Induction Type Meters
The principle of working and construction of induction type meter is very simple and easy to understand that's why these are widely used in measuring energy in domestic as well as industrial world. In all induction meters we have two fluxes which are produced by two different alternating currents on a metallic disc. Due to alternating fluxes there is an induced emf, the emf produced at one point (as shown in the figure given below) interacts with the alternating current of the other side resulting in the production of torque.
Similarly, the emf produced at the point two interacts with the alternating current at point one, resulting in the production of torque again but in opposite direction. Hence due to these two torques which are in different directions, the metallic disc moves.This is basic principle of working of an induction type meters. Now let us derive the mathematical expression for deflecting torque. Let us take flux produced at point one be equal to F1 and the flux and at point two be equal to F2. Now the instantaneous values of these two flux can written as:
Where, Fm1 and Fm2 are respectively the maximum values of fluxes F1 and F2, B is phase difference between two fluxes. We can also write the expression for induced emf's at point one be
at point two. Thus we have the expression for eddy currents at point one is Where, K is some constant and f is frequency. Let us draw phasor diagram clearly showing F1, F2, E1, E2, I1 and I2. From phasor diagram, it clear that I1 and I2 are respectively lagging behind E1 and E2 by angle A. The angle between F1 and F2 is B. From the phasor diagram the angle between F2 and I1 is (90-B+A) and the angle between F1 and I2 is (90 + B + A). Thus we write the expression for deflecting torque as Similarly the expression for Td2 is, The total torque is Td1 - Td2, on substituting the the value of Td1 and Td2 and simplying the expression we get Which is known as the general expression for the deflecting torque in the induction type meters. Now there are two types of induction meters and they are written as follows:
- Single phase type
- Three phase type induction meters.
- Fg which moves on the small reluctance part across the side gaps.
- Fp: It is responsible for the production of driving torque in the aluminium disc. It moves from high reluctance path and is in phase with the current in the pressure coil. Fp is alternating in nature and thus emf Ep and current Ip. The load current which is shown in the above diagram is flowing through the current coil produces flux in the aluminium disc, and due this alternating flux there on the metallic disc, an eddy current is produced which interacts with the flux Fp which results in production of torque. As we have two poles, thus two torques are produced which are opposite to each other. Hence from the theory of induction meter that we have discussed already above the net torque is the difference of the two torques.
Advantages of Induction Type MetersFollowing are the advantages of induction type meters:
- They are inexpensive as compared to moving iron type instruments.
- They have high torque is to weight ratio as compared to other instruments.
- They retain their accuracy over wide range of temperature as well as loads.