We use induction type instrument for AC measurement only. This is because the induction phenomenon only occurs in AC. An induction instrument has multiuse. For example, we use the instruments as ammeters, voltmeters, wattmeters, and energy meters. Induction type instruments essentially have an electromagnet to produce the required magnetic field. AC in the electromagnet produces changing flux between its poles. Here, we place one aluminum disc (or aluminum drum) in the magnetic field. Hence, the changing flux links with the aluminum disc. As a result, the flux induces an eddy current on the disc. This eddy current interacts with the flux which has induced it. Consequently, there is a mechanical torque acting on the disc. This mechanical torque rotates the disc.
Torque in Induction Type Instrument
So, the torque depends on two factors. The first one is the strength of the field of the electromagnet. The second one is the value of eddy current on the disc. Of course, the torque is proportional to the strength of the magnetic field. Also, it is proportional to the eddy current. Again, the strength of the magnetic field depends on the current of the electromagnet. On the other hand, the value of eddy current depends on the strength of the magnetic field. So, we can say, the value of eddy current also depends on the current of the electromagnet.
So, the torque acting on the disc is directly proportional to the square of the current of the electromagnet. In an induction type instrument, we directly feed the measuring current into the coil of the electromagnet. Therefore, the deflecting torque is directly proportional to the square of the measuring current.
Working Principle of Induction Type Instrument
Let us consider the flux, produced by the electromagnet is
The phase angle between that flux and induced eddy current is α. Hence, we can write the expression of the eddy current as
Again, the instantaneous torque is directly proportional to the instantaneous eddy current and the flux. Hence, we can write,
So, the mean torque is as follows,
The above expression tells that the torque is zero if α is 90°.
α is the phase angle between the flux and eddy current.
So, there must be some means in induction type instrument to prevent this phase angle from being 90°. We can achieve this by two methods listed below.
Ferrari’s Type Instrument
For the purpose of making the phase angle less than 90º, we split the winding of the electromagnet into two halves. Then we provide inductance with one half and resistance with the other half. These produce two fluxes. Due to the presence of inductance and reactance in the circuit, there is a phase difference between these two fluxes. But this phase difference cannot be 90º. Then these two fluxes will produce two eddy currents with the same phase difference. The interaction of one flux with the eddy current by the other flux produces a torque. We call those induction type instruments which are based on that principle as Ferrari’s type instrument.
Shaded Pole Type Instrument
Another method of producing two components of flux is providing shaded pole in the electromagnet. We call this type of measuring instruments as shaded pole type instrument. In this method, we split the pole faces into two parts. We fit one copper band on one part of polo faces of either side. We refer the part of the poles provided with the copper band as the shaded pole. As a result, the flux produced by the shaded pole and unshaded pole will have a phase difference. These two fluxes produce two eddy currents on the aluminum disc. The interaction of one flux with the eddy current by other flux causes the deflecting torque.
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