Explanation of Weston M I Type Frequency Meter

Weston (M.I) Type Frequency Meter

Principle:  It is a moving iron instrument which operates on the principle of variation of impedance in an inductive circuit with variation of frequency.

Weston Frequency Meter

Construction: It consists of two coils A and B mounted perpendicular to each other. Each coil is divided into two sections. The connections are as shown in the above fig. The branch of coil has a resistance R₁ connected in the series with it while coil B

has a reactance coil L₂ in series with it. Circuit of coil A is in parallel with a reactance coil L₁ while that coil B is in parallel with a resistance R₂. The moving element is a soft iron needle. The needle is pivoted on a spindle which is also carries a pointer and damping vanes. There is no controlling torque. The series reactance L₀ acts to suppress higher harmonics in the current of instrument and therefore tends to minimize the waveform errors in its indication.

Working: When the meter is connected across the supply, the two coils A & B carries currents and set up two magnetic fields which are at right angle to each other. The magnitude of the field depends upon the value of current flowing in the coil. Both these fields act upon the soft iron needle and the needle takes up a position which depends upon the relative magnitudes of the two fields and hence of the currents.

The meter is so designed that the values of carious resistances and inductances are such that for normal frequency of supply the value of voltage drops across reactance L₁and resistance R₂ send equal currents through coils A and B. Therefore, the needle takes up a position which is at 45^o to both the coils and the pointer is at the centre of the scale as shown in the fig.

Now if the frequency increases above its normal value, reactance of L₁ and L₂ increases while resistances R₁ and R₂ remain same. This means that with an increase in frequency, the voltage impressed upon coil A increases as compared with that across coil B. hence the current in the coil A increases while it decreases in coil B. the tendency of the needle is to deflect towards the stronger field and therefore, it tends to set itself in the line with axis of coil A. Thus the pointer deflects to the left.

When the frequency decreases, an opposite action takes place and the pointer deflects to the right.