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• Carbon brushes(碳刷 ) rest on these rings as they rotate and collect the current for use in an external circuit.
• Current collected in this way will be alternating, that is, changing in direction and rising and falling in value.
• The magnetic field is provided by electromagnets(电磁铁 ) so arranged that adjacent(相邻的 ) poles have opposite polarity(极性 ).
• These ‘field coils(场线圈、励磁线圈 )’, as they are called, are connected in series to an external source or the machine output.
• Three outputs are usually arranged with a phase separation of 120°, to produce a three phase supply.
• The three phase system is more efficient in that for the same mechanical power a greater total electrical output is obtained.
• Each of the three outputs may be used in single phase supplies or in conjunction for a three phase supply.
• So far, alternator construction has considered the armature(电枢 ) rotating and the field coils stationary,
• the same electricity generating effect is produced if the reverse occurs, that is, the field coils rotate and the armature is stationary.
• The field current supply in older machines comes from a low voltage direct current generator or exciter(励磁机 ) on the same shaft as the alternator.
• Modern machines however are either statically excited(静态励磁 ) or of the high speed brushless(无电刷型 ) type.
• The exciter is required to operate to counter(抵消 ) the effects of power factor(功率因素 ) for a given load.
• The power factor is a measure of the phase difference between voltage and current and is expressed as the cosine(余弦 ) of the phase angle.
• With a purely resistance load(电阻性负载 ) the voltage are in phase(同相位 ), giving a power factor of one.
• Therefore the exciter in maintaining the alternator voltage, must vary with load current and also the power factor.
• Hand control of excitation(励磁 ) is difficult so use is made of an automatic voltage regulator (AVR).
• The AVR consists basically of a circuit fed from the alternator output voltage which detects small changes in voltage and
• Stabilizing features are also incorporated in the circuits to avoid ‘hunting(振荡 )’ (constant voltage fluctuations (波动 )) or overcorrecting(过调 ).
• This type of alternator will more accept the sudden loading by direct on-line(轴带的 , 联机的 ) starting of large squirrel cage motors(鼠笼式电机 ).
• The static excitation system uses transformers(变压器 ) and rectifiers(整流器 ) to provide series(串激 ) and shunt(并激 ) components for the alternator field,
• Brushes and sliprings are used to transfer the current to the field coils which are mounted on the rotor.
• and the load current provides the extra excitation to give a steady voltage under any load condition.
• The compensation necessary for speed variation requires that a voltage regulator is also built into the system.
• The brushless high speed alternator was also developed to eliminate d.c. exciters with their associated commutators(整流子 ) and brushgear(电刷机构 ).
• The exciter output is fed to the rectifiers and then through conductors in the hollow shaft to the alternator field coils.
• Three phases alternators arranged for parallel operation require a considerable amount of instrumentations.
• This will include ammeters, wattmeter, voltmeter, frequency meter and a synchronizing device(同步装置 ).
• Reverse power protection(逆功率保护 ) is provided to alternators since current protection cannot be used.
• Alternatively various trips(跳闸机构 ) may be provided in the event of prime mover failure to ensure that the alternator does not act as a motor.
• so for two machines to operate together their voltages must be changing at the same rate or frequency must be reaching their maximum (or any other value) together.
• The synchroscope has two windings(绕组 ) which are connected one to each side of the paralleling switch.
• and the dial is marked for clockwise rotation FAST and anti-clockwise rotation SLOW, the reference being to the incoming machine frequency.
• To parallel an incoming machine to a running machine therefore it is necessary to ensure firstly that both voltages are equal.
• In practice the synchroscope usually moves slowly in the FAST direction and paralleling switch is closed as the pointer reaches the 11 o’clock position.
• The sequence method of lamp connection has a key lamp connected across one phase with the two other lamps cross connected over the other two phases.
• If the frequencies of the machines are different the lamps will brighten and darken in rotation, depending upon the incoming frequency being FAST or SLOW.
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