Short Answer:

An electrodynamometer wattmeter works on the principle of the interaction between magnetic fields produced by a fixed coil (current coil) and a moving coil (voltage coil). When both coils are energized, the magnetic field interaction produces a torque that deflects the pointer, indicating the power consumed in the circuit.

This wattmeter can measure both AC and DC power. The deflecting torque is proportional to the product of voltage, current, and power factor (for AC). Its accuracy and capability to measure true power make it suitable for laboratory and precision applications.

Detailed Explanation:

Working of electrodynamometer wattmeter

An electrodynamometer wattmeter is a type of wattmeter designed to measure true power in both AC and DC circuits. It is widely used in laboratories and precision power measurements because of its accuracy. The instrument consists of two coils: one is fixed, and the other is movable. It works on the principle of electromagnetic interaction between the magnetic fields produced by these coils.

Main Components:

1. Fixed Coil (Current Coil):
   This coil is connected in series with the load and carries the circuit current. It produces a magnetic field proportional to the current in the circuit.
2. Moving Coil (Voltage or Pressure Coil):
   This coil is connected in parallel with the load through a high resistance. It carries a current proportional to the voltage of the circuit.
3. Pointer and Scale:
   The moving coil is attached to a pointer that moves over a calibrated scale to show the power reading.
4. Controlling System:
   A spring is used to provide controlling torque, which balances the deflecting torque and returns the pointer to zero when no power is present.
5. Damping System:
   Air friction damping is usually used to make the pointer movement smooth and stable.

Working Principle:

When the wattmeter is connected to a circuit:

• The fixed coil carries the load current.
• The moving coil carries a current proportional to the load voltage.
• The magnetic field from the fixed coil interacts with the field of the moving coil.
• This interaction creates a deflecting torque.

In a DC circuit:

• The deflecting torque is proportional to the product of voltage and current (P = VI).

In an AC circuit:

• The torque is proportional to VIcosφ, where φ is the phase angle between voltage and current.
• Hence, the wattmeter reads true or real power.

The pointer deflects in proportion to this torque and shows the power on the calibrated scale.

Why it Can Measure Both AC and DC:

• In DC circuits, the magnetic fields are steady and the torque remains constant.
• In AC circuits, the currents in both coils vary with time, but their product (instantaneous power) contains a steady (average) component which results in a constant average torque that deflects the pointer.

Applications:

• Used in laboratories for accurate power measurements.
• Suitable for both single-phase and three-phase circuits (with suitable connections).
• Useful in calibration and standard testing environments.
• Applied in power factor and energy efficiency testing setups.

Conclusion

The electrodynamometer wattmeter operates based on the interaction of magnetic fields from its fixed and moving coils. The resulting torque causes a pointer to move, indicating the real power in the circuit. Because of its ability to measure both AC and DC power accurately, it is commonly used in precise measurement applications in electrical engineering.