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Generator principle
An electrical generator is a machine which converts mechanical energy (or power) into electrical energy (or power). Induced e.m.f is produced in it according to Faraday's law of electromagnetic induction. This e.m.f cause a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are:
a) Magnetic field.
b) Conductor or conductors which can move as to cut the flux.
1.2 Simple loop generator
In fig.(1.1) is shown a single turn rectangular copper coil ( AA′BB′ ) rotating about its own axis in a magnetic field provided by either permanent magnets or electromagnets. The two end of the coil are joined to two slip-rings which are insulated from each other and from the central shaft. Two collecting brushes (carbon or copper) press against
the slip-rings. The rotating coil may be called (armature) and the magnets as (field magnets).One way to generate an AC voltage is to rotate a coil of wire at constant angular velocity in a fixed magnetic field, fig. (1.1). (slip rings and brushes connect the coil to the load). The magnitude of the resulting voltage is proportional to the rate at which flux lines are cut (faraday's law), and its polarity is dependent on the direction the coil sides move through the field. The direction of an induced e.m.f can be predetermined by usingFlemings URUight-hand rule (often called the geneURUator rule) fig.(1.2).
UFUirst finger- UFUield
ThuUMUb – UMUotion
sUEUcond finger – UEU.m.f
Since the rate of cutting flux varies with time, the resulting voltage will also vary with time. For example in (a), since the coil sides are moving parallel to the field, no flux lines are being cut and the induced voltage at this instant (and hence the current) is zero. (this is defined as the 0P°P position of the coil). As the coil rotates from the 0PP position, coil sides AAP⁄P and BBP ⁄P cut across flux lines, hence, voltagebuilds, reaching a peak when flux is cut at the maximum rate in the 90P °Pposition as in (b). Note the polarity of the voltage and the direction of current. As the coil rotates further, voltage decrease, reaching zero at the 180P°Pposition when the coil sides again move parallel to the field as in ©.At this point, the coil has gone through a half-revolution.During the second half-revolution, coil sides cut flux in directions opposite to that which they did in the first half revolution, hence, the polarity of the induced voltage reverses. As indicated in (d), voltage reaches a peak at the 270P °P point, and, since the polarity of the voltage has changed, so has the direction of current. When the coil reaches the 380P °P position, voltage is again zero and the cycle starts over. Fig. (1.1) shows one cycle of the resulting waveform. Since the coil rotates continuously, the voltage produced will be a repetitive, periodic waveform as you saw in fig. (1.1).E.m.f. generated in one side of loop= Blv ⋅ sinφ , and total e.m.f. generated in loop=2 × Blv ⋅ sinφ (volts), where(B): flux density in (teslas), (l ): length in (meters), ( v ): the conductor
velocity, is measured in meters per second.
1.3 Construction of DC Generators
The parts of a simple DC generator are shown in fig.(1.3). The principle of operation of a DC generator is similar to that of the AC generator, which was discussed previously. A rotating armature coil passes through a magnetic field that develops between the north and south polarities of permanent magnets or electromagnets. As the coil rotates, electromagnetic induction causes current to be induced into the coil. The current produced is an alternating current. However, it is possible to convert the alternating current that is induced into the armature into a form of direct current. This conversion of AC into DC is accomplished through the use of a commutator. The conductors of the armature of a DC generator are connected to commutator segments. The commutator shown in fig. (1.3) has two segments, which are insulated from one an other and from the shaft of the machine on which it rotates. An end of each armature conductor is connected to each commutator segment. The purpose of the commutator is to reverse the armature coil connection to the external load circuit at the same time that the current induced in the armature coil reverses. This causes DC at the correct polarity to be applied to the load at all times.
U1.4 Armature Windings
Armature windings can be divided into two groups, depending on how the wires are joined to the commutator. These are called (lap windings) and (wave windings). These windings will be examined individually below, and their advantage and disadvantage will be discussed.
U1.4.1 The Lap Winding;
The simplest type of winding construction used in modern DC machines is the simplex lap winding. A simplex lap winding is a rotor (armature) winding consisting of coils containing one or more turns of wire with the two end of each coil coming out at adjacent commutator segments fig. (1.5). The number of current paths in a machine is :
a = mp lap winging, Where:
a : number of current path in the rotor.
m: plex of the windings (1,2,3,etc….)
p : number of poles on the machines.
Lap wound generators produce high current, low voltage output.
1.4.2 The Wave Winding
The wave winding is an alternative way to connect the rotor(armature) coils to the commutator segments. Fig. (1.6) shows a simple wave winding. In this simplex wave winding, every other rotor coil connects back to a commutator segment adjacent to the beginning of the first coil. Therefore, there are two coils in series between the adjacent
commutator segments. Furthermore, since each pair of coils between adjacent segments has a side under each pole face, all output voltage are the sum of the effects of every pole, and there can be no voltage imbalances. wave windings, generators produce higher-voltage, low current outputs, since the number of coils in series between commutator
segments permits a high voltage to be built up more easy than with
a = 2m multiplex wave
1.5 Electromotive Force (e.m.f) Equation
The induced voltage in any given machine depends on three factors:
1. The flux φ in the machine
2. The speed ω of the machine's rotor.
3. A constant depending on the construction of the machine.
The voltage out of the armature of a real machine is equal to the number of conductors per current path time the voltage on each conductor. The voltage in any single conductor under the pole faces was previously shown to be.e Blv in =Where B , the flux density, is measured in teslas, l , the length of conductor in the magnetic field, is measured in meters, and v , the conductor velocity, is measured in meters per second.The voltage out of the armature of a real machine is thus E ZBlv A =Where ( z ) is the total number of conductors and (a) is the number of current paths. The velocity of each conductor in rotor can be expressed v = rω , where r is the radius of the rotor, ω , angular velocity in radiansper second, soThis voltage can be re-expressed in a more convenient form by noting that the flux of a pole is equal to the flux density under the pole times the pole's area: p φ= BA The rotor of the machine is shaped like a cylinder, so its area is
A = 2πrl
1.6 Types of D.C Generators:
D.C Generators are classified according to the way in which a magnetic field is developed in the stator of the machine. Thus, there are three basic classification DC generators (1) permanent-magnet field (2) separately-excited field and (3) self-excited field.
1) permanent-magnet field
Permanent-magnet DC machines are widely found in a wide variety of low-power applications. The field winding is replaced by a permanent magnet, resulting in simpler construction. Chief among these is that they do not require external excitation and its associated power dissipation to create magnetic fields in the machine the space required for the permanent magnets may be less than that required for the field winding, and thus machine may be smaller, and in some cases cheaper, than their externally excited counter parts. Notice that the rotor of this machines consists of a conventional DC armature with commutator segments and brushes.
3) Self-excited field
Self-excited generators are those whose field magnets are energized by the current produced by the generators themselves. Due to residual magnetism, there is always present some flux in poles. When the armature is rotated, some e.m.f and hence some induced current is produced which is partly or fully passed through the field coils thereby
strengthening the residual pole flux. There are three types of self-excited generators named according to the manner in which their field coils ( or windings) are connected to
armature
) Compound –Wound
The compound-wound D.C generator has two sets of field windings. One set is made of low-resistance windings and is connected in series with the armature circuit. The other set is made of high-resistance wire and is connected in parallel with the armature circuit. A compound wound D.C generator is illustrated in figure (1.11), can be either short-shunt or long-shunt. In a compound generator, the shunt field is stronger than the series field. When series field aids the shunt field, generator is said to be cumulatively-compounded. On the other hand if series field opposes the shunt field, the generator is said to be differentially compounded. Various types of DC generators have been