Understanding Three-Phase Dynamometer Wattmeters and Coils

A dynamometer wattmeter is an instrument used to measure electric power. In a three-phase system, power measurement can be done in several ways. One common method for measuring total three-phase power is the two-wattmeter method. A three-phase dynamometer wattmeter often integrates the components required for this method into a single unit.

Two-Wattmeter Method Principle

The two-wattmeter method can measure the total power in a three-phase three-wire system under both balanced and unbalanced load conditions. This method uses two single-phase wattmeters. Each single-phase wattmeter consists of:

• A current coil (CC) connected in series with the load in one line.
• A pressure coil (PC) or voltage coil (VC) connected across the line in which the current coil is placed and another line.

The algebraic sum of the readings of the two wattmeters gives the total power consumed by the three-phase load.

Coil Configuration in a Three-Phase Wattmeter

When a single dynamometer instrument is designed to measure the total power of a three-phase system using the principle of the two-wattmeter method, it needs to replicate the coil setup of the two individual wattmeters. Therefore, a three-phase dynamometer wattmeter typically contains:

• Two current coils, one for each phase line whose current is being measured.
• Two pressure coils, each connected between the respective current coil line and the third line.

These coils are arranged such that the deflection of the single moving system is proportional to the sum of the powers measured by the two equivalent single-phase wattmeters.

Based on the standard two-wattmeter method commonly integrated into a three-phase wattmeter, the instrument will have two sets of current and pressure coils.