Induction type Energy meter Construction The four main parts of an energy meter are: Driving system Moving system Braking system and Registering system The construction is as shown below: Fig. Construction of Induction type energy meter Driving system: The driving system of the meter consists of two electro magnets. The core of these electromagnets is made of silicon steel laminations. The coil of one of the electromagnets is excited by the load current. This coil is called the current coil. The coil of second electromagnet is connected across the supply and therefore carries a current proportional to the supply voltage. This coil is called the pressure coil. Hence the two electromagnets are known as series and shunt magnets respectively. Copper shading bands are provided on the central limb. The position of these bands is adjustable. The function of these bands is to bring the flux produced by the shunt magnet exactly in quadrature with the applied voltage. Moving system:
The moving system consists of an Aluminum disc mounted on a light alloy shaft. This disc is positioned in the air gap between the series and shunt magnets. The upper bearing of the moving system is a steel pin located in a hole in the bearing cup fixed to the top of the shaft. The moving system runs on a hardened steel pivot, screwed to the foot of the shaft. The pivot is supported by a jewel bearing. A pinion is used to connect the shaft and the registering mechanism. Braking system: A permanent magnet positioned near the edge of the Aluminum disc forms the braking system. The Aluminum disc moves in the field of this magnet and thus provides a braking torque. The position of the permanent magnet is adjustable and hence the braking torque can be varied. Registering mechanism: The function of registering or counting mechanism is to record continually a number proportional to the revolutions made by the moving system. A pointer type registering mechanism is shown below: Operation A simplified functional diagram of the driving system of the meter is shown in the fig. below.
The supply voltage is applied across the pressure coil. The pressure coil winding is highly inductive as it has very large number of turns and its reluctance is small due to presence of air gaps of very small length. Thus the current Ip through the pressure coil is proportional to the supply voltage and lags it by a few degrees less than 90. The current Ip produces a flux Φpt. This flux divides itself into two halves Φg and Φp. The major portion Φg flows across the side gaps, as the reluctance of this path is small. The reluctance to the path of flux Φp is large and hence its magnitude is small. This flux Φp goes across Aluminum disc and hence is responsible for production of driving torque. The flux Φp is in phase with the current Ip and is proportional to it. Therefore Φp is proportional to voltage V and lags it by an angle less than 90. Since Φp is alternative in nature, it induces an eddy emf Eep in the disc, which in turn, produces eddy current Iep. The load current I flows through the current coil and produces a flux Φs. This flux is proportional to the load current and is in phase with it. This flux produces Ies in the disc. This Ies interacts with Φp to produce a torque and the Iep interacts with Φs to produce another torque. These two torques are in opposite direction and the net torque is difference of these. Let V be the applied voltage I be the load current be the phase angle of load Ip be the pressure coil current be the phase angle between supply voltage and pressure coil flux Φp f be the frequency Z be the impedance of the eddy current path α be the phase angle of eddy current paths
Eep be the eddy induced emf by Φp Ees be the eddy induced emf by Φs Iep be the eddy current due to Φp Ies be the eddy current due to Φs Lag Adjustment in energy meters
It is clear from the derivation of the number of revolutions made by energy meter that the meter will register true energy only if the angle is made equal to. I.e. the phase angle between the shunt magnet flux Φp and the supply voltage V should be equal to. This requires that the pressure coil winding should be so designed that it is highly inductive and has a low resistance and the iron losses in the core are small. But even this would not ensure phase of exactly. The phase would still be less than. However, by introducing a magnetic shunt circuit which allows the main portion of the shunt magnet flux to bypass the gap in which the disc is located, it is possible to introduce an MMF in proper phase relation to bring the shunt magnet flux in the air gap in exact quadrature with the voltage. The arrangement for this is shown in fig. below. The required MMF is obtained from a lag coil which is located on the central limb of the shunt magnet close to the disc gap and links with the flux that cuts the disc. The pressure coil is excited by voltage V and carries a current Ip which produces an MMF ATpt, which in turn produces a flux Φpt lagging the voltage by an angle. The flux Φpt divides into two parts Φg and Φp. The flux Φp cuts the disc and also links with the lag coil. A voltage EL is induced in the coil lagging Φp by. This voltage circulates a current IL through the lag coil, the lag between IL and EL depends on the resistance of lag coil. This current produces an MMF ATL. The flux in the disc air gap Φp will thus be created by the combined action of main MMF (ATpt) in phase with Ip and the lag coil MMF (ATL) in phase with IL. Thus the
flux Φp will be in phase with the resultant MMF ATp. Hence it is clear that the phase of flux Φp can be adjusted by varying the MMF of shading coil. The arrangements for adjusting the MMF of lag coil are: Adjustable resistance: A few turns of fairly thick wire are placed around the central limb of the shunt magnet and the circuit is closed through a low adjustable resistance as shown in fig. below. Fig. Lag adjustment with adjustable resistance The resistance of this circuit is altered to adjust the lag angle of Φp. An increase in the resistance decreases the current and MMF in the lag coil and therefore the value of lag angle θ is decreased. The value of lag angle can be increased by decreasing the resistance of the lag coil circuit. The resistance of the lag coil is so adjusted that becomes equal to.