Electrodynamic Wattmeter Principle of Opperation

SCQF Level 6 Electrical Engineering: Electrical Testing & Measurement

Principle Operation of an Electrodynamic Wattmeter

The electrodynamic wattmeter is used to measure (Useful) power taken from ac power supplies. The electrodynamic wattmeter uses the reaction between the magnetic fields of two current-carrying coils, one fixed and the other movable. The voltage and current coils are wound around a laminated iron core to produce a magnetic field across the air gap. When the current through the fixed-position field windings is the same as current through the load and the current through the moving coil is proportional to the load voltage, then the pointer deflection is proportional to the power (P=IV). When the load is disconnected, the meter will still indicate that power is being consumed in the circuit. This can be overcome by two compensating springs, mounted with the primary fixed-coil current coil. These stationary coils are used to produce a magnetic flux proportional to the current through the movable coil. The currents through the primary movable coil and the compensating coil flow in opposite directions. These opposing fields cancel and when the load removed from the circuit, the meter will indicate zero power.

Electrodynamic Wattmeter’s are prone to errors from things like changes temperature, frequency and vibration. Heat through the instrument can cause the springs to lengthen and lose tension and errors are produced. Large currents within the circuit will also produce errors; the maximum current range of electrodynamic wattmeters is normally restricted to about 20 amps. When larger load currents are involved, a current transformer is used along with the wattmeter. The voltage range of Wattmeter’s is generally limited to several hundred volts because of heat dissipation within the voltage circuit. However, we can extend the voltage range by using voltage dividers.

Because electrodynamic wattmeter errors increase with frequency, they are normally used for measuring 50Hz circuits. A wattmeter has current, voltage, and power ratings, damage will be caused if any of these ratings are exceeded.

The electrodynamic wattmeter may be converted into an instrument for measuring reactive power by replacing the resistance normally in series with the voltage coil with a large inductance. A 90° current lag within the voltage coil provides a direct reading proportional to the reactive power in the circuit.

Electronic wattmeters are used for small power measurements or for power measurements at frequencies beyond the range of the electrodynamic wattmeter. They also have a number of other advantages such as they are lighter and more compact to use, they are cheaper to produce and can withstand greater vibrations (knocks and bumps). Digital meters are also easier to read and give a more accurate, user friendly reading.