Fig. 1,981.—Elementary diagram illustrating the principles of constant current transformer as used for series arc lighting.
Since the induced currents in the secondary are repelled by the primary there is a tendency for the secondary coil to jump out of the primary field, and in case of a very large current due to a short circuit in the lamp circuit, the secondary current is quickly reduced to normal by the rapid movement of the coil upward.
By adjusting the counterweight for a given number of amperes required by the arc, the current will be maintained constant by the movement of the secondary coil.
The magnetic field produced by the primary must be kept the same by a constant current from the alternator, therefore, when the lamp load is increased the primary voltage increases similar to that of an ordinary series wound direct current dynamo. In other words the alternator and regulating transformer supply a constant current and variable voltage.
Fig. 1,982.—Mechanism of General Electric air cooled constant current transformer. It operates on the principle explained in the accompanying text and is built to supply 25 to 100 arc lamps at 6.6 to 7.5 amperes. The transformers are interchangeable and will operate on 60 or 125 cycles. The relative positions of the two coils may be changed in order to regulate the strength of the current more closely, by shifting the position of the arc carrying the counterbalance by means of the adjusting screw on it. A dash pot filled with special oil prevents sudden movements of the secondary coil and keeps the current through the lamps nearly constant, when they are being cut in or out of the circuit. In starting up a constant current transformer, it is necessary to separate the two coils as far as possible and then close the primary circuit switch and allow the two coils to come together. If the primary circuit be thrown directly on the generator the heavy rush of current which will follow due to the two coils being too close together might injure the lamps.
Constant current incandescent lighting systems for use in small towns also use this method for automatically regulating the current.
Regulation.—This term applies to the means adopted either to obtain constancy of pressure or current. In the transformer, regulation is inherent, that is, the apparatus automatically effects its own regulation. The regulation of a transformer means, the change of voltage due to change of load on the secondary; it may be defined more precisely as: the percentage increase in the secondary voltage as the load is decreased from its normal value to zero. Thus, observation should be made of the secondary voltage, at full load and at no load, the primary pressure being held constant at the normal value.