INDUCTION TYPE INSTRUMENTS

The operation of induction type instruments depends on the production of torque due to reaction between two magnetic fluxes having some phase difference OR reaction between flux of an AC magnet and the eddy current induced by this flux. These types of instruments are used only foe AC measurements.

Before getting into the constructional and working details of these instruments; let us first have a general look on the torque produced in these instruments.

The torque produced in induction type instruments depends on the two fluxes; the net torque acting on the disc is

                          \[T=K\omega {{\phi }_{1m}}{{\phi }_{2m}}\sin \alpha \]

Where, ${{\phi }_{1m}},{{\phi }_{2m}}$ = maximum fluxes produced by the currents

               $\alpha $ = phase difference between the two fluxes

And if both the fluxes are produced by the same alternating current, then

                                     \[T={{K}_{1}}\omega I_{m}^{2}\sin \alpha \]

Where, I_m= maximum value of current. Therefore, torque is proportional to the square of current for a given value of frequency  and angle . If the disc has spring control, then at some point the controlling torque will be equal to the deflecting torque which will help the disc to attain a steady deflected position. And if the disc is attached to a pointer, then this arrangement can be used for measurement of current.

This was a general discussion about induction type instruments. For the sake of a healthy discussion we will take on these instruments by their types.

Induction type instruments are of two types:

1. Split-phase type
2. Shaded pole type

SPLIT-PHASE TYPE –

The diagram for the split phase type induction instrument is shown here.

SPLIT PHASE TYPE INDUCTION INSTRUMENT

CONSTRUCTION –

In this arrangement, there are two AC magnets M₁ and M₂, which are connected in series. The winding in M₂ is shunted by a resistance R.  The current in the M₂ winding lags with respect to the total line current. This helps to develop the necessary phase angle $\alpha $ between the two fluxes.

DEFLECTION –

If the hysteresis effects are neglected, then deflecting torque is

                      \[{{T}_{d}}\propto {{\phi }_{1m}}{{\phi }_{2m}}\sin \alpha \]

(Where all the signs have their usual meanings as stated before)

Or, ${{T}_{d}}\propto {{I}^{2}}$ (if fluxes are produced by the same current)

Note that, here I is the r.m.s value of current.

Also, ${{T}_{c}}\propto \theta $ (Because spring control is used)

In the final deflected position, T_c=T_d.

 Therefore, deflection ($\theta $)$\propto {{I}^{2}}$

DAMPING –

Eddy current damping is used in this instrument.

SHADED POLE TYPE –

Shaded pole type induction instrument uses a single winding to produce flux. The flux produced by this winding is split up into two fluxes , having phase difference  with respect to each other. The phase difference is usually 40 to 50 degrees and can be varied by varying the size of shading band. This is done by making a narrow slot in the poles of the electromagnet. A copper strip is placed around the smaller of the two areas formed by the slot. This copper shading band acts as a short circuited secondary winding.

CONSTRUCTION –

The constructional features of a shaded pole type induction instrument are shown in the figure below.

The exciting coil is placed on the poles and a current proportional to current or voltage being measured is passed through it. An aluminium disc which is mounted on a spindle is inserted in the air gap of the electromagnet. The spindle carries a pointer and has a control spring attached to it. The controlling torque is provided by this spring only.

DEFLECTION –

As the net driving torque is due to the fluxes and the difference in the phase angle of these fluxes, then deflecting torque can be written as

                         \[{{T}_{d}}\propto {{\phi }_{1}}{{\phi }_{2}}\sin \alpha \]

If are produced by same current I, then ${{T}_{d}}\propto {{I}^{2}}$

As the instrument is spring controlled, ${{T}_{c}}\propto \theta $

For steady deflection, ${{T}_{c}}={{T}_{d}}\Rightarrow \theta \propto {{I}^{2}}$

DAMPING –

Damping is provided by a permanent magnet placed at the opposite side of the electromagnet, so that the disc can be used for production of both deflecting and damping torque.

ADVANTAGES –

• A full scale deflection of over 300 degrees can be obtained.
• Good damping.
• Less effect of stray magnetic fields as the operating fields are large.

DISADVANTAGES –

• Errors are caused due to changes in frequency and temperature.
• Non-uniform scale.
• Large power consumption and high cost.
• Can be used for AC only.