Induction type wattmeter - Engineering notes

Induction type wattmeter:

The induction type wattmeter is used to measure a.c power only.

Principle of Induction type wattmeter:

The principle of operation of an induction wattmeter is same as that of induction ammeters and voltmeters i.e. induction principle. However, it differs from induction ammeter or voltmeter in so far that separate two coils are used to produced the rotating flux in place of one coil with phase split arrangement.

Construction of Induction type wattmeter:

The principle parts of an induction wattmeter is as shown in the fig below. It consists of two laminated electromagnets. One electromagnet, called shunt magnet is connected across supply and carries current proportional to the applied voltage. The coil of this magnet is made highly inductive so that the current in it lags behind the supply voltage by 90 degrees. The other electromagnet, called series magnet is connected in series with supply and carries the load current. The coil of this magnet is made highly non inductive so that the angle of lag or lead is determined fully by the load.

Induction type wattmeter

A thin aluminium disc mounted on the spindle is placed in between the two magnets so that it cuts the fluxes of both the magnets. The controlling torque is provided by spiral springs. The damping is electromagnet and is usually provided by a permanent magnet embracing the aluminium disc. Two or more closed copper rings, called shading rings are provide on the central limb of the shunt magnet. By adjusting the position of these rings, the shunt magnet flux can be made to lag behind supply voltage by exactly 90degrees.

Working of Induction type wattmeter:

When the wattmeter is connected in the circuit to measure a.c power, the shunt magnet carries current proportional to the supply voltage and the series magnet carries the load current. The two fluxes produced by the magnets induce eddy currents in the aluminium disc. The interaction between the fluxes and eddy currents produce the deflecting torque on the disc, causing the pointer connected to the moving system to move over the scale.

 

Vector diagram

Deflecting torque of Induction type wattmeter:

let V = Applied voltage

Ic = Load current carried by the series magnet

Iv = Current carries by the shunt magnet

cos a = Lagging power factor of the load

The vector diagram of this wattmeter is shown in the fig below. the current Iv in the shunt magnet lags the applied voltage V by 90 degrees and so does the flux  av produced by it. The current Ic in the series magnet is the load current and hence lags behind the applied voltage by a' . The flux ac produced by this current Ic is in phase with it. Therefore the two currents Ic in the current coil and Iv in the voltage coil and also corresponding fluxes av and ac are (90 - a') apart.

The flux ac  induces the eddy currents iv in the aluminium disc which lags behind the flux by 90degrees. Similarly, flux ac  induces eddy currents ic which again lags behind flux ac  by 90 degrees.

Mean deflecting torque, Td   proportional  ac sin (90 - a ) 

Td  proportional  V I cos a

Td  proportional  a.c power

Since control is by springs, therefore  

Tc  proportional   deflection

For steady deflected position, Td  = Tc

Deflection  proportional  power

Hence, such instruments have uniform scale.