5.4. Series Generator
The field winding of a series generator is connected in series with the armature winding as shown in Fig. 39. It consists of a few series of heavy wire, capable of carrying the output current of the machine without overheating. The characteristic curves of a D.C. series generator are drawn as given below:
Fig. 39. Connection diagram for obtaining the saturation curve. of a series generator.
The series connected generator illustrated in Fig. 40 must be capable of safely carrying the maximum current to be used, or about 125 per cent of rated load current. A plot of simultaneous readings of generated voltage and field current, taken at a rated speed, yields the magnetisation curve 1 of Fig. 41.
- External Characteristic (curve 2). To obtain the data for this curve, the machine is connected to the load as shown in Fig. 40; ammeter and voltmeter being inserted to read the load current I(= Ia) and the terminal voltage V respectively. The machine is run at constant (rated) speed, a series of simultaneous readings of voltage and current is taken while the load is varied from a minimum value to perhaps 125 per cent of rated load. When these readings are plotted, using Vas co-ordinate and Ia ( = I) as abscissa curve 2 (see Fig. 41) is obtained.
Fig. 41. Characteristic curves of a D.C. series generator.
It may be noted that the readings cannot begin at zero load as with the shunt generator, for if the resistance of the circuit including armature, series field and load is increased beyond a certain critical value, the generator unbuilds and loses its load entirely. Thus, if OA is the resistance line for the circuit the terminal voltage is the ordinate to the curve at A. When the resistance of the circuit is gradually increased, the load falls off along the curve, and A approaches B. When the resistance line finally becomes tangent to the curve, however, operation becomes unstable, and any light further increase in the resistance causes the machine to unbuild its voltage and lose its load. The resistance that brings about this condition is called the critical resistance. Therefore, to begin with, the resistance of the circuit must be reduced below the critical value before the generator delivers any load.
- Internal Characteristic (curve 3). This curve is obtained by adding the resistance drop Ia (Ra + Rse) to the external characteristic curve; Ra and Rse being armature resistance and series field resistance respectively.
The difference between curves 1 and 3 is the reduction in voltage caused by armature reaction.
- It is worth noting that between A and C a considerable change in resistance brings about only a slight change in load current. Over this range the voltage decreases rapidly, owing to increasing armature reaction (particularly when the brushes are shifted forward), while the current remains nearly constant. Thus, between A and C the machine may be used to, supply power to a constant current variable- voltage circuit, such as series arc circuit.
- Owing to initially rising characteristic, the series generator is often used as a voltage booster to give an increase of voltage practically proportional to the current.
- A series generator also finds application in electric traction where ‘dynamic braking’ is employed. The connections of the series traction motors are changed by means of a controller so that they act as generators; the power absorbed in braking the vehicle being dissipated in resistances, which are also used for starting purposes when the machines are reconnected as motors.