All About Electrical Relays, Principles, Classification and Requirements

Protective relays
Relay is a device by means of which an electric circuit can be controlled by the change in the same circuit or in other circuit. The primary function of the protective relay is to sense the fault in the system, compare the signal so obtained with  the reference signal under normal conditions of operation and amplify the error signal such that the trip coil of the circuit breaker is energized and faulty section of the system is disconnected from the rest of the system.Under normal conditions of the system, a protective relay is required to keep itself alert such that in case of any emergency, the action may  be taken almost instantneously.It should remain silent under normal condition. Under certain operating condition, the power system components are required to carry more than rated current on a temporary basis. During peak load condition, the power system components arc intentionally over-loaded. Under such conditions of operation, often known as abnormal condition, it is not necessary to disconnect the section from the system. When the operation parameters are unduly strained, the protective relays are required to energize the alarms such that proper precautions are taken.

The Basic Principles of Relays
In electromechanical relays, there are one or more coils, movable elements, contact system, etc. The operation of such relays depends on whether the operating torque/force is greater than the restraining torque/force.The relay operates, if the net force, F in equation given below is positive. Where F is the net force, Fo is the operating force and Fr is the restraining force. In other words, the relay operates only if the operating force is greater than the restraining force. In electromechanical relays, the operating torque is produced by electromagnetic attraction/electromagnetic induction/thermal effects of electric current. The restraining torque is given by springs.

The Classification of Relays
Protective relays are classified as follows according to their construction and the principle of operation.
1. Electromagnetic relays These are actuated by Direct current or Alternating current quantities.
2. Electomagnetic induction relays
3. Electothermal relays
4. Static relays these employ transistors or magnetic amplifiers to obtain the operating characteristic.
5. Electrodynamic relays these are operate on the same principle as moving coil instruments.
6. Under-voltage, under-current and under-power relays.
7. Over-voltage, over-current and over-power relays
8. Direntional or reverse current relays
9. Differential relays
10. Distance relays

The Basic Requirements of Protective System
The efficient protective relaying system should posses the following characteristics:
1. Speed 
Minimum voltage and minimum fault time operate.
2. Selectivity
Maximum continuity of service by disconnecting the faulty part of the system.
3. Sensitivity
Capability of operating reliably under the actual desired conditions.

Protective Relays
These are the devices that detect abnormal conditions in electrical circuits by measuring the electrical quantities which are different under normal and faulty conditions. Due to abnormal conditions, voltage, current, phase and frequency may change. After detecting the fault, the relay operates to complete the trip circuit which results in the opening of the circuit breaker and isolating the faulty circuit.

All About Electrolysis and Batteries Facts and Tutorials

1. The two main defects of the primary cells are local action and polarization.
2. Local action in the primary cell can be rectified by amalgamating the zinc electrode with mercury.
3. One factor affecting voltages of the primary cell is the types of plates and electrolyte.
4. Distilled or approved water is used in electrolytes because it prevents or slows down local action.
5. In electroplating, the positive electrode is called the anode.
6. The mass of an ion liberated at an electrode is directly proportional to the quantity of electricity which passes through the electrolyte. This statement is associated with laws of electrolysis.
7. The mass of material deposited over an electrode is proportional to quantity of electricity and electro chemical equivalent.
8. The condition of a secondary cell can be determined by the terminal voltage and strength of the electrolyte.
9. Nickle-cadium dry cell is becoming popular in power supplies for electronic calculators because it is rechargeable.
10. One advantage of a secondary cell is that it can be recharged.
11. Cell are connected in parallel to increase the current capacity.
12. Cells are connected in series to increase voltage output.
13. To obtain a high voltage of about 1.9 volts from a dry cell on would use magnesium cell.
14. While charging a battery, charge the battery in an airy room, remove the vent plugs during charging, keep flames etc. away from the battery and keep the charging current rate not more that 3 to 6 amperes.
15. If the internal resistance of a discharged battery is more it is not desirable to leave a lead storage battery in a discharged state for a long time mainly because plates will become sulphated.
16. To keep the terminals of a lead acid storage battery free from corrosion, it si advisable to keep the electrolyte level low.
17. If a sixty ampere hours battery has sixty seven ampere discharge rate, it will provide a current of six amperes for ten hours.
18. The ampere hour capacity of battery depends on the area of the plates.
19. Electrolyte of a storage battery is formed by adding suphuric acid to water.
20. Other types of accumulators besides the lead acid type are nickle cadmium batteries.
21. Electro-chemical equivalent is mass of the element liberated per unit of quantity of hydrogen.
22. Impurities in an electrolyte can cause an internal short circuit condition called local action.
23. The action of a dry cell is to change chemical action to electrical energy.
24. Polarization in dry cell can be got rid of by chemical means.
25. Gassing occurs in the process of charging an accurnuiator.

Terms used in Electrolysis

Anode
The plate or electrode through which the current enters the electrolyte or it may be defined as the plate or electrode connected to the positive terminal of supply.

Anions
The ions having negative charge are known as anions.

Atom
An atom is the smallest particle of matter which takes part in a chemical action.

Atomic weight
The atomic weight of an element is the relative weight of its atom compared with that of an atom of hydrogen. The atomic weight of H atom is taken as unity.

Cathode
The plate or electrode through which the current leaving the electrolyte or the plates connected to the negative terminal of supply mains.

Cations
The ions having positive charge are known as cations.

Chemical equivalent
The chemical equivalent of an element is the mass which is chemically equivalent to a unit mass of hydrogen.

Ions
When the current is passed through electrolyte, the electrolyte gets chemically decomposed, molecules of the electrolyte splits up into two parts known an ions.

Molecule
The molecule is the smallest particle of any substance which is capable of separate existence in a chemical form.

Valency
The valency of an element is the number of hydrogen atoms with which it will combine or with which it will replace in a compound.

Things You Should Know About Direct Current Generator Tutorials

The Definition
We know that Direct Current Generator is a machine which converts the mechanical energy into electrical energy. The generator is usually driven by a steam  engine or a diesel engine or an electric motor which are called prime movers.

The Principle
The principle of Direct Current Generator is it works on the principle of Faraday's Laws of electromagnetic induction. According to this law the conductors or armature are rotated in the magnetic field and electro magnetic force is induced in these conductors which is collected from the commutators fitted on the shaft of armature.

The Loop
The simple loop of a Direct Current Generator when the plane of the coil is at right angles to the lines of flux, the flux linked with the coil is maximum but the rate of change of flux linkage is minimum. As coil continues to rotate further, the rate of change of flux linkage increases, till it attains maximum value of 90 degree to 180 degree, the flux linked with the coil gradually increases resulting in decrease in induced electro motive force till it reduces to zero at 180 degree. A reversal of the trend occurs during next half revolution. For unidirectional current, the ends of the coil are connected commutation whose function is to reverse the connections to the rotating coil through fixed brushes and to collect the electro motive in one direction.

Four Reasons for failure of a generator to build voltage:
1.Defective contact of brushes with conmmutator due to dirt, insufficient pressure, tight brushes, dirty or rough commutators or projecting intersegment micas.
2. High resistance or open circuit in the shunt field circuit, faulty contact or burnt resistance in shunt regulators.
3. Loss of residual magnetism.
4. Reverse field connection or reversed speed.

Ten Reasons for sparking and Bad Bommutations:
1. Overload
2. Projecting intersegment micas.
3. Earth fault on armature
4. Armature short circuit
5. Incorrect brush position
6. Wrong grade of brushes
7. Reversed interpole coils
8. Brushes not properly bedded
9. Brushes not equally spaced
10. Worn Brushes

Question and answer about D.C. Generator
1. The armature of a DC generator is laminated to reduce eddy current loss.
2. In a shunt generator the voltage built up is generally restricted by the saturation of iron.
3. Copper loss in DC generators varies with load.
4. Full load efficiency of the generators is 92.51 percent.
5. Shunt generators are preferred for parallel operations.
6. In DC generator the ripples in the direct electro motive force generated are reduced by using commutator with large number of segments.
7. The functions of an interpole is to neutralize crossfield of armature reaction and obtain ideal commutation.
8. Equalizer connection are required when paralleling two compound generators.
9. A simple method of increasing the voltage of DC generator is to increase the speed of rotation.
10. In the commutation process it is the current which is getting reversed.