Motors are so constructed that when a current is passed through the field and armature coils the armature is rotated. The speed of the armature is regulated by the amount of amperage and voltage that passes through the series of magnets, and this rotating power is called the torque.
Torque is a twisting or turning force, and when a pulley is made fast to the armature shaft, and belted to connect with machinery, this torque, or force, is employed for work.
The speed of an armature when at full work is usually from twelve hundred to two thousand revolutions a minute. As few machines are designed to work at that velocity, a system of speeding down with back gears, or counter-shafts and pulleys, is employed. The motor itself cannot be slowed down without losing power. The efficiency of motors is due to the centrifugal motion of the mass of iron and wire in the armature and the momentum it develops when spurred on by the magnetism of the field-magnets acting upon certain electrified sections of the armature. The armature of a working motor is usually of such high resistance that the current employed to run it would heat and burn out the wires if the full force of the current was permitted to flow through it for any length of time. As the armature rotates it has counter electro-motive force impressed upon it. This acts like resistance, and reduces the current passing through. The higher the speed the less current it takes, so that after a motor has attained its highest, or normal speed, it is using less than half the current required to start it.
Reduction of current in the armature reduces torque, so that the turning force of the armature is reduced as its speed of rotation increases. On the other hand, the momentum, or “throw,” produces power at high speed, together with an actual saving of current. An armature revolving at sixteen hundred revolutions, and giving half a horse-power on a current of five amperes, is more economical than one making three to five hundred revolutions, and giving half a horse-power on a current of fifteen to twenty amperes. Thus, a slowly turning armature takes more current and exerts higher torque than a rapidly rotating one.
To protect the fine wire on the armature from burning, in high-voltage machines a starting-box, or rheostat, is employed. The motor begins working on a reduced current, and as it picks up speed more current is let in, and so on until the full force of the current is flowing through the motor. It is then turning fast enough to protect itself through the counter electro-motive force. This can be understood better after some practical experience has been had in the construction and running of motors. Of the various forms of motors but three will be illustrated and described; but the boy with ideas can readily design and construct other types as he comes to need them.
The Flat-bed Motor
The simplest of all motors is the flat-bed type, illustrated in [Fig. 39]. This is composed of a magnet on a shaft revolving before a fixed magnet attached to the upright board of the base. Where space is no object, this motor will develop considerable power from a number of dry-cells or a storage-battery. Now, in the section relating to dynamos, four different systems of wiring were shown. In motors of the direct-current type but one system will be described—that of the series-winding, illustrated in [Fig. 40]. The current, entering at A, passes to the brush (B), thence through the commutator (C) and the armature coils. It runs on through the brush (B B), the field-coils (F), and out at D. This is the same course the current takes in the series-wound dynamo illustrated in [Fig. 14], page 241, and with such a dynamo current could be generated to run any series-wound, direct-current motor.
A FLAT-BED MOTOR AND PARTS