Induction type instruments

Induction-type instruments are widely used as energy meters.

Principle of induction type instruments

This type of instrument works on the principle of electromagnetic induction. When the conducting material is placed in an alternating magnetic field, it induces eddy current in the material. The interaction between the magnetic field and eddy current produces defecting torque on the disc and makes it rotate.

Phasor diagram of induction instruments is shown below:

Here E1 and E2 lag behind Φ1 and Φ2 by 90°.

Induction type instrument phasor diagram.
E1 and E2 lag Phie1 and Phie2 by 90degrees.
Phasor Diagram of Induction type instruments

Read More: Induction energy meter

Classification of induction instruments

Based on how the deflecting torque is produced, it is classified into the following categories:

Shaded pole induction type instrument

Construction

A laminated electromagnet is present in the instrument and the poles of the electromagnet are split into two parts. A rectangular copper tube is applied on both sides of the slipt parts of the electromagnet. And those rectangular copper tubes are known as cooper shading bands.

The portion of electromagnet around which the copper band is applied is called the shaded part and the remaining free potion of the magnet is known as the unshaded part.

A damping magnet is a present opposite to the electromagnet so that the aluminium disc is used for the production of both deflecting and damping torque.  

Controlling torque is provided by 2 springs that are wound together but in opposite directions to each other. These springs are known as controlling springs.

An aluminium disc rotates between the air gap of the electromagnet. The aluminium disc is mounted on the pivot and jewel bearings. And the controlling spring, scale, and pointer are mounted on the spindle.

Working

When the AC supply is allowed to pass through the coil in the electromagnet, a flux φ is produced in the electromagnet. Because of the copper-shaded bands, the flux φ gets divided into two fluxes.

The two fluxes induce emf E1 and E2 in the disc that lags by 90° with their respective flux. The EMFs induce eddy current I1 and I2.

The interaction of the fluxes and eddy current produces 2 torques on the aluminium disc, this produces deflecting torque. The pointer connected to the disc deflects the pointer on the scale. The pointer comes at rest when the controlling torque becomes equal to the deflecting torque.

Split phase induction type instrument

Also known as Ferris type instrument. Two magnets are present in this instrument and an aluminium disc is placed between the magnets. A highly resistive resistance is connected in series with the coil of magnet A and an inductive coil is connected in series with the coil of magnet B.

The working of this type of instrument is similar to the shaded pole induction type instrument. Except in the shaded pole induction instrument single winding produces 2 fluxes, whereas the fluxes required to produce deflecting torque are produced by two different magnets A and B.

Read More: Induction wattmeter

Errors in induction type instrument

  • Frequency error: frequency error exists because the deflecting torque depends on frequency. If the instrument is not calibrated for the frequency of the electrical quantity to be measured then the instrument can create big effective errors.  
  • Temperature error: variation in temperature because of the eddy current in the instrument creates an error in the induction instrument. For ammeters, using shunt and for voltmeter using a combination of shunt and swamping resistance will compensate for the errors.

Advantages and disadvantages

Advantages

  • High torque to weight ratio
  • Effective and efficient damping
  • For wide load range, this type of instruments are very accurate
  • A negligible amount of stray fields because of the large operating field.
  • Absence of moving iron.

Disadvantages

  • Induction instruments consumes very high power.
  • It is Expensive
  • Scale is non-uniform
  • Only applicable for AC measurement
  • Temperature and frequency variations can cause errors
  • Large deflection produces large stresses in the spring

Applications

It has a wide range of applications in electrical systems. Ammeter, voltmeter, watt-meter, energy meter are the instruments where induction type is commonly used.

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