In this article we will discuss about the commonly used D.C and A.C energy meters.
1. D.C. Energy Meters:
(A) Ferranti Mercury Meter:
It is a very commonly used energy meter of mercury ampere-hour type. The construction and connection of this meter are shown in fig. 68.
Ferranti mercury meter consists of a disc revolving between two permanent magnets,—one acting as a driving magnet and the other as a braking magnet. The disc is made of thin enamelled copper which revolves round a central spindle. This spindle is supported between jewel pivots and is surrounded by a container filled with mercury.
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Current is led into the disc at its edge below the driving magnet from where it goes to the central spindle and then out at its lower extremity. The disc is platinum-plated and enamelled so that it is not amalgamated by mercury. Number of revolutions of the disc is recorded by an arrangement of worms and gears, and one of the worms is attached to the spindle.
The current enters the disc at the edge between the poles of the driving magnet, goes radially to the centre and then out. Thus, it flows through the portion below the driving magnet only and not through that under the braking magnet. As a result of this current flow, the field of the driving magnet exerts a force on the right-hand side of the disc.
The magnitude of the force is directly proportional to the strength of the current and its direction is determined by Flemings’s Left-hand Rule. The driving torque is a function of this force and its distance from the spindle.
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When the disc rotates under the action of this torque, it cuts through the magnetic field produced by the braking magnet on the left-hand side and hence eddy-current is induced in the disc. Eddy-current produces the braking torque which is proportional to the flux produced by the braking magnet and the strength of the eddy-current.
The speed of the disc becomes constant when the driving torque is equal to braking torque. But when the current increases, the strength of the field produced by the driving magnet also increases. This increases the driving torque.
However, the action of the magnetic brake remains the same; for the poles of the driving and braking magnets are so arranged that, when the field produced by the poles of the driving magnet increases, that produced by the poles of the braking magnet diminishes. The compensating coil C helps in bringing this about.
The friction due to mercury also causes retardation of the disc. This friction increases with the speed of the meter. It is compensated by a coil C of a few turns placed around the lower iron cross-bar.
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(B) D.C. Motor Meter:
D.C. motor meters may be either ampere-hour type or watt-hour type. The moving system of the meter is allowed to rotate continuously. The speed of rotation is proportional to the current in the circuit in case of an ampere-hour meter, and to the power in case of a watt-hour meter.
Thus, the number of rotation made by the moving system in a given time is proportional to the quantity of electricity supplied during that time in case of an ampere-hour meter, and to the energy supplied in case of a watt-hour meter.
Fig. 69 shows the different parts and their connections of a commutator motor type watt-hour meter. The meter consists of two-fixed current coils. Each coil is made of heavy copper strip of a few turns. These coils produce a magnetic field the strength of which is directly proportional to the line current.
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The armature core is made of a non-magnetic substance which rotates in the magnetic field produced by the current coils. The armature coils are connected to segments of a small commutator. These coils are connected across the supply through the brushes placed on the commutator and in series with a suitable resistance. Thus, the armature coil acts as the pressure coil of the instrument. In order to reduce friction, the commutator is made of silver and the brushes are silver tipped.
A compensating coil is connected in series with the armature. This coil strengthens the magnetic field of the current coils when the armature current flows through it. The object of this coil is to compensate for friction, and its position is so adjusted that the armature cannot rotate when no current flows to the load circuit, although the pressure coils remain energised.
The current flowing through the pressure coil (i.e. the armature current) is proportional to the voltage across the circuit. The torque rotating the armature is proportional to the product of this armature current and the flux produced by the magnetic field due to current coils. Thus, the torque is proportional to power in watt absorbed by the load circuit.
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The braking torque is provided by an aluminium disc mounted on the same spindle with the armature. It rotates in the air-gap between two permanent magnets as shown in fig. 69. The braking torque due to eddy-current induced in this disc is proportional to the speed of the disc. Therefore the steady speed attained by the moving system of the instrument is proportional to the power in watt absorbed by the load circuit. The number of rotations of the armature is recorded by a train of wheels to which the armature spindle is geared.
Since mercury meters have many advantages over commutator meters, now a day’s this type of meter is rarely used as house-service meter.
2. A.C. Energy Meters:
(A) Single-Phase Induction-Type Meter:
Single-phase induction type energy meter is used extensively as house-service meter to measure electrical energy supplied to single-phase a.c. circuit. Such a meter can be used only with a.c. work.
The operation of the induction type energy meter depends on the flow of alternating current through two coils, namely the current coil and the pressure coil, which produce a rotating magnetic field. This field interacts with a metallic disc and causes the disc to rotate in the air-gap between two electro-magnets excited by the current coil and the pressure coil respectively.
The current coil has less number of turns with comparatively large cross-sectional area. It carries the line current and produces a magnetic field which is in phase with the line current. The pressure coil has large number of turns with comparatively small cross-sectional area. This coil is highly inductive so that the current flowing through it lags behind the supply voltage by about 90°.
Thus, there exists a phase difference of 90° between the magnetic fields produced by the two coils. This sets up a resultant rotating field which interacts with the disc and causes it to rotate. The moving system of the meter consists of a light aluminium or copper disc mounted on a vertical spindle.
The spindle is supported by a cup-shaped jewel bearing at the bottom end and by a spring journal bearing at the top end. There is pointer and the control spring, and the disc rotates continuously due to the action of deflecting torque.
The series magnet (the magnet excited by the current coil) placed below the disc consists of a laminated U-shaped iron core. A thick wire of a few turns is wound on both the limbs of this core. The wound coil is called current coil. It is connected in series with the load so that it carries the load current and produces a magnetic field which is proportional to and in phase with this current.
The shunt magnet (the magnet excited by the pressure coil) placed above the disc consists of a laminated M-shaped iron core. A fine wire with large number of turns is wound on the central limb of this magnet. The wound coil is known as pressure coil which is connected across supply lines or load. The pressure coil carries a current proportional to the supply voltage. In order to produce a deflecting torque, current in the pressure coil should lag behind the supply voltage by about 90°.
The necessary phase shift is obtained by placing a copper shading band around the central limb of the shunt magnet. This copper ring is called compensating band or compensating loop. It acts as a short-circuited secondary of a transformer and causes the circulating current in it to lag behind the supply voltage by about 90°.
The magnetic field set up by the current coil reacts with that produced by the pressure coil. As a result a driving torque is created. The disc rotates under the action of this torque. The speed of the disc is adjusted to the required value by a C-shaped permanent magnet, called the braking magnet.
The braking magnet is so mounted that the disc rotates in the air-gap between the poles of this magnet. During rotation the disc cuts the flux produced by the braking magnet and eddy-current is induced in the disc. The direction of the eddy-current is such that is opposes the rotation of the disc. Since the strength of eddy-current is proportional to the speed of the disc, the braking torque produced by this current is also proportional to the speed of the disc.
The number of revolutions of the disc is a measure of the electrical energy consumed by the load circuit in a given time. In order to register this number of revolutions, a suitable train of reduction gear is engaged with the driving shaft or spindle of the rotating disc.
In case of high voltage and high current supply, a suitable potential transformer is used with the pressure coil and a current transformer with the current coil of the instrument.
(B) Three-Phase Induction-Type Meter:
The energy supplied to a three-phase circuit may be measured by a single energy meter. It is a double element meter of the induction type, each element being similar in construction to a single-phase meter.
The meter has two discs mounted on the same spindle and two separate braking magnets. The spindle drives a single counting train of reduction gear. Each element has phase adjustment and friction compensating device. But one of the elements has an adjustable magnetic shunt across its shunt magnet. This arrangement is essential in order that the driving torque may be made the same in the two elements for the same watts.
Three-phase meter may be either three-wire type or four-wire type. The connections of these meters are shown in fig. 71-(a) and in fig. 71(b) respectively. Three-phase, three-wire meters are used for 3-phase motors’ and other power loads where connection for neutral wire is not necessary.
Three-phase, four-wire meters are used in those circuits in which consumption of power is considerably high and where single-phase lighting loads and three-phase motors draw power from the same supply lines.
Creeping Error of Energy Meters:
Sometimes the disc of an energy meter rotates slowly but continuously when only pressure coil of the meter is energised and no current flows through the current coil (i.e. the circuit is under no load condition). This is called creeping. This error may be caused due to stray magnetic field, incorrect friction compensation, rise in supply pressure etc.
Creeping error may be eliminated by drilling two holes in the disc on opposite sides of the spindle. The disc tends to remain stationary when one of the holes comes under one of the poles of the shunt magnet. It can also be eliminated by attaching a small piece of iron wire to the edge of the disc. The force of attraction of the braking magnet upon this wire will prevent continuous rotation of the disc under no load condition.