Question

history of induction type instruments, review of induction type instruments, current information about induction type instruments

Answer 1

Q. ans.

We use induction type instrument for AC measurement only. This is because the induction phenomenon only occurs in AC. An induction instrument has multi use. For example, we use the instruments as ammeters, voltmeters, wattmeter, and energy meters. Induction type instruments essentially have an electromagnet to produce the required magnetic field. AC in the electromagnet produces changing flux between its poles. Here, we place one aluminum disc (or aluminum drum) in the magnetic field. Hence, the changing flux links with the aluminum disc. As a result, the flux induces an eddy current on the disc. This eddy current interacts with the flux which has induced it. Consequently, there is a mechanical torque acting on the disc. This mechanical torque rotates the disc.

PRINCIPLE:

          When a disc or drum of a non-magnetic conducting material is placed in a rotating magnetic field, eddy currents are induced in disc or drum. The torque produces with the reaction of rotating magnetic field and eddy current. The relation between torque, eddy current, rotating flux is given below:

                          T α φ α i^2

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, F_m1 and F_m2 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 T_d2 is,

The total torque is T_d1 – T_d2, on substituting the the value of T_d1 and T_d2 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.

This instrument convert single phase to two phases. This is also called ferraris type instrument

*It consists of pole which are laminated and placed right angles to each other opposite pole connected in series the two pairs of poles connected in parallel.

*One set of coil is connected to inductance and another with a high resistance to create a phase difference 90. The purpose of both the coils is to measure the current.

*In the centre of yoke, the coil is an aluminium drum, inside drum, to strengthen the magnetic field there is a cylindrical laminated iron core.

WORKING:

      When this instrument is fed with supply due to electromagnets action a rotating magnetic action a rotating magnetic flux produced. This flux induced eddy current in the disc or drum. The reaction between flux and eddy current results in production of torque. This torque deflects the pointer attached to the drum. The controlling torque is provided by spring action.

CONSTRUCTION:

   This instrument consists of magnetic core (shaped same as transformer). One end is placed with band of copper(shaded portion), this makes the two fluxes of shaded and unshaded portion of differ in phase by 90.

*A metallic disc rotates between pole faces. The damping is provided by the another magnet.

CURRENT INFORMATION ABOUT THE INDUCTION TYPE METER

The electromagnetic or induction meters are very

reliable. The tendency for the curve of error displacement

during the operating time is toward negative values, mainly

for low load current values. After 10 years in service, the

negative error is bigger in meters that do not have magnetic

suspension. It is not necessary their mediate substitution,

whenever it is only necessary to measure active energy,

without any additional capability. They admit fraud easily,

due to the little complexity and accessibility to their

mechanisms.

The global measurement tends to be negative, whenever

there has not been intentional human intervention.

The electric energy measurement using induction

equipment is not obsolete, if it delimits it to the active

energy measurement and if NON-LINEAR LOADS

(HARMONIC DISTORTION) are not considered

It should be demanded from the manufacturer clear

information on the methodology and algorithms used for

the energy calculation and accumulation. Also, mainly in

the single-phase meter, the equipment should facilitate the

different measure options, by means of functions

programming by simple and really utility available

methods, similarly as it is carried out with the three-phase

meters.

In this way, meters that already offer the possibilities for

modern commercialization and measurement should be

used, because they should be able t o operate during at least

20 years, with out being obsolete in a short time. For

example, besides the programming options previously

mentioned, as basic conditions, the meters should possess a

communication port with open and standardized pr otocol,

that allow the use of different manufacturers without

altering the reading procedure carried out by using portable

data collectors. It should already be incorporated the option

of implementing a system of automatic meter reading

(AMR).

Therefore, the final conclusion is that a gradual and

coherent migration of the measurement scheme should be

carried out preponderantly from induction to electronic

meters, taking advantage of the current meter capabilities.