No-Load (or Off-load) Tap Changer This tap changer is used for seasonal voltage variations. An elementary form of no-load tap changer is illustrated in Fig. 1.55. It has six studs marked from one to six. The winding is tapped at six points, equal to the number of studs. The tapping leads are connected to six marked stationary correspondingly arranged in circle. The face plate carrying the six studs, can be mounted anywhere on the transformer, say on the yoke or on any other convenient place. The rotatable arm R can be rotated by means of handwheel, from outside the tank.
If the winding is tapped at 2.5% intervals,then with the rotatable arm R;
(a) at studs 1, 2; full winding is in circuit;
(b) at studs 2, 3: 97.5% of the winding is in circuit;
(c) at studs 3, 4; 95% of the winding is in circuit;
(d) at studs 4, 5; 92.5% of the winding is in circuit; and
(e) at studs 5, 6; 90% of the winding is in circuit.
Stop S fixes the final position and prevents the arm R from being rotated clockwise. In the absence of stop S, the arm R may come in contact with studs 1 and 6. In such a case, only the lower part of the winding is cut out of circuit and this is undesirable from mechanical-stress considerations.
The tap-changing must be carried out only after the transformer is disconnected from the supply. Suppose arm R is at studs 1 and 2. For bringing arm R at studs 2 and 3, the transformer is first de-energised and then the arm R is rotated to bridge studs 2 and 3. After this, transtormer is switched on to the supply and now 97.5% of the winding remains in circuit.
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