[Illustration: FIG. 2,435.--Dial of Westinghouse dial type variable ratio voltage regulator. The dial consists of a marble slab, upon which the contacts are mounted in a circle as shown. The contact arm is arranged to move from contact to contact. The alternate small contacts are dummies, serving to prevent the contact arm springing down between contacts when moving from one to another. The panel contains a changing switch which makes it possible to double the range of a regulator, since the transformer connections can be changed to both raise and lower to an extent equal to the full range of the transformer. The total range in voltage from a certain per cent. below to a certain per cent. above the line voltage can be obtained in a number of steps equal to twice the number of divisions into which the secondary winding of the transformer is divided.]
~Ques. What modification is made to adapt dial regulators for heavy current?~
Ans. A dial with a series transformer, and a shunt or auto-transformer are employed as shown in fig. 2,436.
~Ques. Why is such modification desirable?~
Ans. Because, the additional cost of a series transformer is small in comparison with the cost of building a dial with a large current carrying capacity, and the cost of bringing out a number of heavy leads from a small transformer.
[Illustration: FIG. 2,436.--Diagram of connections for Westinghouse ~11 point dial, series transformer and auto-transformer~. The auto-transformer has a number of taps connected across the line, the series transformer is placed in series with one side of the line, and connected to a dial, as shown.]
~Ques. How are dial regulators modified for high voltage?~
Ans. Standard dials may be used with series and shunt transformers similar to the method used for heavy current circuits.
Ans. The primary of the shunt transformer is connected across the line and the secondary has a number of taps which are connected to contacts on the dial. The primary of the series transformer is connected in series with the line and two leads from the secondary winding are connected to the dial.
The connections are similar to those shown in fig. 2,437, except that shunt transformers are used instead of auto-transformers.