Short Answer:

A wattmeter works on the principle of measuring the power consumed in an electrical circuit by calculating the product of voltage and current. It uses two coils: a current coil connected in series to measure current, and a voltage (pressure) coil connected in parallel to measure voltage. The interaction between the magnetic fields of these two coils produces a deflecting torque.

This torque is directly proportional to the true power in the circuit. The pointer attached to the moving system deflects over a calibrated scale to show the power in watts. Wattmeters are used in both AC and DC circuits, depending on their construction.

Detailed Explanation:

Principle of operation of a wattmeter

A wattmeter is an instrument used to measure electrical power (in watts) in a circuit. The electrical power is the product of voltage (V) and current (I), and in AC circuits, it also includes the power factor (cos φ). The working of a wattmeter is based on the principle that when a current flows through a coil and another voltage is applied to a separate coil, the electromagnetic interaction between the two results in a torque. This torque is proportional to the real power consumed in the circuit.

Main Components of a Wattmeter:

1. Current Coil (Series Coil):
   This coil is connected in series with the load. It carries the current of the circuit and produces a magnetic field proportional to that current.
2. Voltage Coil (Pressure Coil):
   This coil is connected in parallel across the circuit. It produces a magnetic field in proportion to the applied voltage.
3. Moving System and Pointer:
   The interaction between the magnetic fields from the two coils creates a mechanical torque on a moving element, usually attached to a pointer, which moves across a calibrated scale.
4. Controlling and Damping Mechanisms:
   A spring provides controlling torque to balance the pointer. Air damping or eddy current damping is used to make the pointer stable.

Working Principle:

The deflecting torque in a wattmeter is produced due to the interaction of magnetic fields from the current and voltage coils. This torque depends on:

Td∝V×I×cos⁡ϕT_d \propto V \times I \times \cos\phiTd​∝V×I×cosϕ

Where:

• VVV is the voltage across the load
• III is the current through the load
• cos⁡ϕ\cos\phicosϕ is the power factor of the load

This means that the torque (and thus the pointer movement) is directly proportional to the real power being used in the circuit.

• In DC circuits, power is simply the product of voltage and current (P = VI), and the wattmeter works by measuring this directly.
• In AC circuits, the wattmeter measures true power by taking into account the phase angle between voltage and current (P = VIcosφ).

Types of Wattmeters Based on Principle:

1. Electrodynamometer Wattmeter:
   Suitable for both AC and DC, works on the principle of interaction between fixed and moving coils.
2. Induction Wattmeter:
   Used only for AC circuits, works on electromagnetic induction like an energy meter.
3. Digital Wattmeter:
   Uses electronic sensors and digital processing to calculate power using instantaneous voltage and current samples.

Applications:

• Measuring power consumed by electrical appliances.
• Power testing in labs and industries.
• Used in electrical panels for monitoring load.
• Essential in energy auditing and efficiency analysis.

Conclusion

The wattmeter operates on the principle of electromagnetic interaction between voltage and current coils. This interaction generates a torque proportional to the real power in the circuit. Whether in AC or DC systems, wattmeters are essential tools for accurately measuring how much electrical power is being consumed or delivered by a load.