Electrical Engineering ⇒ Topic : Induction Type Instruments solve two methods

The induction type instruments are worked on single phase. The question arises how to obtain two fluxes having a phase difference 0 (= 900 as far as possible) from a single phase supply. Thisncan be achieved in two ways viz.

• Splitting the phase (i.e., Ferrari's Principle)
• Shaded-pole arrangement

1. Splitting the phase. In this method, two flux-producing windings are connected in parallel across a single phase supply; an inductive coil L in series with one and a resistance R in series with the other. The values of R and L are so selected that currents through the two windings [i.e., I_R and I_L in Fig.a (i)] have a phase difference of nearly 90⁰. The result is that we have two alternating fluxes with a relative phase shift of 90⁰. These fluxes pass through the aluminium disc and induce currents in it to produce the necessary driving torque.

The fluxes produced by the two currents may be represented as :

                                                                           figure (a)

The two fluxes Φ₁ and Φ₂ will induce e.m.f.s e₁ and e₂ respectively in the disc. Assuming r to be the resistance offered by the disc to each induced e.m.f., the induced currents are given by

The portion of the disc which is traversed by flux Φ₁, and carries current Φ₂ experiences a force F₁ along the direction indicated in Fig. a (ii). The magnitude of this force is given by

2. Shaded-pole arrangement. The shaded-pole structure differs from the split-phase type in that there is only one flux-producing winding connected to a.c. supply. The flux Φ produced by this winding is split into two portions  Φ₁ and Φ₂ having a phase difference of θ by shaded-pole arrangement as shown in Fig.(a). In this arrangement, one-half of each pole (on the same side) embraces a thick short-circuited copper loop called a shading coil. The shading coil acts as a short-circuited secondary and the main winding as a primary. Induced currents in the *shading coil cause the flux Φ₁ in the shaded  portion to lag the flux Φ₂ in the unshaded portion by θ  (= 40° to 50°) This displacement between the two fluxes produces the necessary deflecting torque given by

                                                                   figure (a)

The principle of operation of this type is the same as that of split-phase type