Explanation of Classification of Resistance

Classification of Resistance

Resistance that are required to be measured may have any value from a few micro ohms to mega ohms. From the point of view of measurement, resistances are classified as three main groups.

1.       Low Resistances: resistance of the order of 1Ω or less than 1Ω.

2.       Medium Resistance: This class includes the resistance from 1Ω upwards to about 0.1MΩ.

3.       High Resistance: Resistances of the order of 0.1 MΩ and above are classified as high resistance.

Note: The classification given above is not rigid, but forms a basis for techniques, followed or measurement. Which may be different for different classes.

Different Methods used for Measurement of Resistance are:

1.       For low Resistance Measurement:

               (a)     Ammeter-Voltmeter method

Explanation of Weston (M.I) Type Frequency Meter

Weston (M.I) Type Frequency Meter

Principle:  It is a moving iron instrument which operates on the principle of variation of impedance in an inductive circuit with variation of frequency.

Weston Frequency Meter

Construction: It consists of two coils A and B mounted perpendicular to each other. Each coil is divided into two sections. The connections are as shown in the above fig. The branch of coil has a resistance R₁ connected in the series with it while coil B

Explanation of Power Factor Meter

Power Factor Meter

The power factor of an A.C circuit can be calculated from the following relation by measuring the current, voltage and power. I.e. cosΦ = P/VI.

This method of determining the power factor of an electrical A.C circuit, is however, of low accuracy has a number of draw backs. Hence, it is desirable to have an instantaneous indication of the power factor of an A.C circuit, especially where it is varying continuously.

Therefore, a power factor meter is connected in the A.C circuit to measure power factor which indicate directly by a single reading.

There are two types of power factor meters:

1.       Electrodynamometer type of p.f meters

What are the Errors and their Adjustments in 1Φ energy meter

Errors and their Adjustments in 1Φ energy meter

Energy meters should give correct readings over a period of several years under normal use conditions. Some of the common errors in energy meter and their remedial measures are discussed below.

1.       Phase Error: It is necessary that the energy meter should give correct reading on all power factors, which is only possible when the field setup by shunt magnet lags behind the applied voltage by 90^o. But the flux due to shunt magnet does not lag behind the applied voltage exactly by 90^o because of winding resistance and iron losses.

Adjustment: The flux in the shunt magnet can be made to lag behind the supply voltage by exactly 90^o by adjusting the position of shading band (or shading ring or shading coil) placed round the lower part of the control limb of the shunt magnet.

This adjustment is known as lag adjustment or power factor adjustment (or power factor compensator).

2.       Speed Error: Sometimes the speed of the meter is either fast or slow, resulting in the wrong recording of energy consumption.

Adjustment: An error in the speed of the meter when tested on non-inductive load can be eliminated by correctly adjusting the position of the brake magnet.

Explanation of Single Phase Induction Type Energy Meter

Single Phase Induction Type Energy Meter

Single phase induction type energy meters are extensively used for the measurement of electrical energy in single phase A.C circuits. One can find such meters installed in homes. It is also called as single phase watt-hour meter.

Single Phase Induction Type Energy Meter

Principle: torque is produced to rotate the disc is due to interaction of two fluxes. Since the e.m.f induced in the disc by induction, this instrument operates on A.C only.

Construction: The below fig shows the various parts of a single phase induction type energy meter. It consists of a shunt magnet, a series magnet, a rotating disc, a brake magnet and a counting mechanism.

The shunt magnet consist of a number of M shaped iron laminated core wound with a fine wire of many turns i.e. coils and is connected across the supply. The shunt coil carries a current proportional to supply voltage and is known as Pressure coil.

Explanation of Errors in Dynamometer Type Instrument

Errors in Dynamometer Type Instrument

The main sources of error in a dynamometer type of instruments are due to

1.       Low torque to weight ratio.

2.       Frequency

3.       Eddy currents

4.       External magnetic fields

5.       Temperature change

1.       Low Torque/Weight Ratio: Since the coils, are air cored, therefore, magnetic field produced is of small strength. Hence, large number of turns are to be provided. This results in heavy moving system and therefore, small torque/weight ratio. Hence frictional losses are large as compared to other type of instruments.

2.       Frequency: The frequency error is due to

            (a)     Change in ration of the operating coils and

Explanation of Dynamometer Type Wattmeter

Dynamometer Type Wattmeter

This is also similar in design and principle to the dynamometer type ammeter and voltmeter.

Pic 1

Construction: In the dynamometer type instrument is used as a wattmeter, the fixed coil, which is divided into two equal portions, the fixed coils are connected in series with the load and carry the circuit (or load) current, therefore it is also called as current coil. The moving coils is connected across the load and carries the current proportional to the voltage across the circuit. Therefore, it is known as pressure coil or potential coil.

A high non-inductive resistance ‘R’ is connected in series with the moving coil to limit current through it.

Advantages and Disadvantages of Dynamometer Type Instruments

Advantages and Disadvantages of Dynamometer Type Instruments

Advantages:

1.       These instruments are free from hysteresis losses and eddy current losses.

2.       They have a precision grade accuracy.

3.       These instruments can be used on both A.C and D.C. They are also used as a transfer instruments.

4.       Electrodynamometer voltmeters are very useful where accurate RMS values of voltage, irrespective of waveforms, are required.

Disadvantages:

1.       They have a non-uniform scale.

Explanation of Dynamometer Type Instrument

Dynamometer Type Instrument

ElectroDynamometer Instrument

The electrodynamometer type instrument is a transfer instrument. A transfer instrument is one which is calibrated with a D.C source and used without any modifications for A.C measurements. Such a transfer instruments has same accuracy for A.C and D.C measurements. The electro-dynamometer type instruments are often used in accurate A.C voltmeters and ammeters, not only at the power line frequency but also in the lower audio frequency range with some little modifications, it can be used as a wattmeter for the power measurements.

Principle: Electro-dynamometer type instruments are very similar to PMMC type instrument in which the operating field is produced, not by a permanent magnet but by another fixed coil (usually two fixed air cored coils are used).

The PMMC instrument cannot be used on A.C currents or voltages. If A.C supply is given to these instruments, an alternating torque will be developed. Due to moment of inertia of the moving system, the pointer will not follow the rapidly changing alternating torque and will fail to show any reading. In order that the instrument should be able to read A.C quantities, the magnetic field in the air gap must change along with the change in current. This principle is used in the electro-dynamometer type instrument.

Construction: The below fig shows the construction of the electro-dynamometer type instrument.

Explanation of Extending the Range of Ammeters and Voltmeters

Extending the Range of Ammeters and Voltmeters

The ranges of electrical measuring instruments (whether ammeter, voltmeter or any other type of meters) are limited by the currents, which be carried by the coils of the instruments safely. For example, the moving coils of the instruments can carry maximum current of about 50 mA safely and the potential drop across the moving coil is about 50 mV. Hence, it becomes necessary that the current and voltage being measured be reduced and brought within the range of instrument.

There are four common devices used for extending the range of the instruments, namely \

1.       Shunts

2.       Multipliers

3.       Current transformers

4.       Potential transformers

The let us discuss the first two topics i.e. shunts and multipliers

1.       Shunts: The range of an ammeters can be extended by connecting a low resistance, called shunts, connected in parallel with ammeter. The shunt by passes the extra current and allows only safe current