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Shunt generators may be either separately excited or self - excited. Compound machines may be connected long-shunt or short-shunt, in which the shunt-field circuit is connected directly across the armature, without including the series field.

In general, the three characteristics that specify the steady-stale performance of a dc generator are:

The open-circuit characteristic, also known as the no-load magnetization curve, which gives the relationship between the generated emf and field current at constant speed.

The external characteristic, which gives the relationship between the terminal voltage and load current at constant speed.

The load characteristic, which gives the relationship between the terminal voltage and field current, with constant armature current and speed.

All other characteristics depend on the form of the open-circuit characteristic, the load, and the method of field connection. Under steady state conditions, the currents being constant or, at most, slowly varying, voltage drops due to inductive effects are negligible. While the self-inductance of armature coils undergoing commutation and the mutual inductance between these coils and the rest of the armature winding influence commutation, their effect on load characteristics of conventional dc machines is negligible.

The terminal voltage V of a dc generator is related to the armature current I and the generated emf E by:

Vt = Ea - IaRa

where Ra is the total internal armature resistance, including that of inter-pole and compensating windings as well as that of the brushes. The value of the generated emf Ea, is governed by the direct-axis field flux (which is the function of the field current and armature reaction) and the angular velocity of the rotor.

The open-circuit and load characteristics of a separately excited dc generator, along with its schematic diagram of connections, are shown in Figure 2. It can be seen from the form of the external volt-ampere characteristics, shown in Figure 2 that the terminal voltage falls slightly as the load current increases.

Since the separately generator requires a separate dc field supply, its use is limited to applications where a wide range of controlled voltage is essential.

Let us next consider the case of a self-excited dc shunt generator for which the schematic diagram of connections and the open-circuit characteristic are given in Figure 3 (a) and (b) respectively. With a field resistance, given by a line OR of slope OA / OB, shown in Figure 3 (b), the open-circuit voltage will build up to a value of OA.

It follows that there must be a critical-field resistance, when the corresponding critical-field resistance line ORc is coincident with the linear part of the characteristic. That is the maximum permissible value of field resistance if the armature voltage is to build up. Reasons for failure of voltage buildup can be summarized as follows:

Insufficient residual flux.

Incorrect polarity of the field winding due to which the field current is in a direction as to reduce the residual flux.

Field resistance above critical value if the speed is normal.

Speed ??below critical value if the resistance of the field circuit is normal.

A shunt generator maintains approximately constant voltage on load, and finds wide application as an exciter for the field circuit of large ac generators. The shunt generator is also used sometimes as a tachogenerator when a signal proportional to motor speed is required for control or display purposes.

The schematic diagram and the volt-ampere characteristic of a dc series generator at constant speed are shown in Figure 5. The resistance of the series-field winding must be low for good efficiency as well as for low voltage drop. The series generator was employed in early constant-current systems by operating in a range BC, where the terminal voltage fell off very rapidly with increasing current.

Cumulatively compounded generators, in which the series and field winding mmfs are adding, may be overcompounded, flat-compounded or under compounded, depending upon the strenth of the series field. Overcompounding may be used to counteract the effect of a decrease in the prime-mover speed with increasing load, or to compensate for the line drop when the load is at a considarable distance from the generator. Differentially compounded generators, in which the series winding mmf opposes that of the shunt -field winding, are used in applications where a wide variation in load voltage can be tolerated and where the generator may be exposed to load conditions approaching those of short circuit.

. | | Unit 4. | Brushes and brush rigging | Answer the questions. | Grammar exercises. | MECHANICAL ENERGY FROM ELECTRICITY | Compare the structure and function of a generator to an electric motor | Find the answers to the questions. | Translate into English. |

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