Measurement of Three Phase Power: Three Wattmeter Method

Power measurement in an AC circuit is measured with the help of a Wattmeter. A wattmeter is an instrument which consists of two coils called Current coil and Potential coil.

The current coil having low resistance is connected in series with the load so that it carries the load current.

The potential coil having the resistance is connected across the load and carries the current proportional to the potential difference.

For measuring the power in a 3 phase or Poly Phase system, more than one wattmeter is required, or more than one readings are made by one wattmeter. If more than one wattmeter is connected for the measurement, the process becomes convenient and easy to work with instead of taking various readings with one wattmeter.

The number of wattmeters required to measure power in a given polyphase system is determined by Blondel’s Theorem.

According to Blondel’s theorem – When power is supplied by the K wire AC system, the number of wattmeters required to measure power is one less than the number of wire i.e. (K-I), regardless the load is balanced or unbalanced.

Hence, three wattmeters are required to measure power in three-phase, four-wire system, whereas, only two wattmeters are required to measure the power in 3 phase, 3 wire system. Here in this article, a three wattmeter method of power measurement is discussed.

Three-Wattmeter Method of Three-Phase Power Measurement

Three Wattmeter method is employed to measure power in a 3 phase, 4 wire system. However, this method can also be employed in a 3 phase, 3 wire delta connected load, where power consumed by each load is required to be determined separately.

The connections for star connected loads for measuring power by three wattmeter method is shown below:

The pressure coil of all the three wattmeters namely W₁, W₂ and W₃ are connected to a common terminal known as the neutral point. The product of the phase current and line voltage represents phase power and is recorded by an individual wattmeter.

The total power in a three wattmeter method of power measurement is given by the algebraic sum of the readings of three wattmeters. i.e.

Where,

W₁ = V₁I₁

W₂ = V₂I₂

W₃ = V₃I₃

Except for 3 phase, 4 wire unbalanced load, 3 phase power can be measured by using only Two Wattmeter Method.

Also See: Two Wattmeter Method of Power Measurement