Induction Type Wattmeter - Construction, Working & Torque Equation

Induction type wattmeter is one of the types of wattmeters that works on the principle of mutual induction. Since the principle of induction can only possible with alternating current. Thus induction type wattmeters are used for the measurement of power only in ac circuits. Compared to electrodynamometer type wattmeters where it can be used in both ac and dc circuits, induction type wattmeters can be used only in the circuits having relatively steady values of frequency and voltage.


Construction of Induction Type Wattmeter :

It mainly consists of two laminated electromagnets wound with conductors known as shunt and series magnets. The upper electromagnet is known as a shunt magnet. It consists of three limbs, the side limbs carry the winding and it is connected across the load. These windings are excited by the current proportional to the voltage across the load, hence they are called voltage coils.


The lower electromagnet is connected in series with load in which power is to be measured and is known as a series magnet. It carries the windings called current coils and it is excited proportionally to the load current. The below shows the construction of an induction-type wattmeter.

Induction Type Wattmeter

A thin lightweight disc made up of copper or aluminum is pivoted at the center in the airgap present in between the two electromagnets (shunt & series). It is mounted with the help of a spindle so that the disc can rotate, which in turn rotates the spindle. A pointer is attached at top of the spindle and rotates with the spindle over the scale.


Working of Induction Type Wattmeter :

The working principle of induction type wattmeter is similar to induction type voltmeters and ammeters. The shunt magnet that carries two voltage coils are connected in series and other ends are connected across the load. They are wounded in such a way that, both circulate a flux through the central limb. In order to make the resultant flux lag behind the applied voltage by 90° shading bands are placed on the central limb.


Similarly, the series magnet that carries two current coils are connected in series and are wound in such a way that, they produce flux in the same direction. When the disc cuts the two fluxes i.e., from the shunt and series magnet. The combined effect of these changing fluxes on the disc induces eddy currents in it.


Therefore, a deflecting torque is produced on the disc due to the interaction of fields produced by the eddy currents. This causes to rotate the disc, thereby the spindle and pointer. To control deflecting torque, springs are provided that produce controlling torque.

Torque Equation :

The below shows the phasor diagram on the operation of the induction type wattmeter.

Induction Type Wattmeter

Let,
  • V = Voltage to be measured
  • I = Current to be measured
  • φ = Phase angle between current and voltage
  • φse = Flux produced by series magnet
  • φsh = Flux produced by shunt magnet
  • Esh = Emf induced in the disc by the shunt magnet flux
  • Ish = Eddycurrent in the disc caused by emf Esh
  • Ese = Emf induced in the disc by the series magnet flux
  • Ise = Eddycurrent in the disc caused by emf Ese

Assuming the disc is fully resistive. The eddy current Ise induced by the emf Ese will be in phase with it. So, we can see that eddy current Ise lags behind the current I by 90°. Thus there will be a phase difference of 90° between Ise and φse.


The flux φsh induces an emf Esh in the disc, which lags behind the φsh by 90°. Since the disc is resistive, the eddy current Ish caused by Esh will be in phase with it. Thus there will be a phase difference of 90° between Ish and φsh.


The torque produced by the interaction of current Ise and flux φsh is,
Induction Type Wattmeter
The torque produced by the interaction of current Ish and flux φse is,
Induction Type Wattmeter
Therefore, the resultant torque produced is,
Induction Type Wattmeter
Hence the deflecting torque produced is proportional to the ac power to be measured in the circuit.

Advantages of Induction Type Wattmeter :

  • The scale is uniform.
  • They provide good damping.
  • There is no effect of stray fields.

Disadvantages of Induction Type Wattmeter :

  • Can be used only for ac power measurements.
  • Low accuracy due to heavy moving system.
  • Temperature changes can affect the readings by introducing errors.
  • Power consumption is more.

Difference Between Dynamometer Wattmeter and Induction Type Wattmeter :

Electrodynamometer WattmeterInduction Type Wattmeter
The current coil is split into two parts, but a single pressure coil.Both current and pressure coils split into two parts each, placed on each of two arms of the two magnets.
The pressure coil current is very high.The pressure coil current is small.
The pressure coil is the moving coil.None of the coils are moving, rather there is an aluminum disc placed in between the two electromagnets that move.
Can be used for the measurement of power both in AC as well as DC circuits.Can only be used for the measurement of power in AC circuits.
The pointer is attached to the moving (pressure) coil.The pointer is attached to the aluminum disc.
Can be used in circuits even with fluctuating frequency and voltage.Can be used only with circuits having relatively steady values of frequency and voltage.
Fluid friction damping is used.Eddy current damping is used.

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