As a matter of fact every dynamo is protected against such a calamity by means of safety devices, which will be described in a later chapter—because no matter how careful a person may be, a partial short circuit is apt to occur. Happily, guarding against its disastrous effects is one of the simplest problems in connection with the electric plant.
Direct Current and Alternating Current
When one has mastered the simple Ohm's Law of the electric circuit, the next step is to determine what type of electrical generator is best suited to the requirements of a farm plant.
In the first place, electric current is divided into two classes of interest here—alternating, and direct.
We have seen that when a wire is moved through the field of a magnet, there is induced in it two pulsations—first in one direction, then in another. This is an alternating current, so called because it changes its direction. If, with our armature containing hundreds of wires to "cut" the lines of force of a group of magnets, we connected the beginning of each wire with one copper ring, and the end of each wire with another copper ring, we would have what is called an alternating-current dynamo. Simply by pressing a strap of flexible copper against each revolving copper ring, we would gather the sum of the current of these conductors. Its course would be represented by the curved line in the diagram, one loop on each side of the middle line (which represents time) would be a cycle. The number of cycles to the second depends on the speed of the armature; in ordinary practice it is usually twenty-five or sixty. Alternating current has many advantages, which however, do not concern us here. Except under very rare conditions, a farmer installing his own plant should not use this type of machine.
If, however, instead of gathering all the current with brushes bearing on two copper rings, we collected all the current traveling in one direction, on one set of brushes—and all the current traveling in the other direction on another set of brushes,—we would straighten out this current, make it all travel in one direction. Then we would have a direct current. A direct current dynamo, the type generally used in private plants, does this. Instead of having two copper rings for collecting the current, it has a single ring, made up of segments of copper bound together, but insulated from each other, one segment for each set of conductors on the armature. This ring of many segments, is called a commutator, because it commutates, or changes, the direction of the electric impulses, and delivers them all in one direction. In effect, it is like the connecting rod of a steam engine that straightens out the back-and-forth motion of the piston in the steam cylinder and delivers the motion to a wheel running in one direction.
Such a current, flowing through a coil of wire would make a magnet, one end of which would always be the north end, and the other end the south end. An alternating current, on the other hand, flowing through a coil of wire, would make a magnet that changed its poles with each half-cycle. It would no sooner begin to pull another magnet to it, than it would change about and push the other magnet away from it, and so on, as long as it continued to flow. This is one reason why a direct current dynamo is used for small plants. Alternating current will light the same lamps and heat the same irons as a direct current; but for electric power it requires a different type of motor.
Types of Direct Current Dynamos
Just as electrical generators are divided into two classes, alternating and direct, so direct current machines are divided into three classes, according to the manner in which their output, in amperes and volts, is regulated. They differ as to the manner in which their field magnets (in whose field of force the armature spins) are excited, or made magnetic. They are called series, shunt, and compound machines.