Electric Motors and Generators.
Publié le 11/05/2013
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the armature.
It is often desirable to generate as high a voltage as possible, and rotating armatures are not practical in such applications because of the possibility of sparkingbetween brushes and slip rings and the danger of mechanical failures that might cause short circuits.
Alternators are therefore constructed with a stationary armaturewithin which revolves a rotor composed of a number of field magnets.
The principle of operation is exactly the same as that of the AC generator described, except thatthe magnetic field (rather than the conductors of the armature) is in motion.
The current generated by the alternators described above rises to a peak, sinks to zero, drops to a negative peak, and rises again to zero a number of times eachsecond, depending on the frequency for which the machine is designed.
Such current is known as single-phase alternating current.
If, however, the armature iscomposed of two windings, mounted at right angles to each other, and provided with separate external connections, two current waves will be produced, each of whichwill be at its maximum when the other is at zero.
Such current is called two-phase alternating current.
If three armature windings are set at 120° to each other, currentwill be produced in the form of a triple wave, known as three-phase alternating current.
A larger number of phases may be obtained by increasing the number ofwindings in the armature, but in modern electrical-engineering practice three-phase alternating current is most commonly used, and the three-phase alternator is thedynamoelectric machine typically employed for the generation of electric power.
Voltages as high as 13,200 are common in alternators.
V AC MOTORS
Two basic types of motors are designed to operate on polyphase alternating current, synchronous motors and induction motors.
The synchronous motor is essentially athree-phase alternator operated in reverse.
The field magnets are mounted on the rotor and are excited by direct current, and the armature winding is divided intothree parts and fed with three-phase alternating current.
The variation of the three waves of current in the armature causes a varying magnetic reaction with the polesof the field magnets, and makes the field rotate at a constant speed that is determined by the frequency of the current in the AC power line.
The constant speed of asynchronous motor is advantageous in certain devices; however, in applications where the mechanical load on the motor becomes very great, synchronous motorscannot be used, because if the motor slows down under load it will “fall out of step” with the frequency of the current and come to a stop.
Synchronous motors can bemade to operate from a single-phase power source by the inclusion of suitable circuit elements that cause a rotating magnetic field.
The simplest of all electric motors is the squirrel-cage type of induction motor used with a three-phase supply.
The stator, or stationary armature, of the squirrel-cagemotor consists of three fixed coils similar to the armature of the synchronous motor.
The rotating member consists of a core in which are imbedded a series of heavyconductors arranged in a circle around the shaft and parallel to it.
With the core removed, the rotor conductors resemble in form the cylindrical cages once used toexercise pet squirrels.
The three-phase current flowing in the stator windings generates a rotating magnetic field.
This field induces a current in the conductors of thecage.
The magnetic reaction between the rotating field and the current-carrying conductors of the rotor makes the rotor turn.
If the rotor is revolving at exactly thesame speed as the magnetic field, no currents will be induced in it, and hence the rotor should not turn at a synchronous speed.
In operation the speeds of rotation ofthe rotor and the field differ by about 2 to 5 percent.
This speed difference is known as slip.
Motors with squirrel-cage rotors can be used on single-phase alternatingcurrent by means of various arrangements of inductance and capacitance that alter the characteristics of the single-phase voltage and make it resemble a two-phasevoltage.
Such motors are called split-phase motors or condenser motors (or capacitor motors), depending on the arrangement used.
Single-phase squirrel-cage motorsdo not have a large starting torque, and for applications where such torque is required, repulsion-induction motors are used.
A repulsion-induction motor may be of thesplit-phase or condenser type, but has a manual or automatic switch that allows current to flow between brushes on the commutator when the motor is starting, andshort-circuits all commutator segments after the motor reaches a critical speed.
Repulsion-induction motors are so named because their starting torque depends on therepulsion between the rotor and the stator, and their torque while running depends on induction.
Series-wound motors with commutators, which will operate on director alternating current, are called universal motors.
They are usually made only in small sizes and are commonly used in household appliances.
VI MISCELLANEOUS MACHINES
For special applications several combined types of dynamoelectric machines are employed.
It is frequently desirable to change from direct to alternating current or viceversa, or to change the voltage of a DC supply, or the frequency or phase of an AC supply.
One means of accomplishing such changes is to use a motor operating fromthe available type of electric supply to drive a generator delivering the current and voltage wanted.
Motor generators, consisting of an appropriate motor mechanicallycoupled to an appropriate generator, can accomplish most of the indicated conversions.
A rotary converter is a machine for converting alternating to direct current,using separate windings on a common rotating armature.
The AC supply voltage is applied to the armature through slip rings, and the DC voltage is led out of themachine through a separate commutator.
A dynamotor, which is usually used to convert low-voltage direct current to high-voltage direct current, is a similar machinethat has separate armature windings.
Pairs of machines known as synchros, selsyns, or autosyns are used to transmit torque or mechanical movement from one place to another by electrical means.
Theyconsist of pairs of motors with stationary fields and armatures wound with three sets of coils similar to those of a three-phase alternator.
In use, the armatures ofselsyns are connected electrically in parallel to each other but not to any external source.
The field coils are connected in parallel to an external AC source.
When thearmatures of both selsyns are in the same position relative to the magnetic fields of their respective machines, the currents induced in the armature coils will be equaland will cancel each other out.
When one of the armatures is moved, however, an imbalance is created that will cause a current to be induced in the other armature.The magnetic reaction to this current will move the second armature until it is in the same relative position as the first.
Selsyns are widely used for remote-control andremote-indicating instruments where it is inconvenient or impossible to make a mechanical connection.
DC machines known as amplidynes or rotortrols, which have several field windings, may be used as power amplifiers.
A small change in the power supplied to one fieldwinding produces a much larger corresponding change in the power output of the machine.
These electrodynamic amplifiers are frequently employed inservomechanism and other control systems.
See Automation; Electricity.
Contributed By:Elmer Charles EastonMicrosoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.
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