Basics of Wattmeter - Power Measurement

Wattmeter is probably one of most common electrical engineering device which exists around us. A wattmeter measures the current and voltage passing through any circuit and then determines power through the circuit.
Today' you learn:
  1. Construction of wattmeter
  2. Analog vs Digital Multimeters
  3. Multifunction power meters
  4. AC vs DC Wattmeters calculations
  5. Two wattmeters vs Three wattmeter methods

Construction of wattmeter

From Ohm's law, we know that power is a product of voltage and current. So, a wattmeter should measure the voltage and current.
The wattmeter essentially contains two parts:
  1. A current measuring coil which connects in series and measures the current.
  2. A voltage measuring circuitry which connects in parallel to the circuit.
The figure below displays the block diagram of wattmeter connected across the load. Symbol A represents the ammeter (current measuring part) and V represents voltmeter (voltage measuring circuitry).

Analog vs Digital Wattmeters

Old wattmeters were analog devices which were known as electrodynamometers. Although, they are still employed in many places digital wattmeters are now preferred in most industries.

Multifunction power meters

Commonly used wattmeters can measure the active power. Very often electrical industries employ multifunction power meter. Such meters are designed to measure active, reactive and apparent power. They also measure the power factor.

AC vs DC Wattmeter calculations

The DC power is simply a product of voltage and current. One can simply use a voltmeter and ammeter to measure both and then multiply them to obtain the power.
An AC wattmeter, on the other hand, computes load current, load voltage and the cos of the angle between them.
Let's solve an example to learn the calculation.
Example 1:
If a circuit has:
V = 110 ∠20°
I = 2 ∠40°
What will the wattmeter display?
Since θ = 20° - 40° = -20°
So P = VI cos (θ) = 110 * 2 * cos (-20°)
= 206.73 W

Two wattmeters vs Three wattmeters

Three phase ac power systems are of two types:
  • Four-wire systems
  • Three-wire systems
In four wire systems, three wattmeters are used to measure power, whereas a three wire system uses two wattmeters.

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