Types of Induction Relays Based on Structure
(a) Shaded-pole structure – The general arrangement of shaded-pole structure is shown in fig 7. It consists of pivoted aluminium disc free to rotate in the air-gap of an electromagnet. One half of each pole of the magnet is surrounded by a copper band known as shading ring. The alternating flux Фs in the shaded portion of the poles will lag behind the flux Фu in the unshaded portion by an angle α. These two fluxes differing in phase will produce the necessary torque to rotate the disc. And the driving torque T is given by,
T α Фs Фu sin α
Assuming the fluxes Фs and Фu to be proportional to current I in the relay coil,
T α I2 sin α
This shows that driving torque is proportional to the square of current in the relay coil.
(ii) Watt-hour-meter structure – This structure gets its name from the fact that it is used in watt hour meters. The general arrangement of this type of relay is shown in fig8. It consists of a pivoted aluminium disc arranged to rotate freely between the poles of two electromagnets. The upper electromagnet carries two windings the primary and the secondary. The primary winding carries the relay current I1 while the secondary winding is connected to the winding of the lower magnet. The primary current induces e.m.f in the secondary and so circulates a current I2 in it. The flux Ф2 induced in the lower magnet by the current in the secondary winding of the upper magnet will lag behind Ф1 by an angle α. The two fluxes Ф1 and Ф2 differing in phase by α will produce a driving torque on the disc proportional to Ф1 Ф2 sin α. An important feature of this type of relay is that its in operation can be controlled by opening or closing the secondary winding circuit if this circuit is opened no flux can be set up by the lower magnet whatever be the value of current.
(iii) Induction cup structure – Fig 9 shows the general arrangement of an induction cup structure. It resembles an induction motor, except that the rotor iron is stationary, only the rotor conductor portion being free to rotate. The moving element is a hollow cylindrical rotor, which turns on its axis. Two pairs of coils wound on four poles as shown produce the rotating field. The rotating field induces current; in the cup to provide the necessary driving torque. If Ф1 and Ф2 represent the fluxes produced by the respective pairs of poles, then torque produced is proportional to Ф1 Ф2 sin α where α is the phase difference between the two fluxes. A control spring and the backstop for closing of the contacts carried on an arm are attached to the spindle of the cup to prevent the continuous rotation. Induction cup structures are more efficient torque producers than either the shaded pole or the watt-hour meter structure. So this type of relay has very high speed and may have an operating time less then 0.1 second
Written by arjun on March 14th, 2009 with
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