Induction type wattmeter or simply induction wattmeter belongs to the family of induction type measuring instrument. Since the induction phenomenon can only happen in the alternating current system, this instrument only measures AC power. For that reason, an induction wattmeter differs from a dynamometer wattmeter. Because dynamometer type wattmeter can measure both AC and DC power.
Construction of an Induction Type Wattmeter
Let us go through the constructional details of this type of instrument. Before that, we have to recall the constructional details of the shaded pole type induction instrument. In the shaded pole type induction instrument, we intentionally create two flux of different phase to produce deflecting torque. Instead of using single electromagnet with the shaded pole, we use here two different electromagnets. Therefore, these two electromagnets generate two different fluxes of different phase. In this way, we can also be able to produce deflecting torque in this system.
In actual construction of an induction wattmeter, we use one electromagnet connected in series with the circuit. So, the flux produced by this electromagnet is directly proportional to the current in the circuit. Now we connect another electromagnet in parallel with the circuit. Thus, the current proportional to the voltage of the circuit produces another flux in this second electromagnet.
We place a lightweight circular aluminum disc in between these two electromagnets. Fluxes of the electromagnets produce two eddy currents in the disc. The interaction between eddy currents with the fluxes produces deflecting torque in the instrument. There are two or three copper rings fitted on the central limb of the shunt electromagnet. Due to this arrangement, the flux created by the shunt magnet lags 90 degree behind the circuit voltage. We will discuss the reason behind this lagging later in this article.
Constructional Types of Induction Wattmeter
On the basis of construction, there are two varieties of induction type wattmeter available.
In one variation, the shunt electromagnet has three limbs. Here, two side limbs hold the voltage coil and the central limb holds the copper rings. On the other hand, the series electromagnet has two limbs. Both of the two limbs hold the current coil.
In the second variation of induction wattmeter, series electromagnet has three limbs. The central limb holds the current coil. Whereas the shunt electromagnet has only two limbs. Here, the yoke of the electromagnet holds the voltage coil. The pole faces have come closer. These two pole faces together are surrounded by a single copper ring.
Working Principle of Induction Type Wattmeter
Let us consider V is the circuit voltage. I is the circuit current. The angle between V and I is θ. φse is the flux created by the series electromagnet. This flux will be in phase with the current I. φsh is the flux created by the shunt electromagnet. Ideally, the flux φsh lags behind circuit voltage V, by 90°. The flux φse of series electromagnet induces emf Ese in the disc. This EMF causes the eddy current Ise in the disc. We can assume that the disc is fully resistive. The eddy current Ise caused by induced emf Ese will be in phase with that emf. So, we see that the eddy current Ise also lags the current I by 90°. Thus, the angle between flux φse and eddy current Ise is 90°. The flux φsh induces an EMF Esh in the disc. The induced EMF Esh lags the flux φsh by 90°. As we have already assumed that the disc is fully resistive, the eddy current Ish caused by induced emf Esh will be in phase with that emf. Hence, the eddy current Ish also lags the flux φsh by 90°.
Expression of Torque in Induction Wattmeter
The torque produced by the interaction of current Ise and flux φsh is
The torque produced by the interaction of current Ish and flux φse is
The expression of resultant deflecting torque is
This is nothing but the expression of AC power. So the deflecting torque in induction wattmeter is proportional to the power to be measured.