Auto-transformers.—In this class of transformer, there is only one winding which serves for both primary and secondary. On account of its simplicity it is made cheaply.
Auto-transformers are used where the ratio of transformation is small, as a considerable saving in copper and iron can be effected, and the whole transformer reduced in size as compared with one having separate windings.
Fig. 1,978 illustrates the electrical connections and the relations between the volts and number of turns.
By using the end wire and tapping in on turn No. 20 a current at 20 volts pressure is readily obtained which may be used for starting up motors requiring a large starting current and yet not draw heavily on the line.
Since the primary is connected directly to the secondary it would be dangerous to use an auto-transformer on high pressure circuits. This type of transformer has only a limited use, usually as compensator for motor starting boxes.
Figs. 1,979 and 1,980.—Two winding transformer and single winding or auto-transformer. Fig. 1,979 shows a 200:100 volt transformer having a 10 amp. primary and a 20 amp. secondary, the currents being in opposite directions. If these currents be superposed by using one winding only, the auto-transformer shown in fig. 1,980 is obtained where the winding carries 10 amp. only and requires only one-half the copper (assuming the same mean length of turn). If R be the ratio of an auto-transformer, the relative size of it compared with a transformer of the same ratio and output is ((R - 1) / R):1. For example, a 10 kw. transformer of 400 volts primary and 300 volts secondary could be replaced by an auto-transformer of 10 × (1.33 - 1) / 1.33 = 2.5 kw.; or, in other words, the amount of material used in a 2½ kw. transformer could be used to wind an auto-transformer of 400:300 ratio and 10 kw. output.
Constant Current Transformers for Series Arc Lighting.—The principle of the constant current transformer as used for series arc lighting is readily understood by reference to the elementary diagram shown in fig. 1,981. A constant alternating current is supplied to the stationary primary coil which induces a current in the movable secondary coil. The pressure induced in the coil will depend on the number of lines of flux which pass through it and by changing its position in the magnetic field over the primary a variable e.m.f. can be produced and a constant current maintained in the lighting circuit when the lamps are turned on or off, or if the resistance of the circuit be lowered by the consumption of the carbons.
