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Construction of an Induction Motor
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In an induction motor, the stationary portion of the machine is called a stator; the rotating member, a rotor. Instead of salient poles in the stator distributed windings are used. These are placed in slots around the periphery of the stator.
It is not usually possible to determine the number of poles from visual inspection of an induction motor. A look at the nameplate will usually tell the number of poles. It also gives the value of the RPM (revolutions per minute), the voltage required, and the current needed. This rated value is usually less than the synchronous speed because of slip to be discussed later. To determine the number of poles per phase of the motor, divide 120 times the frequency by the rated speed^
120 x f
P = -----------
N
P is the number of poles per phase, f is the frequency measured in hertz, N is the rated speed in RPM, and 120 is a constant. The result is very nearly the number of poles per phase.
If the number of poles per phase is given on the nameplate, the synchronous peed can be determined by dividing 120 times the frequency by the number of poles per phase.
The rotor of an induction m motor consists of an iron core with longitudinal slots around its circumference, in which heavy copper or aluminum bars are embedded. These bare welded to a very heavy ring of high conductivity on either end. This composite structure is sometimes called s a squirrel cage. Motors containing such a rotor are termed as squirrel cage induction motors.
Slip. Slip is the difference in RPM between a rotor and a rotating field. When the rotor of an induction motor is subjected to the revolving magnetic field produced by the stator windings, a voltage is induced in the longitudinal bars. The induced voltage causes the current to flow through the bars. This current, in turn, produces its own magnetic field. The magnetic field combines with the revolving field in such a way as to cause the rotor to assume the position in which the induced voltage is minimized. As a result, the rotor revolves at a nearly synchronous speed of the stator field. The difference in speed is just enough to induce enough current in the rotor to overcome the mechanical and electrical losses in the rotor. *** If the rotor were to turn at the same speed as the rotating field, the rotor conductors would not be cut by any magnetic lines of force. *** Therefore, no EMF would be induced in them, no current could flow, and there would be no torque. For this reason there must always be a difference between the rotor and the rotating field. This difference in speed is called slip. Slip is expressed as a percentage of the synchronous speed. For example, if the rotor turns at 1750 RPM and the synchronous speed is 1800 RPM, the difference in speed is 50 RPM. The slip is then equal to 2.78%.
Synchronous Motor
A synchronous motor is one of the principle types of AC motors. Like the induction motor, the synchronous motor is designed to take advantage of a rotating magnetic field. Unlike the induction motor, however, the torque developed does not depend upon the induction of currents in the rotor. Briefly, the principle of operation of a synchronous motor is as follows.
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A multiphase source of AC is applied to the stator windings and a rotating magnetic field is produced. A DC is applied to the rotor windings and another magnetic field is produced. The synchronous motor is so designed and constructed that these two fields react upon each other. They act in such a manner that the rotor is dragged along. It rotates at the same speed as that of the rotating magnetic field produced by the stator windings.
An understanding of the operation of the synchronous motor may be obtained by considering the simple motor. Assume that poles A and B are being rotated clockwise by some mechanical means in order to produce a rotating magnetic field. The rotating poles induce poles of opposite polarity, and the forces of attraction exist between corresponding north and south poles. Consequently, as poles A and B rotate, the rotor is dragged along at the same speed. However, if a load is applied to the rotor shaft, the rotor axis will momentarily fall behind that of the rotating field, but will thereafter continue to rotate with the field at the same speed, as long as the load remains constant. If the load is too large, the rotor will pull out of synchronization with the rotating field, and, as a result, will no longer rotate with the field at the same speed. The motor is then said to be overloaded.
Some advantages of a synchronous motor are:
1) when used as a synchronous capacitor, the motor is connected on the AC line in parallel with other motors on the line. It is run either without load or with a very light load. The rotor field is overexcited just enough to produce a leading current which offsets the lagging current of the line with motors operating. A unity power factor (1.00) can usually be achieved. This means the load on the generator is the same as though only resistance made up the load;
2) the synchronous motor can be made to produce as much as 80% leading power factor. However, because a leading power factor on a line is just as detrimental as a lagging power factor, the synchronous motor is regulated to produce just enough leading current to compensate for lagging current in the line.
The synchronous motor is not a self-starting motor. The rotor is heavy, and from a dead stop it is impossible to bring it into magnetic lock with the rotating magnetic field. For this reason, all synchronous motors have some kind of starting device. Such a simple starter is another motor, either AC or DC, which can bring the rotor up to approximately 90% of the synchronous speed. The starting motor is then disconnected and the rotor locks in step with the rotating field.
Another starting method is a second winding of the squirrel cage type on the rotor. This induction winding brings the rotor almost into synchronous speed. When DC is connected to the rotor windings, the rotor pulls into step with the field. The latter method is more commonly used.
Generators
There are a number of ways to produce electricity, some of which are commercially feasible. The use of magnetism is the most common method of generating electricity in large quantities for businesses, homes, industry, hospitals, and other institutions.
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Cells, or batteries, produce direct current (DC). A more economical way of producing DC, however, is with a mechanically driven generator. Mechanical force is used to rotate a wire loop in a magnetic field to generate electricity. The magnetic field is generated by a current-carrying wire, looped around a core.
The voltage generated will depend upon the following: the intensity of the magnetic field, the number of turns of the wire and the speed at which the wire passes through the magnetic field.
The arrangement of a simple generator is as follows: a loop of wire is wrapped around an iron core called an armature. Copper segments called the commutator, which are attached to the ends of the loop of wire, are insulated from the core and from each other. Brushes, conductors that make a sliding contact, are placed so that they contact the commutator and carry any generated electricity to the load, or a consuming device. To produce electricity the armature must be mounted between field coils so that the magnetic force generated by the electromagnet will be cut by the rotating armature. Field coils form an electromagnet when they are wrapped around soft-iron cores known as field poles.
When the armature loop cuts directly across the magnetic field, it is generating its maximum output. When the armature has rotated another one-quarter turn, it will be moving parallel with the magnetic field, and no output will be obtained from the generator. During the time that the armature rotates 3600 (one revolution), it generates a maximum and a minimum twice – maximum when passing across the S pole, and again when crossing the N pole.
Voltage across the loop is represented as alternating current (AC) while voltage across the brushes is shown as pulsating DC. The commutator acts as a reversing switch as the armature rotates in different fields. As a result of the switching action, the current output is the series of maximum and minimum with the current flowing in only one direction.
Types of DC generators.
Generators are devices that generate or produce electricity. A generator may be used to produce either AC or DC. The AC generator is usually called an alternator.
DC generators are classified according to the method used to supply the exciting current to the field winding. When the field current is obtained from a separate source, the generator is said to be separately excited. The self-excited generator has the excitation current supplied by the generator itself. The self-excited generator is further divided into at least three groups: shunt, series and compound.
Shunt generator. The poles of a generator retain some magnetism, known as residual magnetism, when not in operation.
Because residual magnetism produces a weak magnetic field, when the generator is started a small voltage is induced in the armature and appears at the output terminals. Then, because the armature output voltage is connected across the field windings, a small current flows in the windings. This field current, in turn, strengthens the magnetic field, the output voltage increases accordingly, and a larger current flows in the windings. This action is cumulative, and the output voltage continues to rise to a point (called field saturation) where no further increase in output voltage occurs.
If the initial direction of the armature rotation is wrong, the cumulative action does not occur, since the small induced voltage opposes the residual field and there is no build-up of output voltage.
When the field winding of a shunt generator is excited, the machine is said to be separately excited. The terminal voltage of a shunt generator can be controlled by means of a rheostat inserted in series with the field windings. As resistance is increase, the field current is reduced, and consequently the generated voltage is reduced as well. For a given setting of the field rheostat, either for a separately excited or self-excited machine, the terminal voltage at the armature brushes will be approximately equal to the generated voltage minus the voltage drop produced by the load current in the armature. Consequently, the voltage available at the terminals of the generator will drop as the load is applied.
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This voltage drop is greater in a self-excited generator than in a separately excited generator. Certain voltage sensitive devices are available which automatically adjust the field rheostat to compensate for the variation in the load. When these devices are used, the terminal voltage remains essentially constant.
Shunt generators are used primarily in such an application as battery charging, which requires a certain voltage under varying current conditions. Separately excited shunt generators are often used in certain speed-control systems.
Other types of generators
Series generators. The field winding of a series generator is connected in series with the armature output voltage.
The field coils of the series generator are made up of a few turns of heavy wire. All current flowing through the field coil also flows through the armature. Series generators have very poor voltage regulation under changing load conditions. The greater the current through the field coils to the load, the greater will be the induced EMF, and the greater will be the output voltage of the generator. Therefore, when load is increased, voltage will increase. When load is decreased, voltage will also decrease. Because the series generator has such poor regulation, however, only a few are in actual use.
Compound generators. A compound generator has both a series and a shunt field, both windings being on the same pole structure. The series field may be connected to aid or opposes the shunt field.
There are several types of compound generators.
A cumulative compound generator has the series field aiding the shunt field/
1. Flat compounded: voltage remains constant for all loads.
2. Over compounded: voltage rises with increased load.
3. Under compounded: voltage drops with the increased load.
A differentially compounded generator has the series field opposing the shunt field.
Compound generators are usually designed to be over compounded. This feature permits the degree of compounding to be varied by connecting a variable shunt across the series field. Such a shunt is sometimes called a diverter. Compound generators are used where voltage regulation is of prime importance.
Differential generators have somewhat the same characteristics as series generators in that they are essentially constant-current generators. However, they generate rated voltage at no load, with the voltage dropping as the load increases. This constant-current characteristic makes them ideally suited for electric-arc welders, which is the principle application for them.
Three-wire generators
Some DC generators are called three-wire machines. They are designed to deliver 240 volts with a neutral connection that provides 120 volts on either side of the neutral. This is accomplished by connecting a reactance coil to opposite sides of the commutator. The neutral is connected to the midpoint of the reactance coil. Such a reactance coil acts as a low-loss voltage divider. If resistors were used, the I2R loss would be prohibitive unless the two loads were matched perfectly.
The reactance coil may be built into the machine as part of the armature, the midpoint connected to a single slip ring with which the neutral makes contact by means of a brush. Or, the two connections to the commutator may be in turn connected to two slip rings, in which case the reactor is located outside the machine. In either case, the load unbalance on either side of the neutral must be more than 25% of the rated current output of the generator. The three-wire generator permits simultaneous operation of 120-volt lightning circuits and 240-volt motors from the same generator.
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Motors, like generators, are simply a means of transforming energy or power. Motors convert electrical power into mechanical power. The essential feature and parts of a DC generator and DC motor are the same: both have field coils, armature coils, a commutator, and a brush assembly. It is also possible to use generators as motors. Foe example, when a voltage is applied to the terminals of a generator, the currents in the field coils and armature coils, respectively, set up magnetic fields which react to each other and cause the armature to revolve. When this happens, a DC motor is in operation.
Understanding of basic motor action may be obtained by considering the following simple facts concerning magnetic fields. A uniform magnetic field exits between the poles of a magnet when its field coils are connected to a DC source. The lines of flux are directed from the north pole to the south pole.
Briefly, the principle on which motors operate may be stated as: a conductor carrying current in a magnetic field tends to move at right angles to (across) the field.
Transformers (1)
The transformer is a device for changing the electric current from one voltage to another. It is used for increasing or decreasing voltage. A simple transformer is a kind of induction coil. It is well known that in its usual form it has no moving part. It requires very little maintenance if it is not misused and is not damaged by lightning.
We may say that the principle parts of a transformer are two windings, that is coils, and an iron core. They call the coil which is supplied with current the “primary winding”, or just “primary” for short. The winding from which the current is taken is termed the “secondary winding” or “secondary” for short. The former is connected to the source of supply; the latter is connected to the load.
When the number of turns of wire on the secondary is the same as the number of turns on the primary, the secondary voltage is the same, as the primary, and we get what is called a “one-to-one” transformer. In case, however, the number of turns on the secondary winding is greater than those on the primary, the output voltage is larger than the input voltage and the transformer is called a step-up transformer. On the other hand, if the secondary turns are fewer in number than the primary, the transformer is known as a step-down transformer.
The transformer operates equally well to increase the voltage and to reduce it, The above process needs a negligible quantity of power. It is important to point out that the device which is being considered will not work on DC but it rather often employed in direct-current circuits.
Transformers are used in stepping up voltages for distribution or transmission over long distances and then in stepping these voltages down. In some distant locality step-down transformers are used to reduce the value of voltages. A lot of electronic devices including radio sets and television sets cannot be operated without transformers.
a one-to-one transformer – разделительный трансформатор с коэффициентом трансформации, равным единице;
on the other hand – с другой стороны.
Transformers (2)
A transformer is an apparatus for converting electrical power in an AC system at one voltage or current into electrical power at some other voltage or current without the use of rotating parts. A transformer may receive energy at one voltage and deliver it at a higher voltage; in this case it is called a step-up transformer. A transformer may receive energy at one voltage and deliver it at a lower voltage; in this case it is called a step-down transformer. A transformer may receive energy at one voltage and deliver it at the same voltage, then it is called a one-to-one transformer. A transformer has no rotating parts, and, therefore, it requires little attention and its maintenance is low. The transformer windings can be immersed in oil, so it is not difficult to insulate transformers for very high voltages.
Because of these many desirable characteristics, the transformer is a very useful piece of apparatus, and as it can transform from low to high voltage, and from high to low voltage, economically, it is largely responsible for the extensive use of alternation current.
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The transformer is based on the principle that energy may be efficiently transferred by induction from one set of coils to another by means of varying magnetic flux, id both sets of coils are on a common magnetic circuit.
Electromotive forces are induced by a change in flux linkages. In the generator, the flux is substantially constant in magnitude. The amount of flux linking the armature coils is changed by the relative mechanical motion of flux and coils. In the transformer, the coils and magnetic circuit are all stationary with respect to one another. The e.m.fs are induced by the change in the magnitude of the flux with time. The transformer is a modified form of the induction coil.
In practice the transformer is largely used as a “step-down” transformer, that is, the mean potential at which the primary is supplied is many times higher than the mean potential which is induced in the secondary.
a one-to-one transformer – разделительный трансформатор с коэффициентом трансформации, равным единице;
flux-linkages – потокосцепления.
Inductance (1)
When a current flows through a conductor it sets up a magnetic field in theneighbourhood of the conductor. This is negligible in its effects in a number of cases, but there also many cases where this magnetic field exerts a profound effect upon the circuit. The magnetic field created by the current is represented by lines of magnetic flux; these lines consist of closed loops which are interlinked with the electric circuit, which is also necessarily a closed circuit. If the current is steady, the magnetic flux is constant and produces no effect upon the circuit, but if the current changes, then the strength of the magnetic field also changes. If the current increases, the total number of lines of magnetic flux is increased, so that the total number of flux-linkages is also increased. It is, however, a fundamental law that whenever the number of flux-linkages changes, an electromotive force is induced in the circuit linked with the flux. This e.m.f. is proportional to the rate of change of linkages, and one volt is induced when the linkages change at the rate of 108 linkages per second.
The unit of inductance is the Henry, and the circuit possesses an inductance of one Henry if one volt is induced when the current changes at the rate of one ampere per second. The symbol for inductance is L, so that a circuit possesses an inductance of L Henries if L volts are induced due to a rate of change of current of 1 ampere per second.
The induced e.m.f. always acts in such a direction as to oppose the change of current in the circuit, and also the magnetic flux linked with it. Thus, if the current is rising, inductance tends to oppose its growth, and, if the current is falling, inductance tends to oppose its decay. Examples of this effect are found in the field circuit of an ordinary generator or motor and the field circuit is highly inductive. Inductance is a property of a circuit and it is possessed by DC as well as Ac circuits.
flux-linkages – потокосцепления;
a field circuit – схема возбуждения, цепь возбуждения;
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