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Tuesday, February 2, 2010

What about electrical generator?



In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. The reverse conversion of electrical energy into mechanical energy is done by a motor; motors and generators have many similarities. A generator forces electric charges to move through an external electrical circuit, but it does not create electricity or charge, which is already present in the wire of its windings. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air or any other source of mechanical energy.

Electrical developments history:



Before the connection between magnetism and electricity was discovered, electrostatic generators were invented that used electrostatic principles. These generated very high voltages and low currents. They operated by using moving electrically charged belts, plates and disks to carry charge to a high potential electrode. The charge was generated using either of two mechanisms:
* Electrostatic induction
* The triboelectric effect, where the contact between two insulators leaves them charged.
Because of their inefficiency and the difficulty of insulating machines producing very high voltages, electrostatic generators had low power ratings and were never used for generation of commercially-significant quantities of electric power. The Wimshurst machine and Van de Graaff generator are examples of these machines that have survived.
Faraday's disk




In the years of 1831-1832 Michael Faraday discovered the operating principle of electromagnetic generators. The principle, later called Faraday's law, is that a potential difference is generated between the ends of an electrical conductor that moves perpendicular to a magnetic field. He also built the first electromagnetic generator, called the 'Faraday disk', a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage.
This design was inefficient due to self-cancelling counterflows of current in regions not under the influence of the magnetic field. While current flow was induced directly underneath the magnet, the current would circulate backwards in regions outside the influence of the magnetic field. This counterflow limits the power output to the pickup wires, and induces waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.

Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Experimenters found that using multiple turns of wire in a coil could produce higher more useful voltages. Since the output voltage is proportional to the number of turns, generators could be easily designed to produce any desired voltage by varying the number of turns. Wire windings became a basic feature of all subsequent generator designs.
However, recent advances (rare earth magnets) have made possible homo-polar motors with the magnets on the rotor, which should offer many advantages to older designs.

Dynamo





The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into a pulsing direct electric current through the use of a commutator. The first dynamo was built by Hippolyte Pixii in 1832.

Through a series of accidental discoveries, the dynamo became the source of many later inventions, including the DC electric motor, the AC alternator, the AC synchronous motor, and the rotary converter.

A dynamo machine consists of a stationary structure, which provides a constant magnetic field, and a set of rotating windings which turn within that field. On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils.

Large power generation dynamos are now rarely seen due to the now nearly universal use of alternating current for power distribution and solid state electronic AC to DC power conversion. But before the principles of AC were discovered, very large direct-current dynamos were the only means of power generation and distribution. Now power generation dynamos are mostly a curiosity.
Rotating electromagnetic generators



without a commutator, the dynamo is an example of an alternator, which is a synchronous singly-fed generator. With an electromechanical commutator, the dynamo is a classical direct current (DC) generator. The alternator must always operate at a constant speed that is precisely synchronized to the electrical frequency of the power grid for non-destructive operation. The DC generator can operate at any speed within mechanical limits but always outputs a direct current waveform.
Other types of generators, such as the asynchronous or induction singly-fed generator, the doubly-fed generator, or the brushless wound-rotor doubly-fed generator, do not incorporate permanent magnets or field windings (i.e, electromagnets) that establish a constant magnetic field, and as a result, are seeing success in variable speed constant frequency applications, such as wind turbines or other renewable energy technologies.

The full output performance of any generator can be optimized with electronic control but only the doubly-fed generators or the brushless wound-rotor doubly-fed generator incorporate electronic control with power ratings that are substantially less than the power output of the generator under control, which by itself offer cost, reliability and efficiency benefits.
Generator Terminology


The two main parts of a generator or motor can be described in either mechanical or electrical terms:
Mechanical:
* Rotor: The rotating part of an electrical machine
* Stator: The stationary part of an electrical machine
Electrical:
* Armature: The power-producing component of an electrical machine. In a generator, alternator, or dynamo the armature windings generate the electrical current. The armature can be on either the rotor or the stator.
* Field: The magnetic field component of an electrical machine. The magnetic field of the dynamo or alternator can be provided by either electromagnets or permanent magnets mounted on either the rotor or the stator.
Because power transferred into the field circuit is much less than in the armature circuit, AC generators nearly always have the field winding on the rotor and the stator as the armature winding. Only a small amount of field current must be transferred to the moving rotor, using slip rings. Direct current machines necessarily have the commutator on the rotating shaft, so the armature winding is on the rotor of the machine.

Excitation
An electric generator or electric motor that uses field coils rather than permanent magnets will require a current flow to be present in the field coils for the device to be able to work. If the field coils are not powered, the rotor in a generator can spin without producing any usable electrical energy, while the rotor of a motor may not spin at all. Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger.

In the event of a severe widespread power outage where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore customer power service.
Equivalent circuit

The equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generator's VG and RG parameters, follow this procedure: -
* Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance RGDC.
* Start the generator. Before connecting the load RL, measure the voltage across the generator's terminals. This is the open-circuit voltage VG.
* Connect the load as shown in the diagram, and measure the voltage across it with the generator running. This is the on-load voltage VL.
* Measure the load resistance RL, if you don't already know it.
* Calculate the generator's AC internal resistance RGAC from the following formula:
R_{GAC} = {R_L} \left( {{{V_G}\over{V_L}}-1} \right)
Note 1: The AC internal resistance of the generator when running is generally slightly higher than its DC resistance when idle. The above procedure allows you to measure both values. For rough calculations, you can omit the measurement of RGAC and assume that RGAC and RGDC are equal.
Note 2: If the generator is an AC type, use an AC voltmeter for the voltage measurements.
The maximum power theorem states that the maximum power can be obtained from the generator by making the resistance of the load equal to that of the generator. This is inefficient since half the power is wasted in the generator's internal resistance; practical electric power generators operate with load resistance much higher than internal resistance, so the efficiency is greater.
Engine-generator
An engine-generator is the combination of an electrical generator and an engine mounted together to form a single piece of self-contained equipment. The engines used are usually piston engines, but gas turbines can also be used. Many different versions are available - ranging from very small portable petrol powered sets to large turbine installations.

Human powered electrical generators

A generator can also be driven by human muscle power .
Human powered direct current generators are commercially available, and have been the project of some DIY enthusiasts. Typically operated by means of pedal power, a converted bicycle trainer, or a foot pump, such generators can be practically used to charge batteries, and in some cases are designed with an integral inverter. The average adult could generate about 125-200 watts on a pedal powered generator.

Electrical machine

An Electrical machine is a device that converts mechanical energy to electrical energy or vice versa, and changes AC voltage from one level to another level.
Generator
A generator is the device that converts mechanical energy at its prime mover to produce constant electrical energy at its output. In more technical words it is a dynamic electrical energy machine. Generator is classified into two types: AC generator and DC generator.

The basic requirements for a dynamically induced emf to exist are the following: (1) A steady magnetic field
(2) A conductor capable of carrying current
(3) The conductor to move in the magnetic field
AC Generator
AC generator is the generator that converts mechanical energy at its prime mover into AC electricity.
AC generator is classified into several types:
* Asynchronous AC generator or induction AC generator, an AC generator whose field current is supplied by magnetic induction into the field windings. * Synchronous AC generator, an AC generator whose magnetic field current is provided by a separate DC current source, either external DC source or mounted DC source.
DC Generator
DC generator is the generator that produces DC power i.e.,constant power P=V*Iby taking mechanicalenergy as input. Example of a DC generator is dynamo.
Motor
Motor is the device that converts electrical energy at its input to produce mechanical energy. Motor is classified into two types: AC motor and DC motor.
AC Motor
AC motor is the motor that converts AC electrical energy at its input into mechanical energy.
AC motor is classified into several types:
* Asynchronous motor or induction AC motor
* Synchronous motor
DC Motor
DC motor is the motor that converts DC electricity into mechanical energy. Its main components are stator, rotor, windings and commutator.
DC motor is classified into five types:
* Compounded DC motor
* Permanent magnet DC motor
* Separately excited DC motor, a DC motor whose field circuit receives power from a separate constant voltage supply.
* Series DC motor, a DC motor whose field windings consist of relatively few turns and connected in series with the armature circuit.
* Shunt DC motor, a DC motor whose field circuit receives power directly across the armature terminals.

Losses in DC motor are brush drop losses, core losses, mechanical losses and stray losses.
Transformer
Transformer is the device that converts AC voltage from one level to another level higher or lower, or even to the same level without changing the frequency. It works based on the principle of mutual induction, so its power remains approximately constant, where as frequency also remains the same.

Electronic test equipment

Electronic test equipment is used to create signals and capture responses from electronic Devices Under Test (DUTs). In this way, the proper operation of the DUT can be proven or faults in the device can be traced and repaired. Use of electronic test equipment is essential to any serious work on electronics systems.
Practical electronics engineering and assembly requires the use of many different kinds of electronic test equipment ranging from the very simple and inexpensive to extremely complex and sophisticated such as Automatic Test Equipment.
Generally, more advanced test gear is necessary when developing circuits and systems than is needed when doing production testing or when troubleshooting existing production units in the field.
Types of test equipment
Basic equipment

The following items are used for basic measurement of voltages, currents, and components in the circuit under test.
* Voltmeter (Measures voltage)
* Ohmmeter (Measures resistance)
* Ammeter, e.g. Galvanometer or Milliameter (Measures current)
* Multimeter e.g., VOM (Volt-Ohm-Milliameter) or DMM (Digital Multimeter)

The following are used for stimulus of the circuit under test:
* Power supplies
* Signal generator
* Digital pattern generator
* Pulse generator
The following analyze the response of the circuit under test:
* Oscilloscope (Measures all of the above as they change over time)
* Frequency counter (Measures frequency)
And connecting it all together:
* Test probes


Advanced or less commonly used equipment
Meters
* Solenoid voltmeter (Wiggy)
* Clamp meter (current transducer)
* Wheatstone bridge (Precisely measures resistance)
* Capacitance meter (Measures capacitance)
* LCR meter (Measures inductance, capacitance, resistance and combinations thereof)
* EMF Meter (Measures Electric and Magnetic Fields)
* Electrometer (Measures charge)

Probes
* RF probe
* Signal tracer
Analyzers
* Logic analyzer (Tests digital circuits)
* Spectrum analyzer (SA) (Measures spectral energy of signals)
* Protocol analyzer (Tests functionality, performance and conformance of protocols)
* Vector signal analyzer (VSA) (Like the SA but it can also perform many more useful digital demodulation functions)



* Time-domain reflectometer (Tests integrity of long cables)

Signal-generating devices
* Signal generator
* Frequency synthesiser
* Function generator
* Digital pattern generator
* Pulse generator
* Signal injector
Miscellaneous devices
* Continuity tester
* Cable tester
* Hipot tester
* Network analyzer (used to characterize components or complete computer networks)
* Test light
* Transistor tester
* Tube tester
* The Energy Detective
* Electrical tester pen
* Receptacle tester

Electrical equipment

Electrical equipment includes any machine powered by electricity. They usually consists of an enclosure, a variety of electrical components, and often a power switch. Examples of these include:
Major appliance
Microcontroller
Power tool
Small appliances
More specifically, often electrical equipment refers only to components part of the electrical distribution system such as:
Electric switchboards
Distribution boards
Circuit breakers and disconnects
Electricity meter
Transformers