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Electric Motor

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MOTOR, ELECTRIC. A machine for converting energy in the form of an electric current into mechanical work. This is accomplished through the use of two fundamental physical phe nomena, the first, that an electric current passed through a wire surrounding an iron core causes a magnetic field to be set up in that core and, the second, that an electric current flowing through a wire situated in a magnetic field exerts a mechanical force on the wire tending to force it out of the magnetic field. Funda mentally, then, it may be said that an electric motor consists of four main elements, viz., (a) a mechanical structure which sup ports and protects the magnetic, electric and insulating members, and which transmits the mechanical force to the work; (b) a magnetic structure which carries the necessary magnetic flux or field; (c) one set of copper coils or windings for carrying the electric magnetizing current which sets up the magnetic field; (d) a second set of coils for carrying the electric working current which reacts on the magnetic field to produce torque.

Electric motors are first classified with reference to the char acteristics of the supply circuit from which electric energy is to be taken. There are two common forms in which electric energy is commercially distributed. These are direct current and alternating current. In the first of these forms the current is considered as flowing continuously in one direction around the circuit. The distribution of the current requires only two conductors. Or dinarily these two conductors are two copper wires, but in the case of street railway work a copper wire is used for one conductor and the earth or ground is used as the return side of the circuit. This arrangement avoids the necessity of carrying two trolley poles on the car. In the second form, or alternating current, the current is assumed as reversing its direction several times a second throughout the circuit and flowing alternately one way and the other. In the two most widely used systems of alternating cur rent in use in the United States, the current reverses its direction or alternates 5o times per second and 120 times per second. Since two alternations are required to complete a cycle forward and back to the starting point, these two systems are known as 25 cycle systems and 6o cycle systems. This characteristic of alter nating current energy is called frequency. Electric motors are, therefore, broadly classified as : (a) direct current motors and (b) alternating current motors. Direct current motors are further classified as: (I) shunt motors; (2) series motors; (3) compound motors; (4) compensated motors. Alternating current motors are classified as: (I) synchronous motors; (2) induction motors ; (3) commutator motors. Induction motors are further classified as : (I) squirrel cage rotor motors ; (2) phase wound rotor motors.

Structural Description.

In structural detail an electric motor consists of a stationary member and a rotating member, usually called respectively the field or stator and the armature or rotor, as shown in Plate, fig. 7. The stator is composed of

(a) a frame, usually cylindrical, which serves as a mechanical support for the stationary magnetic structure and for the stator windings as shown in Plate, fig. 3. In machines of small and medium capacity, the frame also serves to support the brackets which carry the bearings in which turns the shaft or axle carrying the rotor parts. In direct current motors the frame forms part of the magnetic circuit; it may also support the brushes or carbon blocks which make contact with the rotating commutator and serve as means for introducing line current into the rotating coils on the armature as seen in Plate, fig. 6; (b) the stator magnetic structure consisting of individual or salient steel poles bolted to the frame in direct current motors and of a core built up in the axial direction of many thin sheet steel laminations in alternating current motors. These laminations have punched, in their inner periphery, bore slots or notches in which are placed the stator coils or field windings, as illustrated in Plate, fig. 7; (c) the field coils or stator windings which are electrically connected to the supply circuit and which conduct into the motor the electric current for magnetizing purposes. In alternating current motors the torque or working current is also taken into the stator wind ings, but in direct current motors the working current is taken into the armature or rotor through the stationary brushes and the rotating commutator. The rotating member, called the armature or rotor, shown in Plate, fig. 5, consists of (a) an axle or shaft which turns in the bearings and which serves to transmit the mechanical torque from the rotating magnetic core and windings to the pulley or gear or coupling, which is connected to the driven machine or work which is to be done. Mounted on the shaft and inside the bearings is (b) the rotating magnetic structure. In direct current motors and induction motors this consists of built-up thin sheets of steel laminations. These laminations have punched in their outer periphery slots in which are located the rotor coils. These laminations may be mounted directly on the shaft or carried through an intermediate member, usually of cast iron, called a spider. In the case of synchronous motors the rotor magnetic circuit consists of an annular steel rim to which are bolted salient radially projecting poles of solid or laminated steel seen in Plate, fig. ; (c) the rotor windings of various forms situated in slots in direct current armatures and induction motor rotors as illustrated in Plate, fig. 5, fig. 2, fig. 4, and surrounding the complete pole in the case of synchronous motors; (d) the commutator in the case of direct current motors and alternating current commutator motors or slip rings in the case of phase wound rotor induction motors and synchronous motors. The function of the commutator and slip rings is described later.

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