Induction type wattmeter

Wattmeter are of two types electrodynamo type wattmeter and induction type wattmeter.

Induction type wattmeter works on the principle of mutual induction. It can only be used for the measurement of ac power, whereas the electrodynamometer type can be used for both ac and dc.

Induction wattmeter is useful with circuits having constant supply and frequency.

Construction of induction type wattmeter

The constructional feature of induction-type wattmeter is shown below:

The induction type wattmeter consists of two electromagnets that are placed opposite to each other.

The upper electromagnet is known as a shunt magnet and is connected across the measuring voltage. The other electromagnet is known as a series magnet, is connected in series with the load, carrying load current.

A thin aluminium disc is placed between the 2 electromagnets acted upon by the resultant flux produced by both the electromagnets.

The shunt magnet consists of some strips on its central limb, those strips are shading bands that are made of copper material. The cooper shading bands make the flux produced by the shunt magnet lag behind the applied voltage by 90°.

Working of induction type wattmeter

The induction wattmeter works on the principle of electromagnetic induction. The working principle of induction type ammeter and voltmeters is also similar.

When an AC supply is applied to the wattmeter, an electric current flows through the coil of the shunt electromagnet that is proportional to the supply voltage.

Due to this, both the magnets (shunt and series) produce flux, which induces eddy current in the aluminium disc. When both the fluxes interact with each other a deflecting torque is produced on the disc, which tends to move the disc, and the pointer connected to the disc experiences movement and indicated the reading on the scale.

When the controlling torque becomes equal to the deflecting torque, the pointer comes at-rest position.

Read More: Induction instruments working principle.

Theory

Phasor diagram of this instrument is shown below:

V = voltage to be measured

I = current to be measured

φ = phase angle between current and voltage

φsh = flux by the shunt magnet

φse = flux produces by the series magnet

Esh = emf induced in the disc by the flux of shunt magnet

Ish = eddy current flowing in the disc caused by emf Esh

Ese =  emf induced in the disc by the flux of the series magnet

Ise = eddy current flowing in the disc caused by emf Ese

Interaction of current Ise with flux φsh produces torque, T1

T1 = KIse φsh cosѲ

Interaction of current Ish with flux φse produces torque, T2

T2 = KIsh φse cos(180° – Ѳ)

Therefore, the resultant torque expression is

Td = T1 – T2

Td = KIse φsh cosѲ – KIsh φse cos(180° – Ѳ)

Td = K (KIse φsh cosѲ + KIsh φse cosѲ)

Td = K(K1VIcosѲ + K2VIcosѲ)

Td = KVIcosѲ (K1 + K2)

Therefore,

Td is directly proportional to VIcosѲ

Advantages and disadvantages

Advantages

  • The induction wattmeter produces a uniform scale.
  • Very robust construction
  • Unaffected by the stray magnetic field, because the deflecting torque of this instrument is high.

Disadvantages

  • The biggest disadvantage of this instrument is that it can only be used for AC measurements.
  • It consumes very high power
  • It has low accuracy because of the heavy-moving system.
  • Temperature variation can cause major errors in the instrument.

Quick summary

  • It works on the principle of mutual induction
  • It consists of 2 electromagnets (shunt and series), an aluminium disc, a spindle attached with the aluminium disc and copper shading bands.
  • AC supply produces flux, which induces eddy current and the interaction of eddy current and flux causes deflecting torque.

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