Induction type instruments - Engineering notes

Induction type instruments:

These instruments are based on the principle of induction motor.

Principle of Induction type instruments:

When a drum or disc of a non - magnetic conducting material is placed in a rotating magnetic field, eddycurrents are induced in it. The reaction between the rotating flux and the eddy current produced by it creates a torque which rotates the disc or drum. The rotating flux is produced by the current or voltage to be measured. The eddy current again is proportional to the flux.

The single phase supply is converted into two phases in the instrument, that is done by split phase or shaded pole arrangement. Accordingly induction instruments are classified as

1. Split phase type

2. Shaded pole type

1. Split phase type induction instrument:

Construction of Split phase type induction instrument:

This is also called ferraris type instrument and is shown in the fig. It consists of a laminated magnet with the pairs of poles at right angles to each other.  Coils are wound on the poles, the opposite poles being connected in series. The coils on the two pairs of poles are connected in parallel. One set of coil is connected through an inductance and another with a high resistance to create a phase difference of 90 degrees. The input to both the coils is the current to be measured. In the center of the yoke and coil is an aluminium drum. Inside the drum there is cylindrical laminated iron core to strengthen the magnetic field.

Split phase induction instrument

Working of Split phase type induction instrument:

When the instrument is connected in the circuit diagram flows through the coils. A rotating magnetic field is produced. This field induces eddy currents in the drum and a torque is produced by the reaction of magnetic field and current. This torque deflects the pointer attached to the drum. Controlling torque is produced by spring.

Deflecting torque of Split phase type induction instrument:

deflecting torque produced is given by

T  proportional   IR. IL. f cos a  sin b  / Z

For an ammeter,  T  proportional   I2. cos a. sin b  / Z

For an voltmeter,  T  proportional  V^2  cos a. sin b  / Z

Where IR  = current through resistor

IL = current through inductor

f = supply frequency

Z = impedance of eddy current path

a = phase angle between voltage and current in resistor

b = phase angle between currents in resistor and inductor

a is almost zero and R is very much larger than reactive part of eddy paths. Hence

T  proportional   I^2 . sin b  / R

Obviously  b should be as high as possible for high torque.

2. Shaded pole type induction instrument:

Construction of Shaded pole type induction instrument:

Shaded pole type instrument is as shown in the fig. A band of copper is placed in pole faces, this makes the two fluxes of shaded and unshaded portions differ in phase by 90 degrees. A metallic disc rotates between the pole faces. The damping is provided by another magnet as shown in the fig.

Shaded pole type induction instrument

Working of Shaded pole type induction instrument:

The current flowing through the exciting coil sets up flux. Eddy currents are induced in the copper band. Flux of the eddy current opposes the flux in the magnetic core and a two phase flux same as ferraris type instrument.