Regulation of Series Dynamos.—The series dynamo is ordinarily used for operating arc lamps connected in series. The current generally consumed is about 10 amperes, and it is necessary that it should remain at this strength to keep the lights burning steadily. If it increase, the lights will be too bright, and if it decrease, they will be too dim or flicker.
With all the lamps connected in series it is evident that the resistance of the circuit will vary widely as they are turned on or off, the resistance increasing as the lamps are turned on, and decreasing as they are turned off. It is necessary, therefore, that some means of regulation be provided to enable the dynamo to increase or decrease the voltage in proportion to the load. There are several methods of regulation, as by:
1. Variation of armature speed; 2. Variation of position of brushes; 3. Variation of field strength.
Whatever method be used the necessary regulation should be accomplished by automatic devices, as it would not be practical to station a man in constant attendance to regulate the voltage every time one or more lamps were thrown on or off.
Ques. When is the first method of regulation used?
Ans. It is only used in special cases, as for constant load; if the voltage be not just right to give the required current, it may be adjusted by changing the speed of the engine.
Ques. What may be said of the second method?
Ans. In both the “ring” and “drum” types of armature, rotating in a bipolar field, there are two points situated at opposite extremities of a diameter of the commutator, at one of which the potential is a maximum and at the other a minimum, and it is at these points that the brushes must be placed in order to obtain the greatest difference of pressure, the difference being less at other points. Hence, by rocking the brushes around the commutator the pressure at the terminals of the machine may be varied and regulated as required.
Ques. What difficulty is experienced in rocking the brushes to regulate the voltage?
Ans. Sparking takes place at the brushes when they are moved any considerable distance from the neutral position.