Wattmeter: Complete Overview With 2 Variants

Definition of a wattmeter:      

A wattmeter is made of a current coil that carries the load current and a voltage coil or pressure coil. These coils carry a current proportional to it and in phase with the voltage as shown in the figure.

Voltage coil is often referred as pressure coil. Inductance in Voltage coil, current is minimized as it causes the Voltage coil current to lag behind the applied voltage. To overcome this, a non-inductive resistance is connected in series with the voltage coil as shown in the figure above.

Wattmeter geraet
Wattmeter, Image Credit – UlfbastelWattmeter geraet, marked as public domain, more details on Wikimedia Commons

Working Principle of an Electrodynamic Wattmeter:

A PMMC (permanent magnet moving coil) instrument cannot be used on ac currents or voltages.

To produce an alternating torque, a supply is given to these instruments. But due to the presence of moment of inertia in the moving system, the pointer unable to make this rapid change and it will not display that reading. For this, the magnetic field in the gap must change along with the change of current so instrument is capable to read ac signal. This principle is used in these instruments only the operating field is provided by current carrying coils instead of magnets.

The moving coil carries a current proportional to the supply voltage. It is connected across the supply hence also called voltage coil or pressure coil. The fixed coil is connected in series with the load and carries current proportional to the load. Fixed coil is also called current coil. When fixed coil carries current, it produces its own flux and when current carrying moving coil is placed in this flux, it experiences a force, generating the required deflecting torque to deflect the pointer.


V = supply voltage

I1 = load current = current through fixed coil

I2 = current through moving coil

Now, I2 α V

The fixed coil is air cored hence flux density produced in the coil is directly proportional to the current through the coil.

Hence, B α I1

Now the deflecting torque is proportional to the interaction of two quantities i.e., flux produced by fixed coil and current through the moving coil.

Td α BI2

Td α I1I2

Td α VI­1

But V1 is power consumed by load hence deflecting torque is proportional to the power consumed by the load.


Let e = instantaneous voltage across load

            = Em sin (ωt -Ø)

i1 = instantaneous load current

    = current through fixed coil

If load is inductive in nature,

i1 = Im sin(ωt – Ø)

i2 = instantaneous current through moving coil

V = r.m.s value of voltage across load

I1 = r.m.s value of load current

cosØ = power factor of load

the deflecting torque is proportional to interaction of two fluxes; one produced by current i1 and other by i2

Td α i1i2

But i2 α e as moving coil is across the supply

Td α ei

             α   Em sinωt x Im sin (ωt – Ø)

             α   ½ EmIm [cos (Ø) – cos (2ωt – Ø)]

            α          ½ EmIm cos (Ø) – ½ EmIm cos (2ωt – Ø)

hence the average torque is

Td α ½ EmIm cosØ

             α          .  . cosØ

Td α VI cos (Ø) where V and I1 are r.m.s values. But VI1cos(Ø) is power consumed by the load. Thus the deflecting torque is directly proportional to the power consumed by load.

Tc α θ as spring control

In steady state, Td = Tc

                             Θ α VIcos(Ø)

Dynamometer type Wattmeter vs induction type Wattmeter:

   Dynamometer type Wattmeter            Induction type Wattmeter
This type of wattmeter can be used on both ac and dc system
In carefully designed instruments, it provides high degree of accuracy.
This wattmeter has less power consumption criteria.
Weight of moving system in this system is reasonably low
This is in uniform scale.
It has relatively weaker working torque.
The type of wattmeter can only be used on ac system.
The instrument is less accurate. It is accurate only at stated frequency and temperature
This induction type wattmeter has higher power-consumption requirements.
Weight of moving system in this system is reasonably higher
It has linear scale
The instrument has comparatively stronger working torque  

Explain the working principle of a moving iron instrument:

Most commonly used laboratory ammeters and wattmeter are of moving iron type, there are two types of moving iron instruments-

a. Moving iron attraction type instruments.

b. Moving iron repulsion type instruments

 Moving iron attraction type instruments-

This is having a fixed coil C and an iron piece D. The coil structure is flat and it has a narrow slot type of opening.

It’s a flat disc type, which is mounted eccentrically arranged at the spindle and spindle is in-between the jewel bearing. There is a pointer over the spindle, which moves over a graduated scale. The no. of turns in the fixed coil be subject to on the range of the apparatus. But for the transmission has higher current requirements over the coil, less no of turns are sufficient. The controlling torque is generated by the springs as well as gravity control used for vertically mounted panel type instruments. Damping torque is provided by air friction. The construction is shown below:

Working Principle

When current is passing through the coil is proportional to the quantity to be measured then the coil becomes an electro magnet. The electro magnet attracts soft iron piece towards it; thus, producing deflecting torque.

The soft iron piece linked to the spindle, hence as iron portion becomes attracted, spindle travels and hence pointer to the spindle gets deflected. If the direction of current is opposite, the magnetic field by current carrying coil will be in opposite direction. But for any direction of magnetic field, iron pieces are going to attract towards magnet. Hence deflection torque is always unidirectional. Hence these instruments well suit ac and as well as dc measurements.

 Electrodynamic type Instrument:

  • Fixed Coils type – the essential a uniform field for the operation of this instrument is generated by the fixed coil. Henceforth, a uniform field will create adjacent the Centre of that coil.
  • Moving Coil type– This is wound either as a coil of self-sustainment or non-metallic former type.
  • Controlling torque type – the controlling torque is providing by the springs.
  •  Moving Coil – the moving coil will be mounted on the aluminum spindle. It comprises of counter weight mechanism and a specific pointer.
  • Damping torque – the damping torque is providing by the friction of air. This will be produced by the help of aluminum vanes pair linked to the spindle at the end.
  • Shielding Mechanism– Usually generated field in the instrument is very feeble. Earth’s magnetic field impacts the reading. So, shielding is employed to reduce stray-magnetic field in the selected location.
  • The Cases and Scales – This usually polished wooden or metal cases, and rigid casing. 

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