Wattmeter

Wattmeter:

The wattmeter is an instrument for measuring the electric power(or the supply rate of electrical energy) in watts of any given circuit. It consists of current coil and voltage coil(pressure coil). Wattmeter power measurement is done by the arrangement of voltmeter and ammeter.The pressure coil produces a flux Φ proportional to the voltage (V).The current coil produces a fluxΦ proportional to the current (I).Usually, the current coil is fixed and pressure coil is free to move.So the pressure coil is made an integral part of the moving system. It moves or deflects in response to the VI product or power.

Construction of a Wattmeter

The internal construction of a wattmeter is such that it consists of two coils. One of the coils 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.These coils are named according to the convention because the current of the circuit passes through the current coil and the voltage is dropped across the potential coil, also named as the voltage coil.The needle that is supposed to move on the marked scale to indicate the amount of power is also attached to the potential coil. The reason for this is that the potential coil is allowed to move whereas the current coil is kept fixed.The mechanical construction of a wattmeter is shown in the figure below.

Working of a Wattmeter

When the current passes through the current coil, it creates an electromagnetic field around the coil. The strength of this electromagnetic field is directly proportional to the amount of current passing through it.In the case of DC current, the current is also in phase with its generated electromagnetic field. The voltage is dropped across the potential coil and as a result of this complete process, the needle moves across the scale. The needle deflection is such that it is according to the product of the current passing and the voltage dropped, that is, P = VI.This was the case of DC power. We know that theAc power is given by the formula P = VIcosθ, and we know that this cosθ factor is because of the fact that the current and voltage are not in phase.But the question that arises here is that how will a wattmeter measure the AC power and this power factor? So the wattmeter simply measures the average power in case if AC power is required.

The measurement principle of wattmeter is shown in the figure below:

Applications of Wattmeter

1.It is used for power measurement of ac as well as dc circuits.

2.They are also used in industries to check the power rating and consumption of electrical appliances.

3.Electromagnetic watt meters are used to measure utility frequencies.

4.They are used with refrigerators, electric heaters and other equipment to measure their power ratings.

Types of wattmeter:

Basically,There are three types of wattmeters. They are :

1. Dynamometer types wattmeter
2. Induction types wattmeter
3. Electrostatic type wattmeter

The dynamometer type wattmeters are further divided into two classes namely

1. Suspended-coil torsion wattmeter
2. Pivoted-coil direct-reading wattmeter

• Among these types of a 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.

Dynamometer type;

• It works on the principle of torque production in a current carrying coil lying in a magnetic field created by a fixed set of coils.
• It is very similar to dynamometer type voltmeter or ammeter, except that the two coils ( fixed and moving coils) carry two separate currents.
• Dynamometer wattmeter gives an accurate reading in DC and slightly erroneous reading in ACdue to coil inductance.
• This meter suffers from low torque/weight ratio and a considerable power is required to make the instrument work.

Induction type:

• This type of wattmeter works on the principle of torque production by induced currents in a short-circuited secondary circuit.
• So an induction wattmeter can work on ACcircuits only.
• It has a few decisive advantages like sturdy construction and very long circular scale around 300°.

Electrostatic Type:

• A repulsive force exists on a plate placed in an electric field.This principle is used in this type of wattmeter.
• As the deflecting torque is very small it is not a commercial wattmeter.
• Electrostatic type wattmeter is useful in measuring very small powers in dielectric circuits.
• This type of wattmeter is free from errors due to waveform, frequency, eddy current and low power factors.

References:

1.https://en.wikipedia.org/wiki/Wattmeter

2.http://engineering.electrical-equipment.org/forum/general-discussion/how-does-a-wattmeter-work-back-to-basics