Electrical Law's ( Ask in competitive exams )

1) Ohms Law

The statement of Ohm’s law
If potential difference or voltage is applied across a resistor of a closed circuit, current starts flowing through it.


This current is directly proportional to the voltage applied if temperature and all other factors remain constant. Thus we can mathematically express it as:

Now putting the constant of proportionality we get,

This equation show statement of ohms law.

where,
I= is the current through the resistor measured in Ampere (Ampere, or amps),
V=the Voltage/potential difference applied across the resistor in unit of volt,
R=ohm(Ω) is the unit of measure for the resistance of the resistor R
It’s important to note that the resistance R is the property of the conductor and theoretically has no dependence on the voltage applied, or on the flow of current. The value of R changes only if the conditions (like temperature, diameter length etc.) of the material are changed by any means.

Resistivity of each material is different it depend on material not on the applied voltage and current.

Easy to remember


3) Faraday's Law of Electromagnetic Induction

Faraday's First Law

Any change in the magnetic field of a coil of wire will cause an emf to be induced in the coil. This emf induced is called induced emf and if the Conductor circuit is closed, the Current will also circulate through the circuit and this current is called induced current.

Method to change magnetic field:

1. By moving a magnet towards or away from the coil
2. By moving the coil into or out of the magnetic field.
3. By changing the area of a coil placed in the magnetic field
4. By rotating the coil relative to the magnet

Faraday's Second Law

It states that the magnitude of emf induced in the coil is equal to the rate of change of flux that linkages with the coil. The flux linkage of the coil is the product of number of turns in the coil and flux associated with the coil.

Faraday Law Formula

Consider a magnet approaching towards a coil. Here we consider two instants at time T₁ and time T₂. Flux linkage with the coil at time, T₁ = NΦ₁ Wb Flux linkage with the coil at time, T₂ = NΦ₂ wb Change in flux linkage = N(Φ₂ - Φ₁) Let this change in flux linkage be, Φ = Φ₂ - Φ₁ So, the Change in flux linkage = NΦ Now the rate of change of flux linkage = NΦ / t Take derivative on right hand side we will get The rate of change of flux linkage = NdΦ/dt But according to Faraday's law of electromagnetic induction, the rate of change of flux linkage is equal to induced emf.

Considering Len'z Law

Where, flux Φ in Wb = B.A B = magnetic field strength A = area of the coil
HOW TO INCREASE EMF INDUCED IN A COIL

• By increasing the number of turns in the coil i.e N- From the formulae derived above it is easily seen that if number of turns of coil is increased, the induced emf also gets increased.
• By increasing magnetic field strength i.e B surrounding the coil- Mathematically if magnetic field increases, flux increases and if flux increases emf induced will also get increased. Theoretically, if the coil is passed through a stronger magnetic field, there will be more lines of force for coil to cut and hence there will be more emf induced.
• By increasing the speed of the relative motion between the coil and the magnet - If the relative speed between the coil and magnet is increased from its previous value, the coil will cut the lines of flux at a faster rate, so more induced emf would be produced.

Applictions: Transformer,generator,Induction cookers Etc.

4) Lenz law:

Lenz's 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 an Current that's magnetic field opposes the change which produces it.

The negative sign used in Faraday's law of electomagnetic 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

Reason for Opposing, Cause of Induced Current in Lenz's Law?

• As stated above, Lenz's law obeys the law of conservation of energy and if the direction of the magnetic field that creates the current and the magnetic field of the current in a conductor are in same direction, then these two magnetic fields would add up and produce the current of twice the magnitude and this would in turn create more magnetic field, which will cause more current and this process continuing on and on leads to violation of the law of conservation of energy.
• If the induced current creates a magnetic field which is equal and opposite to the direction of magnetic field that creates it, then only it can resist the change in the magnetic field in the area, which is in accordance to the Newton's third law of motion.

4) Fleming Left Hand Rule

It is found that whenever an current carrying conductor is placed inside a magnetic field, a force acts on the conductor, in a direction perpendicular to both the directions of the current and the magnetic field. In the figure it is shown that, a portion of a conductor of length L placed vertically in a uniform horizontal magnetic field strength H, produced by two magnetic poles N and S. If i is the current flowing through this conductor, the magnitude of the force acts on the conductor is,

F = BiL

Hold out your left hand with forefinger, second finger and thumb at right angle to one another. If the fore finger represents the direction of the field and the second finger that of the current, then thumb gives the direction of the force. While, current flows through a conductor, one magnetic field is induced around it. This can be imagined by considering numbers of closed magnetic lines of force around the conductor. The direction of magnetic lines of force can be determined by Maxwell's corkscrew rule or right-hand grip rule. As per these rules, the direction of the magnetic lines of force (or flux lines) is clockwise if the current is flowing away from the viewer, that is if the direction of current through the conductor is inward from the reference plane as shown in the figure.

Now if a horizontal magnetic field is applied externally to the conductor, these two magnetic fields i.e. field around the conductor due to current through it and the externally applied field will interact with each other. We observe in the picture, that the magnetic lines of force of external magnetic field are from N to S pole that is from left to right. The magnetic lines of force of external magnetic field and magnetic lines of force due to current in the conductor are in same direction above the conductor, and they are in opposite direction below the conductor. Hence there will be larger numbers of co-directional magnetic lines of force above the conductor than that of below the conductor. Consequently, there will be a larger concentration of magnetic lines of force in a small space above the conductor. As magnetic lines of force are no longer straight lines, they are under tension like stretched rubber bands. As a result, there will be a force which will tend to move the conductor from more concentrated magnetic field to less concentrated magnetic field, that is from present position to downwards. Now if you observe the direction of current, force and magnetic field in the above explanation, you will find that the directions are according to the Fleming left hand rule.

5) Fleming Right Hand Rule

As per Faradays law of electromagnetic induction, whenever a conductor moves inside a magnetic field, there will be an induced current it. If this conductor gets forcefully moved inside the magnetic field, there will be a relation between the direction of applied force, magnetic field and the current. This relation among these three directions is determined by Fleming Right Hand Rule This rule states "Hold out the right hand with the first finger, second finger and thumb at right angle to each other. If forefinger represents the direction of the line of force, the thumb points in the direction of motion or applied force, then second finger points in the direction of the induced current.





















5) Gauss Law

There is always Static electric field around positive and negative charge Actually this flux is radiated/from the electric charge. Now amount of this flow of flux depends upon the quantity of charge it is emanating from. To find out this relation, the Gauss's theorem was introduced. This theorem can be considered as one of the most powerful and most useful theorem in the field of electrical science. We can find out the amount of flux radiated through the surface area surrounding the charge from this theorem.This theorem states that the total electric flux through any closed surface surrounding a charge, is equal to the net positive charge enclosed by that surface. Suppose the charges Q₁, Q₂_ _ _ _Q_i, _ _ _ Q_n are enclosed by a surface, then the theorem may be expressed mathematically by surface integral as

Where, D= is the flux density in coulombs/m²
dS= is the outwardly directed vector.

For better explanation lets go through example consider Q be the charge at the center and charge is normal to the subsurface states that total charge emanated from body equal to charge Q coulombs if charge is not place at the center other than center

At that time, the flux lines are not normal to the surface surrounding the charge, then this flux is resolved into two components which are perpendicular to each other, the horizontal one is the sinθ component and the vertical one is the cosθ component. Now when the sum of these components is taken for all the charges, then the net result is equal to the total charge of the system which proves Gauss's theorem.

Proof of Gauss’s Theorem

Let us consider a point charge Q located in a homogeneous isotropic medium of permittivity ε.

The Electric field intensity at any point at a distance r from the charge is

The flex density is given as,

Now from the figure the flux through area dS

Where, θ is the angle between D and the normal to dS

Now, dScosθ is the projection of dS is normal to the radius vector.
By definition of a solid angle

Where, dΩ is the solid angle subtended at Q by the elementary surface are dS. So the total displacement of flux through the entire surface area is

Now, we know that the solid angle subtended by any closed surface is 4π steradians, so the total Electric Flux through the entire surface is

This is the integral form of Gauss's theorem. And hence this theorem is proved.

Basics of circuit breaker mechanism

Title: Basics of Circuit breaker mechanism 

How circuit breaker trip unit works?

                                                                                                                                            Thermal magnetic Trip Unit

                                                               



In addition to providing a means to open and close its contacts manually, a circuit breaker must automatically open its contacts when an overcurrent condition is sensed.

The trip unit is the part of the circuit breaker that determines when the contacts will open automatically.

In a thermal-magnetic circuit breaker, the trip unit includes elements designed to sense the heat resulting from an overload condition and the high current resulting from a short circuit. In addition, some thermal magnetic circuit breakers incorporate a “PUSH TO TRIP” button.

Trip Mechanism

The trip unit includes a trip mechanism that is held in place by the tripper bar. As long as the tripper bar holds the trip mechanism, the mechanism remains firmly locked in place.

      Trip Unit with Trip mechanism

The operating mechanism is held in the “ON” position by the trip mechanism. When a trip is activated, the trip mechanism releases the operating mechanism, which opens the contacts.

Note: the drawings in this section show an AC power source; however, a DC source could also be used.

 The Operating mechanism is held in the “ON”    position by the trip mechanism.

Manual Trip

Some molded case circuit breakers, especially larger breakers, can be manually tripped by pressing the “PUSH TO TRIP” button on the face of the circuit breaker. When the button is pressed the tripper bar rotates up and to the right. This allows the trip mechanism to “unlock” releasing the operating mechanism.

The operating mechanism opens the contacts.

The “PUSH TO TRIP” button also serves as a safety device by preventing access to the circuit breaker interior in the “ON” position. If an attempt is made to remove the circuit breaker cover while the contacts are in the closed (“ON”) position, a spring located under the push button causes the button to lift up and the breaker to trip.

                          Manual trip mechanism

Overload Trip

Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions. When sufficient overcurrent flows through the circuit breaker’s current path, heat build up causes the bi-metalic strip to bend. After bending a predetermined distance, the bi-metalic strip makes contact with the tripper bar activating the trip mechanism.

Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions.

Circuit breaker contacts

A bi-metallic strip is made of two dissimilar metals bonded together. The two metals have different thermal expansion characteristics, so the bi-metallic strip bends when heated. As current rises, heat also rises.

The hotter the bi-metallic becomes the more it bends. After the source of heat is removed, as when the circuit breaker contacts open, the bi-metallic strip cools and returns to its original condition. This allows a circuit breaker to be manually reset once the overload condition has been corrected.

Short Circuit Trip

As previously described, current flow through a circuit breaker’s blow-apart contacts creates opposing magnetic fields. Under normal operating conditions, these opposing forces are not sufficient to separate the contacts. When a short circuit occurs, however, these opposing forces increase significantly.

The current that flows through the contacts also flows through a conductor that passes close to the circuit breaker’s trip unit. At fault current levels, the magnetic field surrounding this conductor provides sufficient force to unlatch the trip unit and trip the breaker.

                                                    Short Circuit Trip

The combined actions of magnetic fields forcing contacts apart while simultaneously tripping the circuit breaker result in rapid interruption of the fault current. In addition, because the magnetic forces are proportional to the current, the greater the fault current, the shorter the time it takes to interrupt the current.

Source: Siemens basics of circuit breaker



Thanks,
Kiran Patil

Testing in substation (Basics)

Title:Testing in HV substation (basics)


Thanks
Kiran

How to make Invertor at home.

Title:How to make Invertor at home.

Apparatus :

1) 9V, Battery ( AH capacity as per required )
2) FET 2N3055 ( 2 nos field effect transistor)
3) Resistors 15 ohm, 10W ( 4 nos ).
4) Invertor power transformer.
5) Load  ( Bulb or any other).
6) Connecting wires 1.5 sqmm.


Procedure:

Do the connection as per above circuit diagram

Note: transformer use in circuit is Inverter power transformer which is for special purpose having 3 input                   terminal and 2 output terminal as per shown in fig.

Thanks

Kiran Patil

How to control speed of motor using VFD

Title: How to control speed of motor using motor VFD (variable Frequency Drive)

Theory:

Now a days most of the industries use AC drives for various application as per requirement,To control the speed of AC drive use of VFD is most common practice now  a day.
       Lets go through some basics about speed and frequency relation


                                                           Speed = 120 x F
                                                                              P

                                                Where,  F=frequency

                                                              N= Speed of the drive

                                                              P = Number of pole

from above relation you can understand Frequency is directly proportional to speed,means if frequency is increase speed is also increase and vice versa.

Using VFD we control the out put frequency & voltage of machine accordingly speed of machine is also vary. lets see the VFD connection and speed control

Procedure:

lets see ABB VfD circuit diagram without using any other external devices (Means locally without remote) 

From Circut dig.

1) Connect the Supply terminal to U1,V1,W1 Respectivly if there is single phase supply you can connect U1 as a phase and V1 as a neutral.

2) connect the motorr terminal to U2,V2,W3.

3) We have to start motor locally without external devices that's why we take loop between Point 10 and pint 11 in sim-miller manner from point 9 to point 13 as per circuit diagram other wise no need of looping in remotely control.

Operation / Settings:

1) From above pic yo can see the key's on VFD controller for understanding numbering is given to all.

2) once connections is done as per circuit diagram we have to start motor by pressing Key LOC / Remort (No-5) press START youre motor will star LOC as per default settings which is also shown on display.(LOC means using only VFD keys not any external source).

3) But if you to change settings then Press ENTER you will see two options PAR and REF, 

4) If You select option REF then again pressing Enter you will see Frequency SET option then using UP & DOWN you can set frequency means you can adjust the speed.( If your frequency is LOCK at certain value which will not go beyond the value then you to follow the bellow steps)

5) To unlock youre frequency setting you have to select PAR then you see certain numbers on display then by pressing UP key set the value at 00-20 then press ENTER after that you will see 2007 or 2008 then make this all zero using UP & DOWN keys then press ENTER. your frequency SET  will UNLOCK after following these steps. 

Using above procedure you can control the speed of AC DRIVE in simple way.

For remotely control you can go with USER Manual of respective VFD manufacturer.  

For reference only (As per manufacturer)

Ref : ABB user manual for VFD

Thanks 

Kiran Patil