Weston Frequency Meter: Working Principle, Construction, Advantages & Applications

Introduction

The Weston type frequency meter operates on the deflection principle to measure the unknown frequency of an input signal. It consists of two main coils: an inductive coil and a resistive coil. When the input frequency deviates from the standard frequency, the current distribution between these coils changes, which results in a corresponding pointer deflection.

Operating Principle

The Weston frequency meter works by passing current through two coils placed 90° apart. The magnetic field created by these currents causes the needle to deflect toward the coil producing the stronger field. This deflection indicates the unknown frequency of the input signal.

Construction

The magnetic axes of the two coils, A and B, are positioned perpendicular to each other. A long, thin soft-iron needle is placed at their center. The needle assembly includes a pointer and damping vanes but has no controlling device.

The overall circuit resembles a Wheatstone bridge balanced at the supply frequency. Coil A is connected in series with resistance R_A and in parallel with reactance L_A. Similarly, coil B is connected in series with resistance R_B and in parallel with reactance L_B. A series inductance L₀ is used to suppress higher harmonics, reducing waveform errors. At standard frequency, the meter is manufactured such that the pointer rests at 45°.

Working

When power is applied, currents begin to flow in coils A and B, creating perpendicular magnetic fields. The needle deflects based on the relative strength of these fields. At normal frequency, equal voltage drops occur across L_A and R_B, resulting in equal current in both coils. Thus, the needle rests at the center of the scale.

When the input frequency increases, reactance’s L_A and L_B increase while resistances remain constant. The rising impedance of coil A reduces its current, while coil B receives comparatively more current. This makes the pointer move toward coil B, indicating a higher frequency. When the frequency decreases, the opposite occurs, and the pointer deflects toward coil A.

Advantages

• Highly sensitive instrument
• Simple construction
• Linear frequency scale
• Readings are unaffected by voltage variations
• Applicable for a wide range of voltages

Disadvantages

• Susceptible to temperature variations
• Rectifiers in the circuit may cause distortion, affecting accuracy

Applications

• Testing radio and communication equipment
• Used in laboratories for measuring frequency-related transducer outputs