Above 90% of motors used in an industry are induction motors, because they are cheap, robust and easy to maintain. For higher HP (>250HP) motors we prefer high voltage, because it will reduce operating current and the size of the motor.Why We Require Protection of Motors?To understand this we need to know the cost associated with the failure of motor, i.e. Loss of production (Cost of production) Replacement of motor (Replacement cost) Cost of repair Cost of man hours due to this emergencyThe

The relays used in power system protection are of different types. Among them differential relay is very commonly used relay for protecting transformers and generators from localised faults.Differential relays are very sensitive to the faults occurred within the zone of protection but they are least sensitive to the faults that occur outside the protected zone. Most of the relays operate when any quantity exceeds beyond a predetermined value for example over current relay operates when current t

In this type of relays, the time of operation depends upon the magnitude of actuating quantity. If the magnitude of actuating quantity is very high, the relay operation is very fast. In other words, the relay operating time that is time delay in the relay is inversely proportional to the magnitude of actuating quantity.The general characteristics of an inverse time relay is shown are in figure below.Here, in the graph it is clear that, when, actuating quantity is OA, the operating time of the re

In a simple way electrical relay is shown in the figure above. Here the fixed coil is fed from the circuit to be protected. When there is a current in fixed coil above the pick up value, the iron plunger is so attracted, it moves upward and makes NO contact closed. The function of this relay is very fast. The normally open (NO) contacts of the relay are closed as soon as current in the fixed coil crosses the pick up value. This is a simplest example of instantaneous relay. Because ideally there

There is one type of relay which functions depending upon the distance of fault in the line. More specifically, the relay operates depending upon the impedance between the point of fault and the point where relay is installed. These relays are known as distance relay or impedance relay.Working Principle of Distance or Impedance RelayThe working principle of distance relay or impedance relay is very simple. There is one voltage element from potential transformer and a current element fed from cur

The coefficient of expansion is one of the basic properties of any material. Two different metals always have different degree of linear expansion. A bimetallic strip always bends when it heated up, due to this inequality of linear expansion of two different metals.Working Principle of Thermal RelayA thermal relay works depending upon the above mentioned property of metals. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over

In an over current relay or o/c relay the actuating quantity is only current. There is only one current operated element in the relay, no voltage coil etc. are required to construct this protective relay.Working Principle of Over Current RelayIn an over current relay, there would be essentially a current coil. When normal current flows through this coil, the magnetic effect generated by the coil is not sufficient to move the moving element of the relay, as in this condition the restraining force

Induction Cup RelayThis relay in nothing but one version of induction disc relay. Induction cup relay work in same principle of induction disc relay. The basis construction of this relay is just like four poles or eight pole induction motor. The number of poles in the protective relay depends upon the number of winding to be accommodated. The figure shows a four pole induction cup relay.Actually when any one replaces disc of induction relay by a aluminum cup, the inertia of rotating system of re

What is a Dry Contact?A dry contact (also known as a volt free contact or potential-free contact) is defined as a contact in which power/voltage is not directly provided from the switch but is instead always being supplied by another source. Dry contacts are known as passive contacts, as no energy is applied to the contacts.Dry contact simply operates like an ordinary switch that opens or closes the circuit. When the contacts are closed the current flows through the contacts and when the contac

What is a Latching Relay?A latching relay (also known as a bistable, keep, impulse, stay relay, or simply a “latch”) is defined as a two-positional electromechanical switch. It is an electrically actuated switch used to maintain its position without power applied to the coil.A latching relay is used to control the large flow of current with a smaller current. The coil of the latching relay consumes power only while the relay is switched ON. And its contact remains in position after the switch h