How to measure Energy?



Energy meter is integrating meters which measures electrical energy consumption in a circuit and registers the amount in the B.O.T. units on a scale or dial and thus permits an evaluation of the energy from time to time.

The essential difference between an energy meter and a wattmeter is that the former is fitted with some type of registration mechanism whereby all the instantaneous readings of power are summed over a definite period of time where as the latter indicate the value at the particular instant when it is read.

Motor Meters:

The meters can be used in d.c. as well as in a.c. circuits. In principle the motor meter is a small motor of d.c. or a.c. type whose instantaneous speed of 6. rotation is proportional to the circuit current in case of an ampere hour meter and to the power of the circuit in case of a watt-hour meter.

The number of revaluations made by the revolving portion in any given time is proportional, in case of A. (Ampere hour) H. meater, to the quantity of electricity supplied during fir that time, and in the case of energy meter to the energy supplied.

A train of wheels connected to the spindle (rotor) count the number of revolutions made and thus registers the A-H or kwh directly. Essential parts of such meters are as follows:

1. An operating torque system which produces a torque and causes the mov­ing system to rotate continuously.

2. A braking device, which is usually a permanent magnet. The permanent magnet, known as brake magnet, induces current in some part of the moving system, which in turn produces the braking torque.

Therefore the braking torque is proportional to the induced currents; whereas induced currents are proportional to the speed of the moving system and hence the braking torque is proportional to the speed of the moving system (disc). The moving system attains a steady speed when the braking torque is equal to the driving torque.

3. A device of registering the number of revolutions made by rotating element. This is obtained by having a worm cut on the spindle of the instrument. The worm engages with a pinion and thus drives the train of wheels and registers A-H or kwh directly.

Motor metre is (i) mercury motor meters: (ii) commentator motor meters and (iii) induction energy meters. Mercury motor meters and commentator motor meters are used in d.c. and induction meters are used in a.c.

Mechanical Details of Construction:

The construction of the meter must he such that the functioning of the meter is not readily interfered with by tilting or any external means. For safety the meters are enclosed in insulating or metal casing provided with glass windows for the purpose of reading the meter read­ings.

House service meters are so designed that both the terminal block and the remainder of the meter can be sealed separately in order to prevent access to both the leads to the meter and to the internal working parts. Thus complete protec­tion is provided against electric shock by accidental contact with the meter.

There are two principal errors common to all motor meters -

The friction error, which is due to friction at the pivots and bearings, plays a considerably more important part than the corresponding error in most indicating instruments, since it continuously operates and affects the speed of the rotating element (disc) for any given value of current or power.

The friction torque which exists, when the disc is just starting to rotate, may prevent it from starting at all, if load is small and will cause its registration to be won small loads.

This part of the friction torque may be assumed to remain constant when the moving system of the meter is in rotation and may be compensated Tor by providing a small constant driving torque on the moving system independent of load.

When the moving system of the meter is rotating normally, a frictional torque proportional to the speed of rotating element also exists but this torque is not of considerable importance since it merely adds a retarding torque and helps the braking magnet action.

In some meters such as those of the mercury motor type, a frictional, torque, which is proportional to the square of the speed also exists and has to be compensated for. In order to reduce the frictional torque to the minimum, the weight of the rotating system should be made as small as possible.

(i) Variation in braking action affects the speed of the rotating element of the meter, for a given' driving torque, and therefore affects the number of revolu­tions made in a given time.

The rotating element of the meter attains steady speed when the braking torque, which is proportional to the speed, is equal to driving torque. The braking torque is also proportional to the strength of the brake magnet.

The braking torque is the flux of brake magnet and is the eddy current induced in the rotating disc due to rotation in the field of the brake magnet.

From the above expression of steady speed it is obvious that the steady speed attained for a constant driving torque Td is directly proportional to the resistance of eddy current path and inversely proportional to the square of the flux of the brake magnet.

Hence it is necessary that strength of the brake magnet should remain constant during the use of the meter. Careful design and treatment of brake magnet during its manufacture are necessary in order to ensure this consistency.

With the increase in temperature, the resistance of eddy current path will increase and therefore the braking torque will decrease and cause an error in the instrument registration.

Though it is somewhat difficult to compensate completely the reduction in braking torque due to increase in temperature but in some meters driving torque also decreases with the increase in temperature and thus automati­cally compensates partially.

(a) Mercury Motor Meters:

The most common used and best form of mercury ampere hour meter is Ferrants mercury motor meter. It can be used only It is essentially a motor the magnetic field is provided by a permanent magnet known driving magnet. The permanent magnet used for driving purpose represented by ND SD in the figure.

Another permanent magnet represented by Na San figure is used for creating braking torque. The pole pieces of the magnets are made of steel.

BB are the circular brass plates into which the pole pieces are fitted. A fibre ring of same external diameter as the plates is placed between the brass plates BB.

The plate fibre ring together from a hollow circular box in which rotates the copper disc and the space is filled with mercury which exerts a considerable upward thrust on the thereby reducing the pressure on the bearings. The disc is mounted on a spindle at the bearing.

The spindle is so weighed that it just sinks in the mercury bath. The upper part of the spindle has a worm cut on it to engage with the recording mechanism.

The operating current enters by the positive terminal C connected to the centre contact of the mercury bath and flows through the mercury and copper disc from periphery to the centre of the disc where from it flows out through the supporting screw D.

When the current flows through the copper disc, the driving torque acting on the spindle is produced due to interaction between the fluxes of the disc where from it flows out through the supporting screw D.

When the current flows through the copper disc, the driving torque acting on the spindle is produced due to interaction between the flux of the permanent magnet and the current flowing in the disc. Under the influence of this driving torque disc starts rotating as an armature of the d.c. motor.

The rotating disc cuts the magnetic field of the braking magnet and there­fore eddy currents are induced in the disc which in turn set up the braking torque. The braking torque so created is proportional to the speed of rotation of the disc. When stable condition of constant disc speed is attained, the driving and braking torque will have reached equality.

Now the former is proportional to the current passing through the meter and the latter is proportional to the rotational speed of the disc. Thus equality of two torques means that the speed of the disc is proportional to the rotational speed of the disc.

Thus equality of two torques means that the speed of the disc is proportional to the current passing through the meter. Thus the number of revolutions made in any given time will be propor­tional to dt i.e. quantity of electricity.

In order to compensate for the mercury friction, two iron bars are placed across the two permanent magnets, one above and another below the mercury bath. The lower bar carries a small compensating coil of a few turns through which the load current passes.

The compensating coil sets up a local magnetic field, which strengthens the magnetic field of the driving magnet and weakens the magnetic field of the braking magnet. Fluid friction is thus compensated for.

This meter is essentially of the ampere hour type; it can, however, be calibrated to read watt-hours or kwh for a constant supply voltage.

To minimise errors on account of changes, with temperature, in the mercury fluid friction, and in the field of permanent magnets, adjustable magnetic shunts of low temperature coefficient of resistance are often used in the ampere hour meters so that the field strengths of the permanent magnets may be varied.

Modification For Watt-hour Measurement - If the permanent magnet, employed for producing operating torque, in the above ampere-hour meter, is replaced by around electro-magnet connected in series with a resistance across the supply, these results in a watt, hour meter.

For small variations in the supply voltage, the driving flux can be made proportional to supply voltage. The driving torque is thus proportional to the product of this flux and current through the disc i.e. proportional to the product of supply voltage and the current and therefore to the power. Thus, with induced current braking, the meter registers energy directly.

In some forms of these meters the disc has radial slots cut in it in order to ensure radial flow of the current in it, this current being led into and out of the disc through mercury contacts as diametrically opposite points.

Such a disc cannot be used for eddy current braking, the braking torque is obtained from a sec­ond disc mounted on the same spindle, but outside the mercury chamber, and rotating between the poles of a permanent magnet, known as brake magnet.

Comspensation fluid friction at high speeds of revolution is provided by taking one or two turns of the current lead round the poles of the electro-magnet so that is field is strengthened thereby when the load is heavy.

Since, due to magnetic hysteresis, in the electro-magnet, there is a lack of strit proportionality between the flux in the magnet and the voltage creating it, so the modified meter is only an ampere hour meter with compensation for voltage variations, rather than a true energy meter.

(b) Commutator Motor Meters:

The Elihu Thomson commutator motor meter. It is essentially a small d.c. motor with a wound armature. It consists of two fixed coils, known as current coils, represented by c.c. in the figure, each made of a few turns of heavy copper strip and connected in series with each other and with the circuit so that they may carry the circuit current. Thus the strength of magnetic field produced by the current coil is proportional to the load current.

The pressure coil consisting of a number of coils connect to the segments of a small commutator connected in series with a suitable resistance across the, supply. Thus the pressure coil carries a current proportional to the load voltage.

The commutator is made of silver and brushes are also silver or gold-tipped in order to reduce the friction at the commutator.

A compensating coil is also connected in series with the pressure coil and is so placed that it strengthens the magnetic field of current coils when the current flowing through the pressure coil flows through it. Thus the friction error is compensated for.

The operating torque is proportional to the product of flux created by current coil and that due to current flowing through the pressure coil.

Since the flux created by current coils is proportional to the load current and current through the pressure coil is proportional to load voltage, therefore, the torque is propor­tional to the product of load current and load voltage i.e. proportional to the power supplied to the circuit.

Such meters are commonly used for switch boards. House service meters are invariably of the mercury type. The advantages of mercury type meters are

(i) Simple construction

(ii) Small voltage drop across the meter

(iii) The considerably large current carrying capacity without use of shunt and

(iv) Small starting friction owing to the very small pressure on the bearing as a result of the up thrust of the mercury on the rotating system.

(c) Motor Meters for A. C. Circuits:

The most commonly used meter on ax. circuits for the measurement of energy is the induction type watt-hour meter, the induction type energy meters have got the following advantages:

(i) cheap in cost (ii) simple in operation (iii) little maintenance (iv) efficient damping (v) robust construction and long life (vi) uneffected by temperature variations (vii) more accurate than the commutator type energy meter on light loads owing absence of a commutator with its accompanying friction and (viii) correct registration even for very low power factor.

Induction Type single Phase Energy Meters:

The brake-magnet and recording wheel-train being committed for clearance. It consists of a shunt magnet, a series magnet a brake magnet and a rotating disc. Shunt magnet consists of a number of M shaped iron laminations assembled together to form a core.

A coil having large number of turns of fine wire is fitted on the middle limb of the shoe magnet. The coil is known as pressure coil and is connected across the supply mains.

The series electro-magnet consists of a number of U-shaped iron laminations assembled together to form a core. Each of the two limbs is wound with a few turns of heavy gauge wire. This wound coil is known as current coil and is connected in one of the lines in series with the load to be metered.

The series electro-magnet is energized and set magnetic field cutting through the rotating disc, when load current flows through the current coil (c.c.). The rotating disc is an aluminums disc mounted on a vertical spindle and support.

The magnetic field produced by shunt electro-magnet is pulsating in charac­ter, cuts through the rotating disc and induces eddy currents there in, but nor­mally does not in itself produce any driving force. Similarly series electro-magnet induces eddy currents in the rotating disc, but does not in itself produce any driving force.

In order to obtain driving force in a single phase meter, phase displacement of 90° between the magnetic field set up by shunt electromagnet and applied voltage V is achieved by adjustments of the copper shading band, also known as power factor compensator or compensating loop.

The reaction between these magnetic fields and eddy current set up a driving torque in the disc which would revolve at a very high speed in the absence of any opposing force.

The brake magnet consists of C Shaped piece of alloy steel bent round to form a complete magnetic circuit, with the exception of a narrow gap between the poles. This magnet is mounted so that the disc revolves in the air gap between the polar extremities.

The movement of rotating disc through the magnetic field crossing the air gap sets up eddy currents in the disc which react with the field and exerts a braking effect. By changing the position of the brake magnet or by diverting some of the flux there from, the speed of the rotating disc may be adjusted.

Errors and Adjustments:

1. Phase and Speed Errors:

It is necessary that the energy meter should give correct reading on all power factors, which is only possible when the field set up by shunt magnet lags behind the applied voltage by 90°. Ordinarily the flux due to shunt magnet does not lag behind the applied voltage exactly by 90 because of winding resistance and iron losses.

The flux due to shunt magnet is made to lag behind applied voltage by 90° with the help of copper shading band provided on the central limb. An error due to incorrect adjustment of shading band will be evident when the meter is tested on a load of power factor less than unity.

An error on the fast side under these conditions can be eliminated by bringing the shading band nearer to the disc and vice-versa.

An error in the speed of the meter when tested on non-inductive load can be eliminated by adjustment of the position of the brake magnet. Movement of the brake magnet in the direction of the spindle will reduce the braking tarque and vice-versa.

2. Friction Compensation:

The two shading bands embrace the flux contained in the two outer limbs of the shunt electro-magnet, and thus eddy currents are induced in them which cause a phase displacement between the enclosed flux and the main gap-flux. As a result, a small driving torque is ex­erted on the disc, this torque being adjusted, by variation of the positions of these bands, to compensate for friction in the instrument.

In some energy meters, the disc continues rotating when the potential coils are excited but with no load current flowing. This defect is known as creep­ing and is prevented by cutting two holes or slots in the disc on opposite sides of the spindle. The disc tends to remain stationary when one the holes come under one of the poles of the shunt magnet.

In some cases, a small piece of iron wire is attached to the disc. The force of attraction of the brake magnet upon this wire is sufficient to prevent continuous rotation of the disc under no-load conditions.

3. Temperature and Frequency Errors:

The errors due to variation in tem­perature are very small, since the various effects due to change in temperature tends to neutralize each other on unity power factor if not on low p. f. (lagging). Since the meters are used normally at fixed frequency, they can be adjusted to have a minimum error at declared supply frequency which is normally 50 Hz.