Shunt Generator

Chapter 7.6 Shunt Generator

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • 240Fundamental Electrical and Electronic Principles It will be seen that the shape of this graph is similar to the magne-tization curve for a magnetic material. This is to be expected, since the emf will be directly proportional to the pole fl ux. The ‘ fl attening ’ of the emf graph indicates the onset of saturation of the machine ’ s magnetic circuit. When the machine is used in practice, the fi eld current would normally be set to some value within the range indicated by If1 and If2 on the graph. This means that the facility exists to vary the emf between the limits E1 and E2 volts, simply by adjusting rheostat R1 . Let the emf be set to some value E volt, within the range specifi ed above. If the load is now varied, the corresponding values of terminal voltage, V and load current I L may be measured. Note that with this machine the armature current is the same as the load current. The graph of V versus IL is known as the output characteristic of the generator, and is shown in Fig. 7.11 . The terminal p.d. of the machine will be less than the generated emf, by the amount of internal voltage drop due to Ra , such that: VEI Raa volt (7.1)IaRaV0voltageEIL Ia Fig. 7.11 Ideally, the graph of E versus I L would be a horizontal line. However, an effect known as armature reaction causes this graph to ‘ droop ’ at the higher values of current. The main advantage of this type of generator is that there is some scope for increasing the generated emf in order to offset the internal voltage drop, Ia R a , as the load is increased. The big disadvantage is the necessity for a separate d.c. supply for the fi eld excitation. 7.6 Shunt Generator This is a self-excited machine, where the fi eld winding is connected in parallel (shunt) with the armature winding. The circuit diagram is