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.
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., IR and IL 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 :
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
The principle of operation of this type is the same as that of split-phase type
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