Motors And Generators
Electric Motors and Generators, group of devices used to convert mechanical energy into electrical energy, or electrical energy into mechanical energy, by electromagnetic means. A machine that converts mechanical energy into electrical energy is called a generator, alternator, or dynamo, and a machine that converts electrical energy into mechanical energy is called a motor.
Two related physical principles underlie the operation of generators and motors. The first is the principle of electromagnetic induction discovered in 1831 by the British scientist and inventor Michael Faraday. If a conductor is moved through a magnetic field, or if the strength of a magnetic field passing through a stationary conducting loop is made to vary, a current is set up or “induced” in the conductor. The converse of this principle is that of electromagnetic reaction, first observed by the French physicist André Marie Ampère in 1820. If a current is passed through a conductor located in a magnetic field, the field exerts a mechanical force on it.
The simplest of all dynamoelectric machines is the disc dynamo developed by Faraday. It consists of a copper disc that is mounted so that part of the disc, from the centre to the edge, is between the poles of a horseshoe magnet. When the disc is rotated a current is induced between the centre of the disc and its edge by the action of the field of the magnet. The disc can be made to operate as a motor by applying a voltage between the edge of the disc and its centre, causing the disc to rotate because of the force produced by magnetic reaction.
Generally, in larger machines, electromagnets are employed. Both motors and generators consist of two basic units: the field, which is the electromagnet with its coils; and the armature, which is the structure supporting the conductors that cut the magnetic field and carry the induced current in a generator, or the exciting current in a motor. The armature is usually a laminated soft-iron core around which conducting wires are wound in coils.
If an armature revolves between two stationary field poles, the current in the armature moves in one direction during half of each revolution and in the other direction during the other half. To produce a steady flow of unidirectional, or direct, current from such a device, it is necessary to provide a means of reversing the current flow outside the generator once during each revolution. In older machines this reversal is accomplished by means of a commutator-a split metal ring mounted on the shaft of the armature. The two halves of the ring are insulated from each other and serve as the terminals of the armature coil. Fixed brushes of metal or carbon are held against the commutator as it revolves, connecting the coil electrically to external wires. As the armature turns, each brush is in contact alternately with the halves of the commutator, changing position at the moment when the current in the armature coil reverses its direction. Thus there is a flow of unidirectional cur!
rent in the outside circuit to which the generator is connected. DC generators are usually operated at fairly low voltages to avoid the sparking between brushes and commutator that occurs at high voltage. The highest potential commonly developed by such generators is 1,500 V. In some newer machines this reversal is accomplished using power electronic devices, for example, diode rectifiers.
Modern DC generators use drum armatures that usually consist of a large number of windings set in longitudinal slits in the armature core and connected to appropriate segments of a multiple commutator. In an armature having only one loop of wire, the current produced will rise and fall depending on the part of the magnetic field through which the loop is moving. A commutator of many segments used with a drum armature always connects the external circuit to one loop of wire moving through the high-intensity area of the field, and as a result the current delivered by the armature windings is virtually constant. Fields of modern generators are usually equipped with four or more electromagnetic poles to increase the size and strength of the magnetic field. Sometimes smaller interpoles are added to compensate for distortions in the magnetic flux of the field caused by the magnetic effect of the armature.
DC generators are commonly classified according to the method used to provide field current for energizing the field magnets. A series-wound generator has its field in series with the armature, and a shunt-wound generator has the field connected in parallel with the armature. Compound-wound generators have part of their fields in series and part in parallel. Both shunt-wound and compound-wound generators have the advantage of delivering comparatively constant voltage under varying electrical loads. A magneto is a small DC generator with a permanent-magnet field.
In general, DC motors are similar to DC generators in construction. When current is passed through the armature of a DC motor, a torque is generated by magnetic reaction, and the armature revolves. The action of the commutator and the connections of the field coils of motors are precisely the same as those used for generators. The revolution of the armature induces a voltage in the armature windings. This induced voltage is opposite in direction to the outside voltage applied to the armature, and hence is known as back emf or counter emf (electromotive force). As the motor rotates more rapidly the back emf rises until it is almost equal to the applied voltage. The current is then small and the speed of the motor will remain constant as long as the motor is not under load and performing no mechanical work except that required to turn the armature. Under load the armature turns more slowly, reducing the back emf and permitting a larger current to flow in the armature. The motor !
is thus able to receive more electric power from the source supplying it and to do more mechanical work.
Because the speed of rotation controls the flow of current in the armature, special devices must be used for starting DC motors. When the armature is at rest, it has virtually no resistance, and if the normal working voltage is applied, a large current will flow, which may damage the commutator or the armature windings. The usual means of preventing such damage is the use of a starting resistance in series with the armature to lower the current until the motor begins to develop an adequate back emf. As the motor picks up speed the resistance is gradually reduced, either manually or automatically.
The speed at which a DC motor operates depends on the strength of the magnetic field acting on the armature, as well as on the armature current. The stronger the field, the slower is the rate of rotation needed to generate a back emf large enough to counteract the applied voltage. For this reason the speed of DC motors can be controlled by varying the field current.
Alternating-Current (AC) Generators (Alternators)
As stated above, a simple generator without a commutator will produce an electric current that alternates in direction as the armature revolves. Such alternating current is advantageous for electric power transmission, and hence most large electric generators are of the AC type. In its simplest form, an AC generator differs from a DC generator in only two particulars: the ends of its armature winding are brought out to solid unsegmented slip rings on the generator shaft instead of to commutators, and the field coils are energized by an external DC source rather than by the generator itself. Low-speed AC generators are built with as many as 100 poles, both to improve their efficiency and to attain more easily the frequency desired. Alternators driven by high-speed turbines, however, are often two-pole machines. The frequency of the current delivered by an AC generator is equal to half the product of the number of poles and the number of revolutions per second of the armature.
It is often preferable to generate as high a voltage as possible. Rotating armatures are not practical in such applications because of the possibility of sparking between brushes and slip rings and the danger of mechanical failures that might cause short circuits. Alternators are therefore constructed with a stationary armature within 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 that the 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 each second, depending on the frequency for which the machine is designed. Such current is known as single-phase alternating current. If, however, the armature is composed of two windings, mounted at right angles to each other, and provided with separate external connections, two current waves will be produced, each of which will 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, current will 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 of windings in the armature, but in modern electrical-engineering practice three-phase alternating current is most commonly used, with the three-phase alternator the dynamoelectric !
machine typically employed for the generation of electric power. Voltages as high as 23,200 V are common in alternators.
Two basic types of motors are designed to operate on polyphase alternating current: synchronous motors and induction motors. The synchronous motor is analogous to a three-phase alternator. The field magnets are mounted on the rotor and are excited by direct current, and the armature winding is divided into three 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 poles of 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 a synchronous motor is advantageous in certain devices. However, in applications where the mechanical load on the motor becomes very great, synchronous motors cannot 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 be made 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 armature of the squirrel-cage motor 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 heavy conductors 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 to exercise pet squirrels. The three-phase current flowing in the stationary armature windings generates a rotating magnetic field, and this field induces a current in the conductors of the cage. 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 the same 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 rota!
tion of the rotor and the field differ by about 2 to 5 per cent. This speed difference is known as slip.
Motors with squirrel-cage rotors can be used on single-phase alternating current by means of various arrangements of inductance and capacitance that alter the characteristics of the single-phase voltage and make it resemble a two-phase voltage. Such motors are called split-phase motors or condenser motors (or capacitor motors), depending on the arrangement used. Single-phase squirrel-cage motors do 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 the split-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, and short-circuits all commutator segments after the motor reaches a critical speed. Repulsion-induction motors are so named because their starting torque depends on the repulsion between the rotor and the stator, and their torque while running depends on induction.!
Series-wound motors with commutators, which will operate on direct or alternating current, are called universal motors. They are usually made only in small sizes and are commonly used in household appliances.
For special applications several combined types of dynamoelectric machines are employed. It is frequently desirable to change from direct to alternating current or vice versa, 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 from the available type of electric supply to drive a generator delivering the current and voltage wanted. Motor generators, consisting of an appropriate motor mechanically coupled to an appropriate generator, can accomplish most of the indicated conversions. A rotary converter is a machine that can be used to convert current from alternating to direct, 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 the machine through a separate commutator. A dynamotor, which is usually used to convert low-voltage direct current to high-voltage direct current, is!
a similar machine that 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. They consist 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 of selsyns 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 the armatures 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 equal and 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-c!
ontrol and remote-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 field winding produces a much larger corresponding change in the power output of the machine. These electrodynamic amplifiers are frequently employed in servomechanism and other control systems.
Torque, a twisting effort applied to an object that tends to make the object turn about its axis of rotation. The magnitude of a torque is equal to the magnitude of the applied force multiplied by the distance between the object’s axis of rotation and the point where the force is applied. In many ways, torque is the rotational analogue to force. Just as a force applied to an object tends to change the linear rate of motion of the object, a torque applied to an object tends to change the object’s rate of rotational motion.
Induction (electricity), in electricity, the creation of an electromotive force (voltage) in a conductor moving across a magnetic field (hence the full name, electromagnetic induction). The effect was discovered by the British physicist Michael Faraday and led directly to the development of the rotary electric generator, which converts mechanical motion into electric energy.
When a conductor, such as a wire, moves through the gap between the poles of a magnet, the negatively charged electrons in the wire will experience a force along the length of the wire and will accumulate at one end of it, leaving positively charged atomic nuclei, partially stripped of electrons, at the other end. This creates a potential difference, or voltage, between the ends of the wire. If the ends of the wire are connected by a conductor, a current will flow around the circuit. This is the principle behind the rotary electric power generator, in which a loop of wire is spun through a magnetic field to produce a voltage and generate a current in a closed circuit (see Electric Motors and Generators).
Induction occurs only if the wire moves at right angles to the direction of the magnetic field. This motion is necessary for induction to occur, but it is a relative motion between the wire and the magnetic field. Thus, an expanding or collapsing magnetic field can induce a current in a stationary wire. Such a moving magnetic field can be created by a surge of current through a wire or electromagnet. As the current in the electromagnet rises and falls, its magnetic field grows and collapses (the lines of force move outward, then inward). The moving field can induce a current in a nearby stationary wire. Such induction without mechanical motion is the basis of the electric transformer.
A transformer usually consists of two adjacent coils of wire wound around a single core of magnetic material. It is used to couple two or more AC circuits by employing the induction between the coils.
When the current in a conductor varies, the resulting changing magnetic field cuts across the conductor itself and induces a voltage in it. This self-induced voltage is opposite to the applied voltage and tends to limit or reverse the original current. Electric self-induction is thus analogous to mechanical inertia. An inductance coil, or choke, tends to smooth out a varying current, as a flywheel smooths out the rotation of an engine. The amount of self-induction of a coil, its inductance, is measured by the electrical unit called the henry, named after the American physicist Joseph Henry, who discovered the effect. The inductance is independent of current or voltage; it is determined only by the geometry of the coil and the magnetic properties of its core.
Transformer, electrical device consisting of one coil of wire placed in close proximity to one or more other coils, used to couple two or more alternating-current (AC) circuits together by employing the induction between the coils (see Electricity). The coil connected to the power source is called the primary coil, and the other coils are known as secondaries. A transformer in which the secondary voltage is higher than the primary is called a step-up transformer; if the secondary voltage is less than the primary, the device is known as a step-down transformer. The product of current times voltage is constant in each set of coils, so that in a step-up transformer, the voltage increase in the secondary is accompanied by a corresponding decrease in the current.
Large devices are used in electricity supply, and small units in electronic devices (see Electronics). Industrial and residential power tranformers that operate at the line frequency (50 Hz in the United Kingdom), may be single phase or three-phase, and are designed to handle high voltages and currents. Efficient power transmission requires a step-up transformer at the power-generating station to raise voltages, with a corresponding decrease in current. Line power losses are proportional to the square of the current times the resistance of the power line, so that very high voltages and low currents are used on long-distance transmission lines to reduce losses. At the receiving end, step-down transformers reduce the voltage, and increase the current, to the residential or industrial voltage levels, usually around 240 volts.
Power transformers must be efficient and should dissipate as little power as possible in the form of heat during the transformation process. Efficiencies are normally above 99 per cent and are obtained by using special steel alloys to couple the induced magnetic fields between the primary and secondary windings. The dissipation of even 0.5 per cent of the power transmitted in a large transformer generates large amounts of heat, which requires special cooling provisions. Typical power transformers are installed in sealed containers that have oil or another substance circulating through the coils to transfer the heat to external radiator surfaces, where it can be discharged to the surrounding atmosphere.
In electronic equipment, transformers with capacities in the order of one kilowatt are largely used ahead of a rectifier, which in turn supplies direct current (DC) to the equipment (see Rectification). Such electronic power transformers are usually made of stacks of steel alloy sheets, called laminations, on which copper wire coils are wound. Transformers in the 1 to 100 watt power level are principally used as step-down transformers to couple electronic circuits to loudspeakers in radios, television sets, and high-fidelity equipment (see Sound Recording and Reproduction). Known as audio transformers, these devices use only a small fraction of their power rating to deliver signals in the audible ranges, with minimum distortion. The transformers are judged on their ability to reproduce sound-wave frequencies (from 20 Hz to 25 kHz) with minimal distortion over the full sound power level (see Frequency; Sound).
At power levels of one milliwatt or less, transformers are primarily used to couple ultra-high-frequency (UHF), very-high frequency (VHF), radio-frequency (RF), and intermediate-frequency (IF) signals, and to increase their voltage. These high-frequency transformers usually operate in a tuned or resonant circuit (see Resonance), in which tuning is used to remove unwanted electrical noise at frequencies outside the desired transmission range.
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