Comprehensive Technical Training For Sugar Mills Staff on Operation & Maintenance of Baggase Based HP Cogeneration System Schedule: 10 th July to 13 th July, 2017
A.C. GENERATOR
Topics Covered. Introduction. Working Principle. Construction. Rotor. Stator. Exciter. Permanent Magnet Generator.(PMG). Rotating Diodes. Automatic voltage. Regulator(AVR). Cooling. Losses. Instrumentation on A.C. Generator.
INTRODUCTION Alternators are synchronous machines used to convert mechanical energy to ac electric power. Alternators generate electricity using the principle, when the magnetic field around a conductor changes, a current is induced in the conductor. Typically, a rotating magnet, called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across the conductors, generating an induced EMF (electromotive force). It always runs at constant speed and has no slip.
WORKING PRINCIPLE It works on the principle of electromagnetic induction. This process can be described in terms of Faraday's law. When you see that the rotation of the coil continually changes the magnetic flux through the coil and therefore generates a voltage. The rotating field cuts the stator conductors thereby inducing voltages in the stator circuit. The induced (Faraday) voltages cause currents to flow in the stator.
CONSTRUCTION An alternator consist of two main parts. ROTOR STATOR
ROTOR Rotor is the rotating part of the machine, It works as a field of synchronous machine. Field system is rotating in the synchronous machine. Field winding is housed in it. Field winding is excited by DC supply through rotating diodes from the exciter.
STATOR The stator is the stationary part of the alternator. It works as a armature in synchronous machine. The armature winding is housed in it. The stator core is made up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses.
EXCITER Generators require direct current to energize its magnetic field. The DC field current is obtained from a separate source called an exciter. There are two types of rotating exciters: Brush and brushless.
PERMANENT MAGNET GENERATOR(PMG) A small PMG often included in system to have the excitation of generator independent of any external power sources. A PMG is a small ac generator with permanent magnets mounted on rotor shaft & a 3 phase winding on stator. It produces power for field circuit of exciter, which in turn controls the field circuit of main machine. With PMG on shaft of generator, no external electric power is required to run generator.
ROTATING DIODES Rotating Diodes are diodes fitted in the rotor of the synchronous machine between the exciter and the main field winding. The function of the Rotating diode is to rectify the AC output of the exciter into DC which can be used to magnetize the main field windings and poles. These diodes are connected in the shape of a normal three phase rectifier with six diodes. The DC output from the rectifier assembly is usually collected from two rings in the assembly.
AUTOMATIC VOLTAGE REGULATOR(AVR) Automatic voltage regulator (AVR) maintains the generator terminal voltage at a given Value automatically by changing the excitation current to the Generator field. The AVR supplies the required D.C. to the Generator field depending on the load, power factor etc. to maintain a constant terminal voltage. The real need of the AVR is to deal with the voltage dip following the sudden connection of extra load and to restore the rated Voltage without undue overshooting and regeneration(voltage hunting).
EXCITATION CIRCUIT
COOLING SYSTEM
INSTRUMENTATION ON A.C. GENERATOR. Stator winding RTDs. Stator core RTDs. Cold air RTDs. Hot air RTDs. Driving end bearing metal RTDs. Non driving end bearing metal RTDs. Water leakage detectors. Moisture detectors.
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