Electric Generator (AC)

The electric generator converts mechanical energy into electrical energy.
The two types of generators are DC and AC generators:

• DC Generators - A cycle dynamo and a car dynamo are examples of DC generators. They produce DC
  
• AC Generators - AC Generators or alternators are used in power stations and industries to produce AC

Principle

When a straight conductor is moved rapidly in a magnetic field, then a current is induced in the conductor. It is based on the phenomenon of electromagnetic induction

Construction


Main Parts of the AC Generator

An AC generator consists of a magnet with concave cylindrical poles, an armature, and a current collecting arrangement.The current collecting arrangement consists of slip rings and brushes.





Armature is a soft iron core on which a coil having a large number of turns of insulated copper wire is wound. Magnetic poles are concave and cylindrical. The concave poles produce a radial magnetic field.
The ends of the armature are connected to two slip rings. They rotate along with the coil. The slip rings are made of metal and are insulated from each other.There are two brushes B1 and B2 made of carbon. One end of each brush is in contact with the rotating slip rings and the other end is connected to an external circuit. Here the brushes are connected to a galvanometer and brushes do not rotate with the coil.
The axle is rotated mechanically from outside by a diesel engine, flowing water, steam or high-speed wind.

Working

• As the armature rotates about an axis perpendicular to the magnetic field, it keeps on changing its relative orientation with respect to the field


• Thus the flux keeps on changing continuously with time


• This change in magnetic flux induces an emf


• If the outer terminals of the armature are connected to an external circuit, an electric current flows through it


• The deflection of the galvanometer needle indicates that an emf is induced


• The direction of the induced emf is reversed after every half rotation of the coil


• Thus in one rotation of the coil, the current changes its direction twice

Such a current which changes its direction after equal intervals of time is called alternating current (AC).To get a direct current (DC) generator a split-ring type commutator must be used. In this arrangement, one brush is at all times in contact with the arm moving up in the field while the other is in contact with the arm moving down. Thus a unidirectional current is produced in such a generator.
The AC current produced in India has a frequency of 50 hertz (Hz). The coil is rotated at the rate of 50 revolutions in 1 second. So in 50 revolutions the current changes its direction 100 times in one second.

Interactive Electric Generator

The animation below shows a simple electric generator. In the animation, the mechanical energy needed to turn the generator comes from the brown hand crank at the front of the generator. In a hydroelectric power plant, the mechanical energy to turn the generator comes from the water turbine, which is turned by the force of falling water. The hand crank in the animation causes the red wire to spin inside a magnetic field (the blue lines). As Faraday learned, moving the wire through the magnetic field causes electric current to flow in the wire. The turning red wire is connected to a volt meter, which shows the amount of electric current that is produced. In a hydroelectric plant, the generator is connected to transmission lines that deliver the electricity to your home or business.
The controls on the animation allow you to control the speed and direction of the generator and turn portions of the animation on and off for greater clarity. You can also use the radio buttons to show a direct current, or DC generator (with commutator) or an alternating current, or AC generator (without commutator).

Here are two pictures of the actual generators in hydroelectric power plants.

Ultra-simple Electric Generator

PARTS:
4 - 1x2x5cm ceramic magnet: Edu. Innv M-700 or Radio Shk #64-1877, or CMS
1 - #30 Magnet wi re 200ft, Rad. Shack 278-1345 $6.59
1 - Miniature Lamp, 1.5V 25mA Rad. Sh. #272-1139 $1.29, or All. LP-3 or #48 lamp
1 - Cardboard strip, 8cm x 30.4cm
1 - Large nail, 8cm long or more
Misc. - Knife or sandpaper to strip the wires
Misc. - tape to hold wire down
Optional: hand drill or electric drill to spin it (hand drill is best) Cheaper: 600ft wire from a $1.50 Electr. Goldmine solenoid (need vise-grips) Also: other sources of wire
This is an AC electric generator which lights up a tiny incandescent light bulb. The generator is made from a hollow-ended cardboard box with a nail through the center. The box has many turns of fine copper wire wound around it, with four large magnets clamped around the nail. When the nail and magnets are spun fast by hand, the little light bulb lights up dimly.
I wrote this article because I found lots of projects for making a simple electric motor, but nobody gave the secret for making a generator. Well, here it is: use strong magnets, lots of fine wire, and a special light bulb which only needs 1/2 volt. Also, don't bother making a "commutator," just hook the wires directly to the bulb. It's much simpler that way, but the generator will produce AC (alternating current).
Before you start, here are some notes: you must use a special light bulb. Normal flashlight bulbs will not work. Also, you must use the large, strong magnets shown in the parts list. Smaller magnets won't work. The wire must be #30 gauge or smaller. Also, you can improve the generator if you buy lots of extra kits of wire and wind it on the cardboard, since the bulb will light up even when the generator spins slowly. Three kits of Radio Shack wire is expensive, it's cheaper to order just one $3 solenoid, but you'll, need vise-grips pliers to pry apart the steel frame and remove the spool of wire.

Human powered electrical generators

A generator can also be driven by human muscle power (for instance, in field radio station equipment).
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. Portable radio receivers with a crank are made to reduce battery purchase requirements, see clockwork radio.

Engine-generator

An engine-generator is the combination of an electrical generator and an engine (prime mover) 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.

Vehicle-mounted generators

Early motor vehicles until about the 1960s tended to use DC generators with electromechanical regulators. These have now been replaced by alternators with built-in rectifier circuits, which are less costly and lighter for equivalent output. Automotive alternators power the electrical systems on the vehicle and recharge the battery after starting. Rated output will typically be in the range 50-100 A at 12 V, depending on the designed electrical load within the vehicle. Some cars now have electrically-powered steering assistance and air conditioning, which places a high load on the electrical system. Large commercial vehicles are more likely to use 24 V to give sufficient power at the starter motor to turn over a large diesel engine. Vehicle alternators do not use permanent magnets and are typically only 50-60% efficient over a wide speed range.[2] Motorcycle alternators often use permanent magnet stators made with rare earth magnets, since they can be made smaller and lighter than other types. See also hybrid vehicle.
Some of the smallest generators commonly found power bicycle lights. These tend to be 0.5 ampere, permanent-magnet alternators supplying 3-6 W at 6 V or 12 V. Being powered by the rider, efficiency is at a premium, so these may incorporate rare-earth magnets and are designed and manufactured with great precision. Nevertheless, the maximum efficiency is only around 80% for the best of these generators - 60% is more typical - due in part to the rolling friction at the tire-generator interface from poor alignment, the small size of the generator, bearing losses and cheap design.
Sailing yachts may use a water or wind powered generator to trickle-charge the batteries. A small propeller, wind turbine or impeller is connected to a low-power alternator and rectifier to supply currents of up to 12 A at typical cruising speeds.

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:
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.

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.

Terminology

The two main parts of a generator or motor can be described in either mechanical or electrical terms[citation needed]:
Mechanical:
Rotor: The rotating part of an alternator, generator, dynamo or motor.
Stator: The stationary part of an alternator, generator, dynamo or motor.
Electrical:
Armature: The power-producing component of an alternator, generator, dynamo or motor. 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 alternator, generator, dynamo or motor. 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. (For a more technical discussion, refer to the Field coil article.)
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.

MHD generator

A magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a steam power plant. The first practical design was the AVCO Mk. 25, developed in 1965. The U.S. government funded substantial development, culminating in a 25MW demonstration plant in 1987. In the Soviet Union from 1972 until the late 1980s, the MHD plant U 25 was in regular commercial operation on the Moscow power system with a rating of 25 MW, the largest MHD plant rating in the world at that time. [1] MHD generators operated as a topping cycle are currently (2007) less efficient than combined-cycle gas turbines.

Other 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.

Historic developments

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.

Generators Types and Features

We recommend diesel due to their longevity and lower operating costs;1. 1800 rpm water cooled diesel operate on average 20-30,000 hours before major engine maintenance is required.2. 1800 rpm water cooled gas normally operate 6-10,000 hours because they are built on a lighter duty gasoline engine block.3. 3600 rpm air-cooled gas engines are normally replaced – not overhauled at 500 to 1500 hours. Because gas engines burn hotter (higher btu of the fuel), you will see significantly shorter lives. Our gensets are from the world’s top manufacturers. We offer products for high-end residential customers with larger homes to industrial and military customers seeking prime power or critical emergency backup generators.Electric equipment is designed to use power with a fixed frequency: 60 Hertz (Hz) in the United States and Canada, 50 Hertz in Europe and Australia. The frequency output depends upon a fixed engine speed. To produce 60 Hz electricity, most diesel engines operate at 1800 or 3600 RPM. Each speed has its advantages and drawbacks. At 1800 RPM, four pole electric power sets are the most common and least expensive. They offer the best balance of noise, efficiency, cost, and engine life. At 3600 RPM, two pole generators are smaller and lightweight; they are best suited for portable, light-duty applications. In simple terms it’s like operating your car at 90 mph, versus 45mph — at 45mph your car will last longer; it is quieter, requires less maintenance, and has a longer life. Most 3600 rpm gensets are twin cylinder air-cooled lawn mower engines, while the water cooled 1800 rpm units are comparable to those found in diesel forklift and diesel tractor engines. The 1800 rpm water cooled engine will last longe r, offerless maintenance problems, and be more fuel efficient. We recommend diesel! Gensetcentral Systems can provide for all your backup power needs from 2 Mega Watt generator for industrial, residential and military applications. We can provide you with high quality gensets. We offer only top quality products that are priced at or below our competition who offer lesser brands Our diesel products are powered by world class diesel engines - we feature SDMO, Lister Petter, Katolight, Baldor, Kubota, John Deere and Cummins power plants

Human powered electrical generators

Main article: Self-powered equipment
A generator can also be driven by human muscle power (for instance, in field radio station equipment).
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. Portable radio receivers with a crank are made to reduce battery purchase requirements, see clockwork radio.

How Generators Work