Dynamometer Wattmeter Moving Coil Function

A dynamometer wattmeter is an instrument designed to measure the electrical power consumed by a circuit. Its operation is based on the principle of mutual electromagnetic interaction between two coils: a fixed coil and a moving coil.

Understanding the Wattmeter's Components

To understand the role of the moving coil, let's look at the typical construction:

• Fixed Coils: These coils are stationary and are connected in series with the load. They are designed to carry the full load current, denoted as $I$. Because they carry the load current, they are known as the current coils. They are usually wound with thick wire and have fewer turns.
• Moving Coil: This coil is free to rotate and is connected across the voltage source, in parallel with the load. It carries a current proportional to the voltage across the load, $V$. This coil is wound with fine wire having many turns and possesses high resistance. Due to its connection across the voltage supply, it is referred to as the potential coil or voltage coil.

Princ of Operation and Coil Roles

The fundamental principle involves the interaction between the magnetic fields produced by these coils:

• The current $I$ flowing through the fixed current coils produces a magnetic field. The strength of this field is proportional to the current $I$.
• The voltage $V$ across the load causes a current $I_p$ to flow through the moving potential coil. Since the coil has high resistance $R_p$, the current is $I_p = V / R_p$.
• The torque generated on the moving coil is proportional to the product of the magnetic field strength (from current coils) and the current in the moving coil (potential coil). Therefore, the torque ($T$) is proportional to $I \times I_p$.
• Substituting $I_p = V / R_p$, we get $T \propto I \times (V / R_p)$. Since $R_p$ is a constant, the torque is proportional to the product $VI$.
• The power $P$ consumed by the load is given by $P = VI$. Thus, the deflecting torque is directly proportional to the power being measured ($T \propto P$).
• A spring mechanism provides the restoring torque, which is proportional to the angle of deflection ($\theta$). At equilibrium, deflecting torque equals restoring torque, so $P \propto \theta$. This means the pointer's deflection angle is proportional to the power.

Identifying the Moving Coil

Based on its connection across the voltage supply and the current it carries (proportional to voltage), the moving coil in a dynamometer wattmeter is correctly identified as the potential coil.

While other types of coils like compensating coils or low power factor coils might be used in specific wattmeter designs for error correction, the fundamental component that moves and interacts with the field of the current coils is the potential coil.