The shunt-wound generator is shown diagrammatically in Figure below. A small part of the total current, the exciting current, is shunted through the fields. The exciting current varies from possibly 5 percent of the total current in small machines to 1 percent in large ones.

The exciting current is determined by the voltage at the brushes and the resistance of the field winding. Residual magnetism in the field cores permits a shunt generator to “build-up.” This small amount of magnetism that is retained in the field cores induces a voltage in the armature (William H. Timbie, Elements of Electricity).

This voltage sends a slight current through the field coils, which increases the magnetization. Thus, the induced voltage in the armature is increased. This in turn increases the current in the fields, which still further increases the magnetization, and so on, until the normal voltage of the machine is reached and conditions are stable. This “building-up” action is the same for any self-excited generator and often requires 20 to 30 s.

If a shunt generator (Timbie) runs at a constant speed, as more and more current is drawn from the generator, the voltage across the brushes fails slightly. This fall is due to the act that more and more of the generated voltage is required to force the increasing current through the windings of the armature; i.e., the armature IR drop increases.

This leaves a smaller part of the total emf for brush emf, and when the brush voltage falls, there is a slight decrease in the field current, which is determined by the brush voltage. This and armature reactions cause the total emf to drop a little, which still further lowers the brush potential. These causes combine to lower the voltage gradually, especially at heavy overloads.

The curve in Fig.III, shows these characteristics. For small loads the curves is nearly horizontal, but at heavy overloads it shows a decided drop. The point at which the voltage of a commercial machine drops off rapidly is beyond the operating range and is of importance only for short-circuit conditions.

The voltage of a shunt machine can be kept fairly constant by providing extra resistance in the field circuit, which may be cut out as the brush potential falls.

This will allow more current to flow through the field coils and increase the number of magnetic lines set up in the magnetic circuit. If the speed is kept constant, the armature conductors cut through the stronger magnetic field at the same speed and thus induce a greater emf and restore the brush potential to its former value. This resistance can be cut out either automatically or by hand.

Shunt-wound generators give a fairly constant voltage, even with varying loads, and can be used for any system which incorporates constant-potential loads. This will operate well in parallel because the voltage of the machines decreases as the load increases. Shunt generators running in parallel will divide the load well between themselves if the machines have similar characteristics.

The necessary change in connections when reversing the direction of rotation of a shunt wound machine is indicated in figure below. Rotation is clockwise when, facing the commutator end of a machine, the rotation is in the direction of the hands of a clock.

Counterclockwise rotation is the reverse. When changing the direction of rotation, do not reverse the direction of current through the field windings. If the direction is reversed, the magnetism developed by the windings on starting will oppose the residual magnetism and the machine will not “build up.”

No comments:

Post a Comment