Principles of Dc Instruments and measurements

Principles of Dc Instruments and measurements

MEGGER TESTERBRIDGE METHODWATTMETER METHODCALIBRATION PRINCIPLES1MEGGER TESTERMEGGER TESTER(in Engineering) an instrument that generates a high voltage in order to test the resistance of insulation, etc.The Megger test is a method of testing making use of an insulationthe Megger test, or insulation resistance test, is going to be useful in a host of different situations and for different purposes. It will be possible to detect leaks on bad motors, generators, transformers, and many other devices. In addition, it is possible to test the wiring in buildings.BRIDGE METHODBRIDGE METHODA bridge circuit is a type of electrical circuit in which two circuit branches (usually in parallel with each other) are "bridged" by a third branch connected between the first two branches at some intermediate point along them. The bridge was originally developed for laboratory measurement purposes and one of the intermediate bridging points is often adjustable when so used. Bridge circuits now find many applications, both linear and non-linear, including in instrumentation, filtering and power conversion. The bridge circuit is useful in measuring impedances (resistors, capacitors, and inductors) and in converting signals from transducers to related voltage or current signals.TYPES OF BRIDGEWheatstone bridge. A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer.

TYPES OF BRIDGEThe Wien bridge is a type of bridge circuit that was developed by Max Wien in 1891. The bridge comprises four resistors and two capacitors.

TYPES OF BRIDGEA Maxwell bridge (in long form, a Maxwell-Wien bridge) is a type of Wheatstone bridge used to measure an unknown inductance (usually of low Q value) in terms of calibrated resistance and capacitance. It is a real product bridge.

TYPES OF BRIDGE

Wattmeter methodVarious methods are used measurement of three phase power in three phase circuits on the basis of number of wattmeter used. We have three methods1. Triple wattmeter method2. Double wattmeter method3. Single wattmeter method.Triple Wattmeter method

Triple Wattmeter methodPressure coils of all the three wattmeter are connected to common point at neutral line. Clearly each wattmeter will give reading as product of phase current and line voltage which is phase power.

Double Wattmeter methodIn this method we have two types of connections:(a)Wye connection of loads(b)Delta connection of loads.

Double Wattmeter methodWYEDELTA

Double Wattmeter methodFor WYE connected load clearly the reading of wattmeter one is product phase current and voltage difference (V2-V3). Similarly the reading of wattmeter two is the product of phase current and the voltage difference (V2-V3).

Double Wattmeter methodWYEDELTA

Double Wattmeter methodFor DELTA connected load.The reading of wattmeter one can be written as

and reading of wattmeter two is

but V1+V2+V3=0, hence expression for total power will reduce to V1I1+V2I2+V3I3.

Single Wattmeter method

Single Wattmeter methodLimitation of this method is that it cannot be applied on unbalanced load. So under this condition we have I1=I2=I3=I and V1=V2=V3=V

Single Wattmeter methodTwo switches are given which are marked as 1-3 and 1-2, by closing the switch 1-3 we get reading of wattmeter as

Similarly the reading of wattmeter when switch 1-2 is closed is

CALIBRATION PRINCIPLES

CALIBRATION PRINCIPLES

Calibration is a fundamental (but often overlooked) step in the process of measurement. It ensures that the instrument which is used for testing accurately indicates the required measurement parameter and that the instrument is performing to its specification.is a comparison between measurements. one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device. The device with the known or assigned correctness is called the standard. The second device is the unit under test, test instrument, or any of several other names for the device being calibrated.

Conversion analysis of galvanometer using shunt and series resistorsConversion analysis of galvanometer using shunt and series resistorsGalvanometer into voltmeter analysisGalvanometer into ammeter analysisPower rating of resistor analysis

Galvanometer into voltmeter analysisGalvanometer - is a type of sensitive ammeter: an instrument for detecting electric current. It is an analog electromechanical actuatorthat produces a rotary deflection of some type of pointer in response to electric current through its coil in a magnetic field.

Galvanometer into voltmeter analysisTo convert a galvanometer to volt meter, a large resistance called the multiplier is connected in series with the coil of the galvanometer.

Galvanometer into voltmeter analysis.

Galvanometer into voltmeter analysisThe figure show a galvanometer G connected in series with high resistor Rm across a component in the circuit. We are interested to know the potential differenceV across the component. If I is the maximum current through the galvanometer then according ohms lawThe total voltage V is equal to the sum of the potential difference across the galvanometer and the potential difference across the multiplier.

Galvanometer into voltmeter analysisV = Vg + VmV = IgRg+ IgRmV = Ig ( Rg + Rm) (Rm+Rg)=V/IRm=(V/I) -Rg

Galvanometer into voltmeter analysisSAMPLE PROBLEM(easy)Lets say we need a voltmeter of to measure voltage between 0 - 10V, and we have galvanometer specifying that its resistance is 20ohms and maximum permissible current is 0.1mA.

Galvanometer into voltmeter analysisSolution:

R = __10V___ - 20ohms 0.1 x 10-3

Galvanometer to Ammeter Analysis Since Galvanometer is a very sensitive instrument therefore it cant measure heavy currents. In order to convert a Galvanometer into an Ammeter, a very low resistance known as "shunt" resistance is connected in parallel to Galvanometer. Value of shunt is so adjusted that most of the current passes through the shunt. In this way a Galvanometer is converted into Ammeter and can measure heavy currents without fully deflected.

Galvanometer to Ammeter Analysis VALUE OF SHUNT RESISTANCELet resistance of galvanometer = Rg and it gives full-scale deflection when current Ig is passed through it. Then, Vg = IgRg -------(i)

Galvanometer to Ammeter Analysis Let a shunt of resistance (Rs) is connected in parallel to galvanometer. If total current through the circuit is I.

Galvanometer to Ammeter Analysis Example Problem A galvanometer coil has a resistance of 12 and it shows full scale deflection at a current of 3 mA. How would you convert it into an ammeter of range 0 -6A.

Power rating of resistor analysisResistor Power Rating - is sometimes called the Resistors Wattage Rating and is defined as the amount of heat that a resistive element can dissipate for an indefinite period of time without degrading its performance.

Power rating of resistor analysis The power rating of a resistor :is measured in watts usually somewhere between W (0.125W) and 1W. Resistors with power ratings of more than 1W are usually referred to as power resistors, and are used specifically for their power dissipating abilities.can vary a lot from less than one tenth of a watt to many hundreds of watts depending upon its size, construction and ambient operating temperature. Most resistors have their maximum resistive power rating given for an ambient temperature of +70 degrees Celcius or below.

Power rating of resistor analysis

Power rating of resistor analysisResistor Power - if a resistance is subjected to a voltage, or if it conducts a current, then it will always consume electrical power and we can superimpose the three quantities of power, voltage and current.

Power rating of resistor analysis

Power rating of resistor analysisFormula of Power Dissipation given the resistance:

Power rating of resistor analysis

Power rating of resistor analysis