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.