Fig. 237.
On two-pole machines there are two brush-holders, each containing one or more brushes. On the four-pole machine there may be either two or four brush-holders, and on a six-pole machine, either two, four, or six brush-holders.
A single path of the current through the commutator and armature winding is shown by the arrows on [Fig. 237]. The brushes B and C are placed on the top side of the commutator to make them more accessible, and this shows a peculiar but simple armature winding.
For the sake of simplicity, the batteries I and J, of [Fig. 236], are not used on common forms of generators or motors, but the current that flows from the armature through the commutator is made to flow through the electro-magnets either in whole or in part. If all of the armature current flows around the electro-magnets or fields of the machine, it is a “series” machine; if only a part of the current is used in this way, it is a “shunt” machine; that is, some of the current is “shunted” through the fields. Sometimes both the shunt and series windings are used, and in that case the machine is called a “compound wound” machine. Such a machine has a large wire through which the main current passes, and a fine wire through which the shunted current flows. Fig. 237 shows how the commutator and the fields are connected, and how the current flows from the wires in the armature through the commutator in a series machine.
If the current delivered by a dynamo does not flow in the desired direction, it can be reversed by shifting the wires in the binding posts or by throwing a switch. If the motor does not revolve in the desired direction, it can be made to do so by reversing the connections to the armature or field-coils; so that, without knowing which way a current of electricity is to be generated, any practical man can make a motor revolve in a proper direction by simply changing its connections.
It is natural that a machine which gives out electric energy when driven by an external power, should, when electric energy is delivered to it, reverse its action and give out mechanical power and do work.
Perhaps the simplest way to explain the cause of the movement of an electric motor, when supplied with a current, is to compare its action to the well-known attraction of unlike poles or magnets and the repulsion of like poles. Unlike poles are North and South; like poles are two North or two South. In all motors a current through the field causes a North or South pole to be maintained, and a current through the armature and brushes causes an opposite polarity. These constantly-maintained unlike poles attract each other and pull the armature around on its axis.
It has been explained that if a motor be driven by a belt an electro-motive force is produced and the machine acts as a dynamo. It is also a fact that an electro-motive force is produced whether the power for driving the machine is received from a belt or from the electric current,—that is, whether the machine be driven as a dynamo or as a motor. In a dynamo, however, the current follows the direction in which the electro-motive force is acting. In a motor, the electro-motive force produced has a direction opposed to that of the flow of current. This may be illustrated by the following experiment.
Two similar machines are driven independently at 600 revolutions and give an electro-motive force of 100 volts. Similar terminals of the two machines are connected together; no current flows between the machines, because the two pressures are the same and are in opposite directions. If now the belt be thrown off from one machine, its speed will begin to fall; this will lower its electro-motive force below that of the other machine or dynamo, but will not change the direction of the force. There will now be a difference of pressure in favor of the machine which is driven, and it will deliver a current through the other machine and run it as a motor. The speed of the motor will continue to fall until the difference in pressure or electro-motive force between the two machines is only sufficient to cause the flow of enough current to keep the motor running against whatever frictional resistance, and other resistance there may be. The electro-motive force generated in the motor, which is against, or counter to that of the current in the circuit, is called the “counter electro-motive force.”