5.2. Building up the Voltage of Self-excited Shunt Generator
One of the simplest forms of ‘self-excited’ generator is the shunt-wound machine, the connection diagram (without load) of which is shown in Fig. 32. The manner in which a self-excited generator manages to excite its own field and build a D.C. voltage across its armature is described with reference to Fig. 33 in the following steps:
- Assume that the generator starts from rest, i.e., prime-mover speed is zero. Despite a residual magnetism, the generated e.m.f. E, is zero.
- As the prime-mover rotates the generator armature and the speed approaches rated speed, the voltage due to residual magnetism and speed increases.
- At rated speed, the voltage across the armature due to residual magnetism is small, E1 as shown in the figure. But this voltage is also across the field circuit whose resistance is Rf Thus, the current which flows in the field circuit I1, is also small.
Fig. 33. Building up the voltage of a shunt generator.
- When I1 flows in the field circuit of the generator of Fig. 32, an increase in m.m.f results (due to IfTf Tf being field turns which aids the residual magnetism in increasing the induced voltage to E2 as shown in Fig. 33.
- Voltage E2 is now impressed across the field, causing a large current I2 to flow in the field circuit. I2Tf is an increased m.m.f., which produces generated voltage E3·
- E3 yields I3 in the field circuit, producing E4. But E4 causes I4 to flow in the field producing E5 ;and so on, up to E8, the maximum value,
- The process continues until that point where the field resistance line crosses. the magnetization curve in Fig. 33. Here the process stops. The induced voltage produced, when impressed across the field circuit, produces a current flow that in turn produces an induced voltage of the same magnitude. Eg. as shown in the figure.
- In the above description a particular value of field resistance Rf was used for building up of self-excited shunt generator. If the field resistance were reduced by means of adjusting the field rheostat of Fig, 32 to a lower value say Rf1 shown in Fig. 33, the build-up process would take place along field resistance line Rf1 and build-up a somewhat higher value than Ea, i.e. the point where Rf1 intersects the magnetisation curve, E9. Since the curve is extremely saturated in the vicinity of E9, reducing the field resistance (to its limiting field winding resistance) will not increase the voltage appreciably. Conversely, increasing the field rheostat resistance and the field circuit resistance (to a value having a higher slope than Rf in the figure) will cause a reduction of the maximum value to which build-up can possibly occur.
- The field resistance may be increased until the field circuit reaches a critical field resistance. Field circuit resistance above the critical field resistance will fail to produce build-up.
This critical field circuit resistance, Rc, is shown as tangent to the saturation curve passing through the origin, O, of the axes of the curve of Fig. 33. Thus a field circuit resistance higher than Rc will produce an armature voltage of E1 approximately (and no more).
Reasons for Failure of Self-excited Shunt Generator to Build-up Voltage. The reasons
why a self-excited generator may fail to build-up voltage are given below:
- No residual magnetism. The start of the build-up process requires some residual magnetism in the magnetic circuit of the generator. If there is little or no residual magnetism, because of inactivity or jarring in shipment, no voltage will be generated that can produce field current. To overcome this difficulty, a separate source of direct current is applied to the field for a short pedal, of time and then removed. The magnetic field should now be sufficient to allow the voltage to build-up. The application of a separate source of direct current to the field is called ‘flashing the field’.
- 2. Field connection reversed. The voltage generated due to residual magnetism is applied to the field. Current should flow in the field coils in such a direction as to produce lines of flux in the same direction as the residual flux. If the field connections are reversed, the lines of flux produced by the current flow will oppose the residual flux so that the generated voltage will decrease rather than increase when the field circuit is dosed, In this instance it is necessary to reverse the field
connections with respect to the armature.
- Field circuit resistance too. high. A field circuit resistance greater than critical value will prevent an appreciable build-up. At no load, resistance greater than the critical may be caused by the following:
- Open field circuit connection. The effects of an open circuit are apparent. The field circuit resistance is much greater than the critical value; hence generator will not build- up.
- Dirty commutator. A dirty commutator does not permit good contact between the brushes and the commutator. This poor contact shows up as a high resistance to the flow of current in the field circuit and produces the same effect as a high field circuit resistance -.