GENERATORS

Saturday, April 12, 2008

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INTRODUCTION

GENERATORS

An electrical generator is a device that produces electrical energy from a mechanical energy source using electromagnetic induction. The process is known as electricity generation.Generators and alternators are rotating machines driven by some mechanical force. They use the electromagnetic induction principle to convert the mechanical force to electrical energy. There are ac and dc generators. The ac generators usually called an alternator.

Direction of induced current;

Current from spinning generator will always move in a direction determined by what is called the left hand rule for generators. The rule says: If the thumb, forefinger, and middle finger of the left hand are held at right angles to each other, and the thumb is pointing in the direction the wire is moving, while the forefinger is pointing in the direction of the magnetic field (north to south) then the middle finger will point in the direction of the induced electron flow. This statement is also known Fleming’s Rule.

Theory of Operation

A simple AC generator consists of: Figure 3 Simple AC Generator (a) a strong magnetic field, (b) conductors that rotate through that magnetic field, and (c) a means by which a continuous connection is provided to the conductors as they are rotating (Figure 3). The strong magnetic field is produced by a current flow through the field coil of the rotor. The field coil in the rotor
receives excitation through
the use of slip rings and brushes. Two brushes are spring-held in contact with the slip rings to provide the continuous connection between the field coil and the external excitation circuit. The armature is contained within the windings of the stator and is connected to the output. Each time the rotor makes one complete revolution, one complete cycle of AC is developed. A generator has many turns of wire wound into the slots of the rotor. The magnitude of AC voltage generated by an AC generator is dependent on the field strength and speed of the rotor. Most generators are operated at a constant speed; therefore, the generated voltage depends on field excitation, or strength.


Requirements Diagram

This is a drawing of an acceptable connection of a standby generator.


ADVANTAGES AND DISADVANTAGES OF USING DC GENERATOR

Advantages;

  • Simple and pre-assembled, some are good at low rpm.

  • Using permanent magnets are lower building cost and much simpler construction.

  • For independently exited generator, this method is a constant, strong field strength.

  • By using the combination of field windings, the advantages of each can be utilized.

Disadvantages:

§ High maintenance, most are not good at low rpm, large sizes very hard to find, small ones have limited power output.

§ Using the permanent magnets arethe limited amount of output voltage and current that can be produced.

For an independently exited generator is the need for a separate dc voltage source. The energy used to develop magnetic field is not applied to the load

ALTERNATORS

An electrical generator is a machine which converts mechanical energy into electrical energy by electromagnetic induction. A generator which produces alternating current is referred to as an ac generator and, through combination of the words "alternating" and "generator," the word "alternator" has come into widespread use. In some areas, the word "alternator" is applied only to small ac generators. This text treats the two terms synonymously and uses the term "alternator" to distinguish between ac and dc generators.

The major difference between an alternator and a dc generator is the method of connection to the external circuit; that is, the alternator is connected to the external circuit by slip rings, but the dc generator is connected by a commutator.

Types of Alternators

Alternators are classified in several ways in order to distinguish properly the various types. One means of classification is by the type of excitation system used. In alternators used on aircraft, excitation can be affected by one of the following methods:

A direct connected, direct current generator. This system consists of a dc generator fixed on the same shaft with the ac generator. A variation of this system is a type of alternator which uses dc from the battery for excitation, after which the alternator is self excited.

By transformation and rectification from the ac system. This method depends on residual magnetism for initial ac voltage buildup, after which the field is supplied with rectified voltage from the ac generator.

Integrated brushless type. This arrangement has a direct current generator on the same shaft with an alternating current generator. The excitation circuit is completed through silicon rectifiers rather than a commutator and brushes. The rectifiers are mounted on the generator
shaft and their output is fed directly to the alternating current generator's main rotating field.

Another method of classification is by the number of phases of output voltage. Alternating current generators may be single phase, two phase, three phase, or even six phase and more. In the electrical systems of aircraft, the three phase alternator is by far the most common.

Still another means of classification is by the type of stator and rotor used. From this standpoint, there are two types of alternators: the revolving armature type and the revolving field type. The revolving armature alternator is similar in construction to the dc generator, in that the armature rotates through a stationary magnetic field. The revolving armature alternator is found only in alternators of low power rating and generally is not used. In the dc generator, the e.m.f. generated in the armature windings is converted into a unidirectional voltage (dc) by means of the commutator. In the revolving armature type of alternator, the generated ac voltage is applied unchanged to the load by means of slip rings and brushes.

The revolving field type of alternator (figure 9-34) has a stationary armature winding (stator) and a rotating field winding (rotor). The advantage of having a stationary armature winding is that the armature can be connected directly to the load without having sliding contacts in the load circuit. A rotating armature would require slip rings and brushes to conduct the load current from the armature to the external circuit. Slip rings have a relatively short service life and arc over is a continual hazard; therefore, high voltage alternators are usually of the stationary armature, rotating field type. The voltage and current supplied to the rotating field are relatively small, and slip rings and brushes for this circuit are adequate. The direct connection to the armature circuit makes possible the use of large cross-section conductors, adequately insulated for high voltage.

Since the rotating field alternator is used almost universally in aircraft systems, this type will be explained in detail, as a single phase, two phase, and three phase alternator.


Single Phase Alternator

Since the e.m.f. induced in the armature of a generator is alternating, the same sort of winding can be used on an alternator as on a dc generator. This type of alternator is known as a single phase alternator, but since the power delivered by a single phase circuit is pulsating, this type of circuit is objectionable in many applications.

Figure 9-35 single phase alternator

A single phase alternator has a stator made up of a number of windings in series, forming a single circuit in which an output voltage is generated. Figure 9-35 illustrates a schematic diagram of a single phase alternator having four poles. The stator has four polar groups evenly spaced around the stator frame. The rotor has four poles, with adjacent poles of opposite polarity. As the rotor revolves, ac voltages are induced in the stator windings. Since one rotor pole is in the same position relative to a stator winding as any other rotor pole, all stator polar groups are cut by equal numbers of magnetic lines of force at any time.

As a result, the voltages induced in all the windings have the same amplitude, or value, at any given instant. The four stator windings are connected to each other so that the ac voltages are in phase, or "series adding." Assume that rotor pole 1, a south pole, induces a voltage in the direction indicated by the arrow in stator winding 1. Since rotor pole 2 is a north pole, it will induce a voltage in the opposite direction in stator coil 2 with respect to that in coil 1.

For the two induced voltages to be in series addition, the two coils are connected as shown in the diagram. Applying the same reasoning, the voltage induced in stator coil 3 (clockwise rotation of the field) is the same direction (counterclockwise) as the voltage induced in coil 1. Similarly, the direction of the voltage induced in winding 4 is opposite to the direction of the voltage induced in coil 1. All four stator coil groups are connected in series so that the voltages induced in each winding add to give a total voltage that is four times the voltage in any one winding.

Three Phase Alternator

A three phase, or polyphase circuit, is used in most aircraft alternators, instead of a single or two phase alternator. The three phase alternator has three single phase windings spaced so that the voltage induced in each winding is 120° out of phase with the voltages in the other two windings.

A schematic diagram of a three phase stator showing all the coils becomes complex and difficult to see what is actually happening.

A simplified schematic diagram, showing each of three phases, is illustrated in figure 9-36. The rotor is omitted for simplicity. The waveforms of voltage are shown to the right of the schematic. The three voltages are 120° apart and are similar to the voltages which would be generated by three single phase alternators whose voltages are out of phase by angles of 120°. The three phases are independent of each other.

figure 9-36

Rather than have six leads from the three phase alternator, one of the leads from each phase may be connected to form a common junction. The stator is then called wye or star connected. The common lead may or may not be brought out of the alternator. If it is brought out, it is called the neutral lead. The simplified schematic (A of figure 9-37) shows a wye connected stator with the common lead not brought out. Each load is connected across two phases in series. Thus, RAB is connected across phases A and B in series; RAC is connected across phases A and C in series; and RBC is connected across phases B and C in series. Therefore, the voltage across each load is larger than the voltage across a single phase. The total voltage, or line voltage, across any two phases is the vector sum of the individual phase voltages. For balanced conditions, the line voltage is 1.73 times the phase voltage. Since there is only one path for current in a line wire and the phase to which it is connected, the line current is equal to the phase current.
A three phase stator can also be connected so that the phases are connected end to end as shown in B of
figure 9-37. This arrangement is called a delta connection. In a delta connection, the voltages are equal to the phase voltages; the line currents are equal to the vector sum of the phase currents; and the line current is equal to 1.73 times the phase current, when the loads are balanced.

For equal loads (equal kw. output), the delta connection supplies increased line current at a value of line voltage equal to phase voltage, and the wye connection supplies increased line voltage at a value of line current equal to phase current.

figure 9-37



Dynamo Current (DC) Dynamo Construction

The rotor of the dynamo consists of:

  • The armature shaft, which imparts rotation to the armature core, winding and commutator.
  • The armature core, constructed of laminated layers of dynamo steel, providing a low reluctance magnetic path between the poles. The laminations serve to reduce eddy currents in the core, and the dynamo steel used is of such a grade as to produce a low hysteresis loss.
  • The armature winding, consisting of insulated coils, insulated from one another and from the armature core, and embedded in the slots.
  • The commutator, which, by virtue of the shaft rotation, provides the necessary switching for the commutation process. The commutator consists of copper segments, individually insulated from one another and from the shaft, electrically connected to the armature winding coils.

The rotor armature of the dc dynamo performs four major functions:

· Permits rotation for mechanical generator action or motor action.

· By virtue of rotation, produces the switching action necessary for commutation.

· Contains the conductors that induce a voltage or provide an electromagnetic torque.

· Provides a low-reluctance magnetic flux path.

DIFFERENCES BETWEEN GENERATORS AND MOTORS


GENERATORS

MOTORS

produce

Use mechanical energy to produce electricity

Use electrical energy to produce mechanical force

supply

Rotary motion is supplied to by a prime mover to produce relative motion between the conductors of the armature and the magnetic field of the dynamo in order to generate electrical energy

Electrical energy is supplied to the conductors and the magnetic field winding of the dynamo as well in order to produce an electromagnetic force between them and thus produce mechanical energy

AC

AC generator uses slip rings and brushes

AC motors fall into two general categories, induction and synchronous

DC

Operates in similar manner to the AC generator

Does not necessarily mean that the voltage to it

action

The action of the basic generator can be represented by a coil of wire rotating in a magnetic field

Motor action is the attraction and repulsion of like and unlike magnetic field

RESULTS AND CONCLUSION

The prototype demonstrated waveform generation of the switching action of the inverter to run an ac induction motor. The converter rectified the ac signal into a dc signal to power the high-voltage inverter. The small-signal waveforms from the PWM shaped the high voltage sinusoidal output waveform that ran the motor. The PWM generated a sine wave that matched the frequency of the output waveform from the inverter. The PWM generated a triangle wave that matched the switching frequency by the power transistors (IGBTs) of the inverter. The output waveform varied from 45 Hz to 75 Hz. The speed of the ac induction motor varied in a direct relationship to the frequency.

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