Rectifier type instrument measures the alternating voltage and current with the help of rectifying elements and permanent magnet moving coil type of instruments. However the primary function of rectifier type of instruments work as voltmeter. Now one question must arises in our mind why we use rectifier type of instruments widely in the industrial world though we have various other AC voltmeter like electrodynamometer type instruments, thermocouple type instruments etc? The answer to this questi

Working Principle of Electrostatic Type InstrumentsAs the name suggests the electrostatic type instrument use static electrical field to produce the deflecting torque. These types of instrument are generally used for the measurement of high voltages but in some cases they can be used in measuring the lower voltages and powers of a given circuit. Now there are two possible ways in which the electrostatic force can act. The two possible conditions are written below,Construction of Electrostatic Ty

What is a Permanent Magnet Moving Coil (PMMC)?A Permanent Magnet Moving Coil (PMMC) meter – also known as a D’Arsonval meter or galvanometer – is an instrument that allows you to measure the current through a coil by observing the coil’s angular deflection in a uniform magnetic field.A PMMC meter places a coil of wire (i.e. a conductor) inbetween two permanent magnets in order to create stationary magnetic field. According to Faraday’s Laws of electromagnetic induction, a current carrying conduc

In order to understand the concept of errors in measurement, we should know the two terms that defines the error and these two terms are written below:True ValueIt is not possible to determine the true value of a quantity by experiment means. True value may be defined as the average value of an infinite number of measured values when average deviation due to various contributing factor will approach to zero.Measured ValueIt may be defined as the approximated value of true value. It can be found

Basically there are three types of measuring instruments and they are Electrical measuring instruments Mechanical measuring instruments. Electronic measuring instruments.Here we are interested in electrical measuring instruments so we will discuss about them in detail. Electrical instruments measure the various electrical quantities like electrical power factor, power, voltage and current etc. All analog electrical instruments use mechanical system for the measurement of various electrical quant

The principle of working and construction of induction type meter is very simple and easy to understand that’s why these are widely used in measuring energy in domestic as well as industrial world. In all induction meters we have two fluxes which are produced by two different alternating currents on a metallic disc. Due to alternating fluxes there is an induced emf, the emf produced at one point (as shown in the figure given below) interacts with the alternating current of the other side resulti

Electromagnetic RelayElectromagnetic relays are those relays which are operated by electromagnetic action. Modern electrical protection relays are mainly micro processor based, but still electromagnetic relay holds its place. It will take much longer time to replace all electromagnetic relays by micro processor based static relays. So before going through detail of protection relay system we should review the various types of electromagnetic relays.Electromagnetic Relay WorkingPractically all th

Definition of Protective RelayA relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.Now let’s have a discussion on some terms related to protective relay.Pickup Level of Actuating Signal:The value of actuating quantity (voltage

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