Measuring the Flow of Current: 7 Instruments | Electrical Engineering

The following points highlight the top seven instruments used for measuring the flow of current. The instruments are: 1. Ammeter 2. Voltmeter 3. Wattmeter 4. Ampere-Hour Meter 5. Watt-Hour Meter or Kilowatt-Hour Meter 6. Insulation Testing Megger or Megger Testing Set or Megger 7. Earth Testing Megger or Megger Earth Tester or Meg Earth Tester.

Instrument # 1. Ammeter:

The instrument by which the flow of current through a line is measured in ampere is called an ammeter or an ampere meter. It is nothing but a low resistance galvanometer. The meter remains connected in series with the line so that the full line current flows through the coil of the instrument. Therefore, the coil should have a -cross-section sufficient enough to carry the line current continuously without being burnt out or excessively heated.

The coil of an ammeter must have very low resistance also. Otherwise a considerable amount of voltage will be dropped across the instrument, and the load circuit will receive power at a voltage much below the rated value. As a result the loads will not operate properly.

In different electrical installations usually two types of ammeters are used,—Moving Coil Ammeter and Moving Iron Ammeter. Moving coil ammeter can be used only in d.c. system. The scale of this meter is uniformly divided. But such instrument cannot be used for a.c. measurement. When connected in series with an a.c. line, the pointer of the meter will move to and fro on the scale if the supply frequency is very low. For normal or high frequency of supply, the pointer will rest on zero reading.

Moving iron ammeters can be used both in d.c. as well as in a.c. circuits. Such an instrument has non-uniform scale. The markings on the scale are very close to each other at the beginning; the gap between two markings gradually increases and becomes maximum at the end of the scale.

If an ammeter is connected across two lines:

An ammeter is usually connected in series with the live line or the phase wire of a supply system. If this meter is connected across a live line and the neutral, it will cause a short-circuit between two lines at the point of connection. Since the coil of an ammeter has negligible resistance, a large current will flow through the meter coil from the live line towards the neutral causing the meter to be burnt out if the fuse wire of the supply line is not blown off immediately.

Instrument # 2. Voltmeter:

The instrument used to measure supply pressure or potential difference between two points in a circuit is called a voltmeter. It is connected across the live line and the neutral or between two live lines, and it measures the potential differences in volt between those two lines.

Voltmeter is a high resistance galvanometer. The coil of the instrument has large number of turns and is made of wire having very small cross-sectional area. Since a voltmeter is connected across two lines, the coil of the instrument must have very high resistance.

If the instrument had a low resistance, it would form a short-circuit between the lines at the point of connection, a large current would flow through the coil of the instrument burning it out, and the load circuits would not operate properly due to insufficient current supply. Hence, the coil of the instruments is made to have a high resistance so that a small fraction of the line current flows through it.

In different electric installations usually two types of voltmeters are used—Moving Coil Voltmeter and Moving Iron Voltmeter. Moving coil voltmeter is suitable only for d.c. supply system. The scale of the instrument is uniformly graduated. If this meter is connected across a.c. lines, the pointer will move to and fro on the scale if the supply frequency is very low. But for normal and high frequency, the pointer will rest on zero reading.

Moving iron voltmeter can be used in both d.c. and a.c. supply lines. The meter has non-uniform scale. The markings are close to each other in their lower parts, and spread out at the top.

If the voltmeter is connected in series with a supply line:

Since a voltmeter is connected across two lines, the coil of the instrument has very high resistance so that it does not cause a short-circuit between the lines at the point of connection. Often a high resistance is connected in series with the coil. Hence, if a voltmeter were connected in series with a supply line, it would remain in series with the, load circuit.

As a result a considerable amount of voltage would be dropped across the instrument, the load circuit would receive power at a voltage much below the rated value and also less current would flow through the circuit. Therefore the loads connected in the circuit would not operate properly.

Instrument # 3. Wattmeter:

The instrument which measures the power consumed by an electric circuit in watt or kilowatt is called a wattmeter. It works on the principle that mechanical force exists between two current carrying conductors.

A wattmeter essentially consists of two coils, namely, Current Coil and Potential Coil or Pressure Coil. The current coil is connected in series with the load and carry the circuit current. It has a few turns and comparatively large cross-sectional areas.

The potential coil is connected across the load with its one terminal joined with a terminal of the current coil and the other terminal with a line in which there is no current coil. It carries a current proportional to the supply voltage. Potential coil has large number of turns. It is made of wire having very small cross-sectional area. The connections of a wattmeter with supply line is shown in fig. 60(a).

If a current of I-ampere flows through the current coil and the pressure across the potential coil is V-volt, then in case of d.c. supply power indicated by the pointer on the scale of the instrument is VI watt while in case of a.c. supply power indicated is VI Cos 8 watt, where 0 is the angle of phase difference between V and I.

If the supply current and the supply voltage are both high, in a d.c. instrument a shunt is connected in parallel with the current coil and a swamp resistance in series with the potential coil. In an a.c. instrument usually a current transformer is used with the current coil and a potential transformer with the potential coil.

Since the potential coil has large number of turns, it has very high inductance. This high inductance does not affect the operation of a d.c. instrument, but it adversely affect the accuracy of an a.c. instrument. In order to reduce this bad effect of high inductance, therefore, a high resistance is connected in series with the potential coil of the instrument. This is shown in fig 60(b).

Usually three types of wattmeters are in use.

These are:

(a) Dynamo-meter type wattmeter,

(b) Induction type wattmeter, and

(c) Electro static type wattmeter.

Out of these three types dynamometer type and induction type instruments are widely used in different electrical installations. But induction type instruments can be used only in a.c. circuits. It cannot be used in d.c. supply system.

Instrument # 4. Ampere-Hour Meter:

Ampere-hour meter is an energy meter which can be used only in d.c. circuits. The area where d.c. supply is available, these meters are used as house service meters to record the electrical energy in kilowatt-hour consumed per month by the consumers.

Ampere-hour meter has only one coil which is current coil. It has no pressure coil. The current coil has a few turns. It is made of wire having large cross-sectional area. The coil is connected in series with the live line. Therefore the meter has only two terminals.

With one terminal the live line of the supply is connected, from the other terminal the live line goes to the load circuit. Neutral line of the supply system has no connection with this meter. It goes direct to the load circuit.

The connection of an ampere-hour meter is shown in fig. 61. The quantity of electricity consumed by the load circuits during a certain period of time is recorded by the meter. The load current flows through the current coil. This current in ampere multiplied by the time in hour during which the current flows gives the quantity of electricity in ampere-hour which is recorded by the energy meter, and as such it is called ampere-hour meter.

But the scale of the instrument is graduated in kilowatt-hour which is calculated as follows:

Kilowatt-hour = [(ampere-hour consumed) x (supply pressure)]/1000

Supply pressure is measured in volt. It is assumed to be constant so long the load circuit consumes electrical energy. If the supply pressure varies, the meter cannot read correct amount of energy.

There are different types of ampere-hour meter, such as mercury meter, motor meter etc. Out of these meters Ferranti Mercury Meter has the largest application in different d.c. circuits.

Instrument # 5. Watt-Hour Meter or Kilowatt-Hour Meter:

Kilowatt-hour meter is an energy meter which may be used both in d.c. circuit as well as in a.c. circuit. A.C. meters may be either single-phase type or three-phase type. Single-phase a.c. meters are usually used as house-service meters in those localities where supply is available in a.c.

So far as construction is concerned, there are many similarities between a watt-hour meter and a wattmeter. A watt-hour meter has also two coils—one current coil and the other potential coil or pressure coil. Current coil has a few turns and is made of wire having comparatively large cross-sectional area. It is connected in series with the live line and the total load current flows through this coil.

The potential coil is made of wire having comparatively small cross-sectional area. It has large number of turns. It is connected across the supply lines with one terminal joined with a terminal of the current coil and the other terminal with that line in which there is no current coil (i.e. with the neutral line). Fig. 62 shows the connection of a kilowatt-hour meter.

The disc of the meter rotates continuously as long as current flows through the current coil and as long as there is potential difference across the potential coil. The disc is mounted on a vertical spindle. The spindle is supported by a cup-shaped jewelled bearing at the bottom and has a spring journal bearing at the top. There is no pointer and control spring so that the disc makes continuous rotation under the action of deflecting torque.

A train of wheels is attached with the disc. If a pointer and a clock dial are attached with each individual disc., the dial of the instrument is called pointer dial. Here each clock indicates a digit and all the digits together complete a number which gives the total energy consumed by the circuit.

In place of pointer dial another type of dial is also used in energy meters. This is called cyclometer dial. In such a dial there are several holes and a number appears in each hole. All the numbers together indicate directly the total amount of energy consumed. Now a day’s cyclometer dial is preferred over pointer dial in house service meters.

If V-volt be the potential difference across the pressure coil and I-ampere be the current which flows through the current coil continuously for t hours, then

energy recorded by a d.c. meter = VI t/1000 kilowatt-hour, and

energy recorded by an a.c. meter = (VI Cos θ x t)/1000 kilowatt-hour,

where θ is the phase angle between V and I and cos θ is the power factor of the load circuit.

Four different types of energy meters are usually used in electric circuits.

These are:

(i) Mercury Watt-hour Meter,

(ii) Commutator Motor Meter,.

(iii) Clock Meter, and

(iv) Induction-type Watt-hour Meter.

Out of these four types mercury watt-hour meter and commutator motor meter can be used only in d.c. circuits, clock meter may be used both in d.c. and a.c. circuits and induction-type watt-hour meter can be used only in a.c. circuits.

Instrument # 6. Insulation Testing Megger or Megger Testing Set or Megger:

The instrument by which the insulation resistance of a conductor can be measured is known as Insulation Testing Megger or in brief Megger. Insulation resistance has very high values. Usually this resistance is measured in megohms, and hence the instrument is called a ‘megger’.

The different parts of a megger and the connections between them are shown in fig. 66. In this instrument there are two permanent magnets MM of equal strength. One end of each magnet is used to supply field for a hand driven generator and the other end for the moving coil system of the instrument.

The armature G of the generator with its iron pole pieces is bridged across one pair of poles of the two magnets and the iron pole pieces and the moving system of the instrument are bridged across the other pair of poles. The generator is hand driven and is usually a 500-volt machine, but for high-test potential, ratings as high as 1000 volts and 2000 volts are also available.

Megger works on the principle of an ohmmeter. The moving system of a megger consists primarily of two coils C and P. C is the current coil and P is the pressure coil. Sometimes an additional coil A is connected is series with P. This coil acts as a compensating coil to make the scale of the instrument more evenly divided. Coil C is called the current coil.

One terminal of this coil is connected to the negative brush of the generator and the other terminal with a resistance R’ to the line terminal L of the megger. When a resistance is connected across the line and the earth terminals of the megger, the current coil, the resistance R’ and the resistance under measurement are all in series across the armature brushes. The resistance R’ protects the current coil when the instrument is short-circuited.

Coil P is called the pressure coil or potential coil. It is connected across the armature brushes in series with a suitable resistance R. Pressure coil is narrower than the current coil, and when it moves, it encircles a part of the C-shaped iron core in some positions. All the three coils A, P & C and the pointer N are rigidly connected, so that they move together.

The moving system of the instrument is mounted on spring-supported jewel bearings and is free to rotate on its axis. Current is led into the coils by flexible conducting ligaments having negligible tension. Therefore, when the generator is not operated, the pointer floats over the scale and may remain in any position whatsoever.

When current flows through the current coil, it tends to rotate in a clockwise direction. But the current in the pressure coil causes a rotation in anti-clockwise direction. If the line and the earth terminals (L & E respectively) of the megger are open or if a resistance of infinite value is connected across them, no current flows through the current coil. In that case the pressure coil alone controls the movement of the moving system.

The coil P then takes a position opposite the gap in the core, and the pointer N will indicate infinity (INF). But when a resistance is connected between the terminals, current flows through both the coils and the moving system takes up an intermediate position. As the two toils Eire rigidly connected, a steady deflection is obtained when the torque produced by the coil C is just equal and opposite to that produced by the coil P.

Therefore the scale may be calibrated in terms of resistance, and the instrument becomes direct reading. The change in the supply voltage affects both the coils P and C in the same proportion, and hence the position of the moving system is independent of the voltage.

When the terminals of the megger are short-circuited or when a resistance of low value is connected between the terminals, current flowing through the current coil will be much greater than that flowing through the pressure coil. As a result coil C alone will control the movement of the moving system, and this coil will take up such a position that the pointer will indicate zero (ZERO).

A guard wire is also provided so that any leakage Current over the terminals or within the megger itself is shunted to the negative terminal of the generator without passing through the current coil. Due to this arrangement the leakage current cannot affect the indications of the megger.

Difference between Megger and Other Measuring Instruments:

When no current flows through the coil of an instrument like ammeter, voltmeter or wattmeter, the pointer of the instrument rests on zero reading due to tension of the controlling spring. But the system is quite different with a megger. In an insulation testing megger the moving system is mounted on spring-supported jewel bearings and is free to rotate on its axis. Current is led into the coils by flexible conducting ligaments having negligible tension.

When the hand-driven generator of the instrument is not operated, i.e. when no current flows through the current coil and the pressure coil of the instrument, the pointer floats over the scale and may remain in any position whatsoever. Therefore the statement of a person that “the pointer of an insulation testing megger rests on zero reading when not in use” is totally wrong.

Testing of a Megger before Use:

Before use for different testing work of an installation, it is always imperative to test the accuracy of an insulation testing megger. Many important tests like insulation resistance test, short-circuit test, continuity test are carried out with the help of a megger. If there is defect in the working system of the instrument, accurate test results cannot be obtained. This may result in fatal accident or disaster at any time.

Hence, before use, the handle of a megger is to be rotated at first keeping its two terminals open. The pointer must rest on Infinity’ mark of the scale at this time. Afterwards the two terminals of the instrument are joined together (short-circuited) with a small piece of wire and the handle is rotated as before. This time the pointer should rest on ‘Zero’ reading.

If the results of these two tests are satisfactory, it may be assumed that the working system of the megger is in good condition and the instrument may be used for different testing work of an installation.

Instrument # 7. Earth Testing Megger or Megger Earth Tester or Meg Earth Tester:

After completion of earthing of an electrical installation, the resistance of the earthing system is measured by earth testing merger.

The construction and the working principle of an earth testing megger are discussed below:

Megger earth tester or earth testing megger is a combination of an ohm meter and a hand driven generator which supplies the testing current. Ohmmeter is made in such a way that the pointer of the instrument reads the earth resistance directly on the scale.

Earth testing megger works on the principle of the fall of potential method. The test requires two temporary electrodes,—one current electrode and the other potential electrode. The arran­gement is such that the readings of the instrument are not affected either by stray current or by electrolytic back e.m.f.

The ohmmeter consists of two coils, namely the current coil and the potential coil. These two coils are mounted on a common axle at a fixed angle to each other. The current coil carries a current proportional to the circuit current. The potential coil carries a current proportional to the potential difference across the resistance under test.

Thus, the current coil acts as an ammeter, while the potential coil acts as a voltmeter working on the fall of potential method. Since the deflection of the pointer of the instrument is proportional to the ratio of currents flowing through the potential and the current coils, it indicates the earth resistance directly on the scale.

Hand driven generator produces direct current. But to eliminate the bad effect of electrolytic back e.m.f., it is necessary to convert this direct current into alternating current before the current flows from the instrument into the earth. To change d.c. into a.c., therefore, a rotary current reverser is mounted on the same shaft with the generator. Direct current comes out from the positive terminal of the hand driven generator and flows through the current coil of the ohmmeter.

It then goes to the current reverser from where it flows to the earth electrode following a cyclic change in magnitude as well as in direction. As a result the potential drop across the earth electrodes is alternating. This alternating potential cannot be applied across the potential coil of the instrument as an ohmmeter is a moving coil instrument and a moving coil instrument can work only with d.c.

Hence, for changing alternating current drop into direct current drop synchronous rotary rectifier is also mounted on the same, shaft as shown in fig 67. This rotary rectifier supplies direct potential across the potential coil of the ohmmeter.

Sometimes it is observed that the pointer of the instrument vibrates while taking readings. This happens only at the instant when the frequency of the stray alternating current is the same as the frequency of the generated current. In such occasion the speed of the hand driven generator should be either increased or decreased a little.