Understanding the Dynamometer Wattmeter Pressure Coil

A dynamometer type wattmeter is an instrument used to measure electrical power. It operates on the principle of the interaction between two magnetic fields produced by current-carrying coils.

It consists essentially of two coils:

• Current Coil (CC): This coil has a low resistance and few turns of thick wire. It is connected in series with the load, so the main load current passes through it.
• Pressure Coil (PC): This coil has a high resistance and many turns of fine wire. It is connected in parallel with the load, across which the voltage is to be measured.

The torque produced in the wattmeter is proportional to the product of the instantaneous currents in the two coils, which, for a purely resistive AC load or DC load, is proportional to the power being measured.

Dynamometer Wattmeter Pressure Coil Characteristics

The primary function of the pressure coil (or voltage coil) is to draw a current proportional to the voltage across the load it is connected to. According to Ohm's Law, the current ($I$) is given by $I = V/Z$, where $V$ is the voltage and $Z$ is the impedance.

To ensure the current in the pressure coil accurately represents the voltage, its impedance ($Z_{PC}$) should ideally be as high as possible and primarily resistive. This is achieved by:

• Using many turns of fine wire, which increases resistance.
• Adding a high resistance ($R_{series}$) in series with the pressure coil.

The total impedance of the pressure coil circuit is $Z_{PC} = R_{series} + Z_{coil}$. The coil itself ($Z_{coil}$) has resistance and inductance ($R_{coil}$, $L_{coil}$). The impedance is $Z_{coil} = R_{coil} + j\omega L_{coil}$.

By adding a large series resistance ($R_{series} \gg R_{coil}$ and $R_{series} \gg \omega L_{coil}$), the total impedance becomes dominated by the added resistance. This makes the pressure coil circuit highly resistive.

Why is high resistance important?

• Minimizes Current Draw: A high resistance ensures that the current drawn by the pressure coil from the supply is small and does not significantly affect the load current or the circuit being measured.
• Reduces Phase Angle Error: The pressure coil circuit impedance is $Z_{PC} = R_{total} + j\omega L_{coil}$, where $R_{total} = R_{series} + R_{coil}$. For accurate power measurement, the current in the pressure coil should be in phase with the voltage across it. If the inductance ($L_{coil}$) is significant, the current will lag the voltage, introducing a phase error. Making the circuit highly resistive (i.e., $R_{total} \gg \omega L_{coil}$) minimizes this phase angle, ensuring the current is nearly in phase with the voltage.

While designers aim to minimize the inductance of the pressure coil itself (often using non-inductive windings), the defining characteristic achieved through the series resistor is that the pressure coil circuit behaves as a highly resistive element.

Therefore, the pressure coil of a dynamometer type wattmeter is designed to be highly resistive.