The principle of working and construction of induction type meters is very simple and easy to understand hence they are widely used in energy measurement in domestic and industrial world. In all induction meters we have two fluxes produced by two different alternating currents on the metal disc. There is an induced emf due to alternating flux, the emf generated at one point (as shown in the figure below) interacts with the alternating current at the other side resulting in a torque.
Similarly, the emf generated at point two interacts with the alternating current at point one, resulting in torque again but in the opposite direction. So due to these two torques which are in different directions, the metal disc moves.
This is the basic working principle of induction type meter. Now let us derive the mathematical expression for turning torque. Let us take the flux generated at point one equal to F1 and the flux at point two equal to F2. Now the instantaneous values of these two fluxes can be written as:
Where, K is some constant and f is the frequency.
Let us draw the phasor diagram clearly showing F1, F2, E1, E2, I1 and I2. From the phasor diagram, it is clear that I1 and I2 lag behind E1 and E2 by an angle A respectively.
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.
Similarly, Td2 is expressed as,
Substituting the value of Td1 and Td2 and simplifying the expression we get the total torque is Td1 – Td2
which is known as the general expression for the deflecting torque in induction type meters. Now we know that there are two types of induction meters and they are written as:
- Single phase type
- Three phase type induction meters.
Here we are going to discuss single phase induction type in detail. Below is a picture of a single phase induction type meter.
A single phase induction type energy meter consists of four main systems which are as follows:
The driving system consists of two electromagnets on which pressure coils and current coils are wound, as shown in the diagram above. A coil that carries the load current is called a current coil, while a coil that is in parallel with the supply voltage (i.e., the voltage across the coil is equal to the supply voltage) is called a pressure coil. The shading bands are wound as shown in the diagram above to make an angle equal to 90 degrees between the flux and the applied voltage.
A floating shaft energometer is used to reduce friction to a greater extent, friction is reduced to greater extinction as the rotating disc, which is made of a very light material such as aluminum, is in contact with any surface. It doesn’t happen. It floats in the air. A question must arise in our mind that how does the aluminum disc float in the air? To answer this question we have to look at the construction details of this special disk, which actually consists of tiny magnets on both the top and bottom surfaces. The upper magnet is attracted to the electromagnet in the upper bearings while the lower surface magnet is also attracted to the lower bearing magnet, so these opposing forces cause the light rotating aluminum disc to float.
A permanent magnet is used to generate the braking torque in single-phase induction energy meters which are located near the corner of the aluminum disc.
The numbers marked on the meter are proportional to the revolutions made by the aluminum disc, the main function of this system is to record the number of revolutions made by the aluminum disc. Now let us see the working operation of single phase induction meter. To be the understand the working of this meter let us consider the diagram given below:
Here we have assumed that the pressure coil is highly inductive in nature and consists of a very large number of turns. The current flow in the pressure coil is Ip which lags the voltage by an angle of 90 degrees. This current produces a flux F.The F is divided into two parts Fg and Fp.
Fg which moves on the small reluctance section.
Fp: It is responsible for producing the driving torque in the aluminum disc. It moves through a highly reluctance path and is in phase with the current in the pressure coil. Fp is changing in nature and thus emf Ep and current Ip. The load current shown in the above diagram flowing through the current coil induces a flux in the aluminum disc and this alternating flux creates an eddy current on the metal disc which interacts with the flux Fp which The resulting torque as we have two poles, thus creating two torques that are opposite to each other. So from induction meter theory we have already discussed above the net torque is the difference of two torques.
Advantages of induction type meters:
The Following are the advantages of induction type meters:
- They are cheaper than iron type devices.
- They have a higher torque to weight ratio than other devices.
- They maintain their accuracy over a wide range of temperatures as well as loads.