How does a wattmeter work?
The wattmeter is an instrument which is used for measuring the electric power of a circuit(or the supply rate of electrical energy) in watt of any given circuit. Electromagnetic wattmeters are used for measurement of utility frequency and audio frequency power; other types are required for radio frequency measurements.
A modern digital wattmeter samples the voltage and current thousands of times a second. For each sample, the voltage is multiplied by the current at the same instant; the average over at least one cycle is the real power. The real power divided by the apparent volt-amperes (VA) is the power factor. A computer circuit uses the sampled values to calculate RMS voltage, RMS current, VA, power (watts), power factor, and kilowatt-hours. The readings may be displayed on the device, retained to provide a log and calculate averages, or transmitted to other equipment for further use. Wattmeters vary considerably in correctly calculating energy consumption, especially when real power is much lower than VA (highly reactive loads, e.g. electric motors). Simple meters may be calibrated to meet specified accuracy only for sinusoidal waveforms. Waveforms for switched-mode power supplies as used for much electronic equipment may be very far from sinusoidal, leading to unknown and possibly large errors at any power. This may not be specified in the meter's manual.
Construction
The internal construction of a wattmeter is such that it consists of two coils. One of the coil is in series and the other is connected in parallel. The coil that is connected in series with the circuit is known as the current coil and the one that is connected in parallel with the circuit is known as the voltage coil.
Types of wattmeters
There are three types of wattmeters
- Dynamometer type wattmeter
- Induction type wattmeter
- Electrostatic type wattmeter
The dynamometer type wattmeter are further divided into two classes namely
- Suspended-coil torsion wattmeter
- Pivoted-coil direct-reading wattmeter
- Among these types of wattmeter, the suspended-coil torsion wattmeter is used as a standard meter in test laboratories only.
- The electrostatic wattmeter is suitable for high voltage low power factor circuits like dielectric loss measurement in capacitors.
- They are used in very small power measurements.
- The induction type wattmeter and pivoted coil direct reading wattmeters are very commonly used for power measurement.
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