaft and insulated from it. Electrical connections from the rotating part of the system, such as the rotor of a generator, are made to the ring. Fixed contacts or brushes run in contact with the ring, transferring electrical power or signals to the exterior, static part of the system.

STATOR

The stator consists of 3-ph winding forms wound 'poles' that carry the supply current to induce a magnetic field that penetrates the rotor. In a very simple motor, there would be a single projecting piece of the stator (a salient pole ) for each pole, with windings around it; in fact, to optimize the distribution of the magnetic field, the windings are distributed in many slots located around the stator, but the magnetic field still has the same number of north-south alternations. The number of 'poles' can vary between motor types but the poles are always in pairs

SLIP RING ROTOR

The slip ring induction motors usually have “Phase-Wound” rotor. This type of rotor is provided with a 3-phase, double-layer, distributed winding consisting of coils used in alternators. The rotor core is made up of steel laminations which has slots to accommodate formed 3-single phase windings. These windings are placed 120 degrees electrically apart. The rotor is wound for as many poles as the number of poles in the stator and is always 3-phase even though the stator is wound for 2-phase. These three windings are “starred” internally and other end of these three windings are brought out and connected to three insulated slip-rings mounted on the rotor shaft itself. The three terminal ends touch these three slip rings with the help of carbon brushes which are held against the rings with the help of spring assembly These three carbon brushes are further connected externally to a 3-phase star connected rheostat Thus these slip ring and external rheostat makes the slip ring induction motors possible to add external resistance to the rotor circuit, thus enabling them to have a higher resistance during starting and thus higher starting torque. 

DC MOTOR

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A  DC motor  is a mechanically commutated  electric motor  powered from  direct current  (DC). The stator is stationary in space by definition and therefore the current in the rotor is switched by the commutator  to also be stationary in space. This is how the relative angle between the stator and rotor magnetic flux is maintained near 90 degrees, which generates the maximum torque.

DC motors have a rotating armature winding (winding in which a voltage is induced) but non-rotating armature magnetic field and a static field winding (winding that produce the main magnetic flux) or permanent magnet. Different connections of the field and armature winding provide different inherent speed/torque regulation characteristics. The speed of a DC motor can be controlled by changing the voltage applied to the armature or by changing the field current.

Principle of DC Motor

This DC or  Direct Current Motor  works on the principal, when a current carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move. This is known as motoring action. If the direction of   electric current   in the wire is reversed, the direction of rotation also reverses. When magnetic field and electric field interact they produce a mechanical force, and based on that the working principle of  dc motor  established.   The direction of rotation of a this motor is given by Fleming’s left hand rule, which states that if the index finger, middle finger and thumb of your left hand are extended mutually perpendicular to each other and if the index finger represents the direction of magnetic field, middle finger indicates the direction of   electric current then the thumb represents the direction in which force is experienced by the shaft of the  dc motor . Structurally and construction wise a  Direct Current Motor  is exactly similar to a D.C. Generator, but electrically it is just the opposite. Here we unlike a generator we supply electrical energy to the input port and derive mechanical energy from the output port

• Shunt motor

The field coil and the armature windings are connected in shunt or parallel across the power source. The armature winding consists of relatively few turns of heavy gauge wire. The voltage across two windings is the same but the armature draws considerably more current than the field coil. Torque is caused by the interaction of the current caring armature winding with the magnetic field produced by the field coil. If the DC line voltage is constant, the armature voltage and the field strength will be constant. The speed regulation is quite good; the speed is a function of armature current and is not precisely constant. As the armature rotates within the magnetic field, an EMF is induced in its wining. This EMF is in the direction opposite to the source EMF and is called the counter EMF (CEMF), which varies with rotational speed. Finally, the current flow through the armature winding is a result of the difference between source EMF and CEMF. When the load increases, the motor tends to slow down and less CEMF is induced, which in turn increases the armature current providing more torque for the increased load

Motor speed is increased by inserting resistance into the field coil circuit, which weakens the magnetic field. Therefore, the speed can be increased from “basic” or full-load, full-field value to some maximum speed set by the electrical and mechanical limitations of the motor.

Series motor

The field coil and armature windings are connected in series to the power source. The field coil is wound with a few turns of heavy gauge wire. In this motor, the magnetic field is produced by the current flowing through the armature winding; with the result that the magnetic field is weak when the motor load is light (the armature winding draws a minimum current). The magnetic field is strong when the load is heavy (the armature winding draws a maximum current). The armature voltage is nearly equal to the PS line voltage (just as in the shunt wound motor if we neglect the small drop in the series field)

Consequently , the speed of the series wound motor is entirely determined by the load current. The speed is low at heavy loads, and very high at no load. In fact, many series motors will , if operated at no load , run so fast that they destroy themselves . The high forces , associated with high speeds , cause the rotor to fly apart , often with disastrous results to people and property nearby . The torque of any DC motor depends upon the product of the armature current and the magnetic field. For the series wound motor this relationship implies that the torque will be very large for high armature currents, such as occur during start-up. The series wound motor is, therefore, well adapted to start large heavy-inertia loads, and is particularly useful as a drive motor in electric buses, trains and heavy duty traction applications. Compared to the shunt motor, the series DC motor has high starting torque and poor speed regulation.

SINGLE PHASE INDUCTION MOTOR

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The single phase ac motors are further classified as:
• Single phase induction motor s or   asynchronous motor s
• Single phase synchronous motors
• Commutator motors
This article will provide fundamentals, description and   working principle of single phase induction motor

Construction of Single Phase Induction Motor

Like any other electrical motor asynchronous motor also have two main parts namely rotor and stator.

Stator: As its name indicates stator is a stationary part of induction motor. A single phase ac supply is given to the stator of Single phase induction motor.

Rotor: The rotor is a rotating part of induction motor. The rotor is connected to the mechanical load through the shaft. The rotor in single phase induction motor is of squirrel cage rotor type.

Stator of Single Phase Induction Motor

The stator of the single phase induction motor has laminated stamping to reduce eddy current losses on its periphery. The slots are provided on its stampings to carry stator or main winding. In order to reduce the hysteresis losses, stamping are made up of silicon steel. When the stator winding is given a single phase ac supply, the magnetic field is produced and the motor rotates at a speed slightly less than the synchronous speed N s   which is given by

  except there are two dissimilarity in the winding part of the Single phase induction motor.

Rotor of Single Phase Induction Motor

The construction of the rotor of the single phase induction motor is similar to the
Squirrel cage three phase induction motor. The rotor is cylindrical in shape and has slots all over its periphery. The slots are not made parallel to each other but are bit skewed as the skewing prevents magnetic locking of stator and rotor teeth and makes the working of motor more smooth and quieter. The squirrel cage rotor consists of aluminium, brass or copper bars. These aluminium or copper bars are called rotor conductors and are placed in the slots on the periphery of the rotor. The rotor conductors are permanently shorted by the copper or aluminium rings called the end rings. In order to provide mechanical strength these rotor conductor are braced to the end ring and hence form a complete closed circuit resembling like a cage and hence got its name as ‘squirrel cage induction motor”. As the bars are permanently shorted by end rings, the rotor resistance is very small and it is not possible to add external resistance as the bars are permanently shorted. The absence of slip ring and brushes make the construction of single phase induction motor very simple and robust.

Working Principle of Single Phase Induction Motor

When single phase ac supply is given to the stator winding of single phase induction motor, the alternating current starts flowing through the stator or main winding. This alternating current produces an alternating flux called main flux. This main flux also links with the rotor conductors and hence cut the rotor conductors. According to the Faraday’s law of electromagnetic induction, emf gets induced in the rotor. As the rotor circuit is closed one so, the current starts flowing in the rotor. This current is called the rotor current. This rotor current produces its own flux called rotor flux. Since this flux is produced due to induction principle so, the motor working on this principle got its name as induction motor. Now there are two fluxes one is main flux and another is called rotor flux. These two fluxes produce the desired torque which is required by the motor to rotate.

Why Single Phase Induction Motor is not self starting?

According to double field revolving theory, any alternating quantity can be resolved into two components, each component have magnitude equal to the half of the maximum magnitude of the alternating quantity and both these component rotates in opposite direction to each other. For example - a flux, φ can be resolved into two components

Each of these components rotates in opposite direction i. e if one φ m   / 2 is rotating in clockwise direction then the other φ m   / 2 rotates in anticlockwise direction.

When a single phase ac supply is given to the stator winding of single phase induction motor, it produces its flux of magnitude, φ m . According to the double field revolving theory, this alternating flux, φ m   is divided into two components of magnitude φ m   /2. Each of these components will rotate in opposite direction, with the synchronous speed, N s . Let us call these two components of flux as forward component of flux, φ f   and backward component of flux, φ b . The resultant of these two component of flux at any instant of time, gives the value of instantaneous stator flux at that particular instant.

Now at starting, both the forward and backward components of flux are exactly opposite to each other. Also both of these components of flux are equal in magnitude. So, they cancel each other and hence the net torque experienced by the rotor at starting is zero. So, the single phase induction motors are not self starting motors.

SINGLE PHASE TRANSFORMER

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A transformer is a static piece of apparatus by mean of which electric power in on e circuit is transformed into electric power of the same frequency in another circuit .it can raise or lower the voltage in a circuit but with a corresponding decrease or increase in current. the physical basis of a transformer is mutual induction between two circuit linked by a common magnetic flux .in its simplest form it consist of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance .the two coils posses high mutual inductance. If one coil is connected to a source of alternating voltage ,an alternating flux is setup in the laminated core, most of which is linked with the other c oil in which it produces mutual inducted emf according to faraday’s laws of electromagnetic induction E =MdI/dt .If the second coil circuit is closed ,a current t flows in it and so electric energy is transferred from the first coil to the second coil. The first coil in which electric energy is fed from the ac supply mains called primary winding and other from which energy is drawn out is called secondary winding

The rate of change of flux linkage depends upon the amount of linked flux, with the second winding. So it desired to be linked almost all flux of primary winding, to the secondary winding. This is effectively and efficiently done by placing one low reluctance path common to both the winding. This low reluctance path is core of transformer, through which maximum number of flux produced by the primary is passed through and linked with the secondary winding. This is most basic   theory of transformer .

constructional parts of transformer

• Primary Winding of transformer - which produces magnetic flux when it is connected to electrical source

• Magnetic Core of transformer - the magnetic flux produced by the primary winding, will pass through this low reluctance path linked with secondary winding and creates a closed magnetic circuit

• Secondary Winding of transformer - the flux, produced by primary winding, passes through the core, will link with the secondary winding. This winding is also wound on the same core and gives the desired output of the transformer

                     

• The  transformer  which increases the   a.c. voltage   is called   step up   transformer .

• The  transformer  which decreases  the   a.c. voltage   is called   step down transformer .

E s is the voltage generated in the secondary coil.

E p  is the voltage given to primary coil.

N s  is the number of the turns in the secondary coil.

N p  is the number of the turns in the primary coil. 

where   K  is called as transformation ratio.

• If K>1 then the transformer as step up transformer.

• if K<1 then the transformer as  step down transformer.

FLEMINGS LEFT HAND RULE

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Fleming's left-hand rule   (for motors), and  Fleming's  right-hand rule  (for generators) are a pair of visual  mnemonics. They were originated by  John Ambrose Fleming, in the late 19th century, as a simple way of working out the direction of motion in an  electric motor, or the direction of electric current in an  electric generator.

When current flows in a wire, and an external magnetic field is applied across that flow, the wire experiences a force perpendicular both to that field and to the direction of the current flow. A left hand can be held, as shown in the illustration, so as to represent three mutually orthogonal axes on the thumb, first finger and middle finger. Each finger is then assigned to a quantity (mechanical force, magnetic field and electric current). The right and left hand are used for generators and motors respectively.

To work out the  direction  of force experienced we use  Fleming's Left Hand Rule.

• Your  first  finger points in the  direction of the magnetic field  (North to South).

• Your  second  finger points in the  direction of conventional current  (positive to negative).

• Your  thumb  points in the direction of the  thrust or force  on the conductor.

LEN'Z LAW

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Lenz's law is named after the German scientist H. F. E. Lenz in 1834.   Lenz's law  obeys Newton's third law of motion (i.e to every action there is always an equal and opposite reaction) and the conservation of energy (i.e energy may neither be created nor destroyed and therefore the sum of all the energies in the system is a constant)
Lenz law is based on   Faraday's law   of induction so before understanding Lenz's law one should know what   Faraday’s law of induction   is. When a changing magnetic field is linked with a coil, an emf is induced in it. This change in magnetic field may be caused by changing the magnetic field strength by moving a magnet toward or away from the coil or moving the coil into or out of the magnetic field as desired. Or in simple words we can say that the magnitude of the emf induced in the circuit is proportional to the rate of change of flux

Lenz law states that when an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it.

The negative sign is used in   Faraday's law of electromagnetic induction, indicates that the induced emf ( ε ) and the change in magnetic flux ( δΦ B   ) have opposite signs.

Where
ε = Induced emf
δΦ B   = change in magnetic flux
N = No of turns in coil