# How wattmeter works?

How does wattmeter work?

In this post we are going to understand how wattmeter works... before that just refresh about the electrical power.
Power Definition:
Electrical Power(P) is defined as the rate at which work is done in an electrical circuit.
Power = Voltage x Current
P = V.I
The basic unit of power is watt.
Watt definition:
The power consumed in a circuit is 1 watt if a potential difference of one volt causes 1 ampere of current to flow through the circuit.
Watt is denoted by a letter W and is measured by an instrument called as wattmeter.

Wattmeter - Introduction:

• Wattmeter is an instrument used to measure the electrical power.
• The traditional form of wattmeter is called as dynamometer wattmeter.
• This device is available in a purely electronic form too which is known as digital wattmeter.

Wattmeter Construction & Working Principle:

• The dynamometer wattmeter works on the motor principle.
• As shown in the above figure, the wattmeter has two sets of coils.
• One coil is fixed and is made in two identical parts.  It is made upon heavy gauge copper wire. So it has low resistance. This is named as current coil.
• The other coil which is known as voltage coil,  is wound from fine gauge wire. So it has relatively high resistance. The voltage coil is mounted on a circular manner. It is placed between the two parts of the current coil.
• The manner of connecting the wattmeter into a circuit is given in the 2nd figure.
• As shown, The current coil is connected in series with the load so that the circuit current flows through it.
• Similar to the voltmeter circuit, the voltage coil is connected in parallel with the load.
• Both voltage and current coils will produce magnetic fields.
• These fields interact to each other and produce a deflecting torque on the voltage coil.
• The interacting fields are proportional to the circuit voltage(V) and current(I)  respectively.
• So the produced deflecting torque is proportional to the product of circuit voltage and current. ie, VI which is nothing but circuit power.
• Restoring torque is provided by contrawound spiral springs, as in the moving coil meter.
• The voltage coil may be connected either on the supply side or the load side of the current coil. The choice of connection depends on other factors concerning the load.

A dynamometer wattmeter reads both ac and dc powers

1. It can be used to measure power in dc as well as ac circuits.
3. As both coils are air-cored it is free from hysteresis and eddy current losses.
4. The scale is uniform

1. As the coils are air-cored, the magnetic fields created are not strong. This requires more turns in moving coil and hence increased power loss.
2. The torque weight ratio is small.
3. It will be affected by stray magnetic field.
4. At low power factors the inductance of pressure coil introduces serious error. So a compensation is mandatory.
5. It is costlier than an induction instrument of the same ratings.

Wattmeter on DC circuits:

• For d.c. circuits, strictly speaking  wattmeter is not necessary.
• As power in dc circuit is P= VI watt, the  can be measured separately by a multimeter.
• Simply by multiplying V and I the power can then be calculated.
• But in AC circuit, this simple technique will not be applicable as we need to consider the power factor too ( ie, P = VIcosΦ)