Gilbert Richardson, age 13, of Hagerstown, Md., for his question:
HOW MANY KINDS OF ELECTRIC MOTORS ARE THERE?
Four electric motors are the most common ones used today: the series motor, the shunt motor, the compound motor and the induction motor.
Simplest and most widely used electric motor is the series motor. The current passes through both the armature and the field magnet in a series, that is, one after the other. The current increases the strength of the magnets. When the current is increased, the armature current is increased and the magnetism is increased. The magnet's strength increases greatly.
For light loads, the torque or turning force of the motor is proportional to the square of the strength of the current that is moving through the motor at the time. If two amperes will move a load of four pounds, four amperes will carry a load of 16 pounds.
On heavy loads the magnets reach the limit of their magnetic strength, called saturation. The armature current is increased when the current is increased. The torque increase of the motor is nearly in proportion to the current increase when the field magnets are saturated.
The shunt motor divides the current so that one part goes through the magnet and the other part goes through the armature. one wire is wound around the magnet many times because the magnetism is increased each time the current flows through the magnet.
The compound motor is a combination of the series and the shunt motors. One branch of the current goes through the magnet, as in the shunt motor. The other branch has a low resistance. It goes around both the magnet and the armature, as in the series motor.
The induction motor has no electrical connection to the armature. Therefore, it needs no commutator or brushes. It uses an alternating current. The armature is an iron core with slots around its outer edge. In the slots are placed copper bars fastened to copper rings at the ends. An alternating current produces a rotating magnetic field in the field magnet, thus inducing a magnetic field in the armature.
The induction motor is easy to build and is widely used in household appliances, such as electric clocks.
Compound motors are used in places where the load may be increased very suddenly.
The series motor is used where great torque is needed to start the motor. In the operation of a machine, it is used where a fairly uniform speed is required.
Electric motors are machines that change electric energy into mechanical power to perform work. The electric motor is used for almost every purpose that requires electric power to run machinery.
Electric motors range in size from the tiny devices that operate sewing machines to the great engines in heavy locomotives. In almost all modern factories, each machine is run by its own motor. Even a machine that generates its own electricity must be driven by a motor.