ELECTRIC MOTORS

the name given to that division of dynamoelectric machinery in which electrical | power is converted into mechanical power. Electric motors are cla ified as direct-current motors or alternating current motors, according as_ the electric power taken by the motor is in the form of a direct current or an alternating current. Further subdivisions of each cla are made on the basis of differences in the operating characteristics of the various types. Direct-current Motors. The motor consists of a fixed magnetic field system with a rotating armature, which carries the conductors through which the supply current is pa ed. The magnetic field, produced in the air-gap between the poles and the armature, reacts with the current carrying conductors of the armature and produces the mechanical turning moment or torque. At the same time the motion of the conductors through the magnetic field generates an_H.M.F. in the conductors. This H.M.F. is in the opposite direction to the applied E.M.F., and is, therefore, called the back E.M.F. of the motor. The current taken by the motor is equal to the difference between the applied and back E.M.F.’s divided by the resistance of the armature winding. Since the armature resistance is always low, and the back H.M.F. is zero at starting, some form of starter is nece ary in order to limit the current to a safe value. E entially the starter consists of a suitable resistance connected in series with the armature. As the motor gains speed this resistance is gradually reduced to zero. The speed at which a D.C. motor runs varies inversely as the air-gap flux per pole, and very approximately, directly as the applied E.M.F. (directly as the back E.M.F. actually). The torque produced is proportional to the product of the air-gap flux per pole and the armature current. The torque and speed characteristics of a D.C. motor, therefore, depend on the manner in which the air-gap flux per pole varies with the load current. Series Motor. In this type the field magnet windings are connected in series with the armature winding, i.e. the same current flows in both windings. The air-gap flux per pole, therefore, depends on the current taken by the motor. Consequently, at light loads the speed of ' the motor is very high, and there is a very large fall in speed as the load increases. The torque increases rapidly with load for the same reason. At starting, a large torque is obtained at a low speed. These characteristics are specially suitable for traction purposes, for crane motors, and for the motors for certain machine tools. Shunt Motor. In this case the field magnet windings are connected as a shunt to the armature windings, i.e. the current in the field coils depends upon the applied voltage, and is, therefore, constant in normal operation. Apart from the slight etfect of the armature magnetomotive force, the air-gap flux per pole, therefore, remains almost constant at all loads. This means that the speed is practically constant at all loads (a very slight fall in speed with load occurs), and the torque, therefore, is almost directly proportional to the load current. The shunt motor is, therefore, suitable for all cases where an approximately constant speed at all loads is required. Alternating-current Motors. There are wide differences between the various types, »oth in construction 108 ELECTRIC MOTORS The type most commonly used is the polyphase induction motor. In this motor both the field system and the armature consist of a slotted core built up of iron laminations. The field system is called the stator, and the armature the rotor. Both carry conductors in their slots, and these conductors in each case form a polyphase winding. Current is supplied to the stator winding only. The currents in the rotor winding are induced by the action of the rotating magnetic field set up by the stator currents. Hence the name induction motor. For starting, a polyphase resistance completes the circuits of the rotor winding. This resistance is gradually and operation. reduced to zero as the motor attains its jull speed. The rotor circuits are, therefore, =- ~~ SII Synchronous motors are seldom used except for special purposes. They are exactly similar to the ordinary synchronous generator or alternator in construction, and the field system is almost invariably the rotating part. As their name implies, these motors have the characteristic of running at synchronous speed at all loads. If through overloading, or for any other reason, the motor is unable to maintain its synchronous speed, it immediately falls out of step and stops. Alternating-current Commutator Motors. These motors are in general appearance similar to the induction motor, but the rotor is fitted with a commutator. According to the electrical connections, these motors may be given characteristics similar to direct-current series or shunt motors. re Diagram of a Simple Motor C, Conductor on surface of iron core A, which is free to rotate between the poles N § of an electro-magnet elosed upon themselves in normal operation. In many motors (especially small ones which are started unloaded) the rotor winding consists of a series of copper bars brazed to solid end-rings at. each end of the core, thus forming a permanently short-circuited winding. Such a rotor is known as a squirrel-cage rotor. The speed characteristic of the induction motor closely resembles that of the shunt D.C. motor, and induction motors are suitable for similar purposes. The induction motor gives its maximum torque at a speed only slightly below the synchronous speed (corresponding to the number of poles in the stator winding and the frequency of the supply); and the torque decreases very rapidly as the speed rises towards synchronism. The maximum torque has a definite value for a given motor, and if the load demands a greater torque than this, the motor slows down and stops. Single-phase commutator motors with series characteristics are used on the Southern Railway electric trains. ELECTRIC POWER TRANSMISSION AND DISTRIBUTION. In the public supply of electric power in this country, the usual practice is to use alternating-current generators in the power stations, and to transmit the power at a high voltage to substations. The substation plant reduces the pre ure to a value suitable to the consumer, and in many instances also converts the alternating current into direct current. From the substations the power is distributed to the consumers. For a Like amount of power transmitted the cro -section of the cables required varies inversely as the square of the voltage. In order to reduce the outlay on cables, it is important that the transmi ion voltage should be as high as the circumstances permit. Naturally this 109 / ELECTRIC TELEGRAPH becomes more and more important as the distance over which the power has to be transmitted increases. In America, where large amounts of power are transmitted over very great distances, the pre ure used is in some cases 150,000 volts, and the tendency is to raise this still further, as switch gear, insulators, and other apparatus capable of withstanding this high pre ure are becoming ayailable. For high-tension underground cables, the pre ure now coming into common use is 20,000 volts. The nature of the low-voltage distribution from the substations, whether alternating current or direct current, depends largely on the requirements of the consumers. In the Thury system of power transmi ion high-voltage direct current ig used throughout. Only one supply in this country is of this kind, but several are in operation on the Continent. Pre ures up to 100,000 volts are used. The Electricity (Supply) Act, 1926, established a Central Electricity Board, charged with the duty of supplying electricity to authorized undertakers in Great Britain. Its chief functions are to construct main transmi ion lines (‘The Grid’), to concentrate generation at standard frequency in selected stations, and to supply electricity in bulk to duly constituted authorities. Schemes covering nearly the whole country have been adopted. ELECTRIC TELEGRAPH. See TELEGRAPH.