DC GENERATOR
A DC generator or motor has two main parts which can be described in terms of either mechanical or electrical.
In Mechanical terms:
Rotor: This is the rotating part of the electrical machine
Stator: This is the stationary or non-moving part of the electrical machine
In Electrical terms:
Armature: This is the part or component of the electrical machine that produces the power. It can be on the stator or rotor. In generators, alternators, or dynamos the windings in the armature produces the electric current.
Field: This is where the magnetic field is produced which is given by permanent magnets or electromagnets mounted on the rotor or the stator. The power transmitted into the field circuit is lesser than that in the armature circuit. Due to this, the AC generators have lesser field windings on the stator or rotor as armature windings. Using slip rings, small amount of field current is transmitted to the moving rotor. In direct current (DC) equipments, a commutator is required on the rotating shaft to convert AC to DC from the armature. The field windings is at the rotor part of the dynamo.
Excitation
In an electric generator or electric motor using field coils instead of permanent magnets, electric current is needed in the field coils so that the motor or generator can produce power. The main reason for this is that the rotor may spin but not produce any stable electrical power or the rotor may not spin if the field coils are not electrically powered.
Self-excitation occurs with small generators. This means that the current produced by the generator itself powers the field coils which are connected either in series or parallel to the armature windings. When the generator is started, the magnetism present in small amounts in the iron core gives a magnetic field necessary to start the machine, thereby producing smaller current in the armature which in turn flows to the field coils making larger magnetic fields, and thereby generating larger current in the armature. This process called “bootstrap” carries on until the magnetic field levels off in the core as a result of saturation. Then the generator reaches a steady state, producing steady power.
In cases of large power station generators, smaller generators are used in order to excite the field coils of the larger station generators. In instances where islanding of power stations occur, the stations have to work a black start to excite the fields of the larger ones in order to restore power.
Vehicle-mounted generators
Motor vehicles before the year 1960 used DC generators with electromechanical regulators. Now, modern vehicles are using alternators with built-in rectifier circuits. The new circuit and alternator are lighter in weight and cost less. The alternators provide power to the electrical systems of the vehicle and recharge the battery after starting the engine. The output rating is within 50-100 A at 12V, which is dependent on the electrical load of the vehicle. Modern cars have power-steering and air conditioning unit which is burdened on the high load of the electrical system. Commercial vehicles use 24 V in order to power the starter motor and turn a large diesel engine. Permanent magnets are not typically used by vehicle alternators. The efficiency over a wide range of speed is between 50-60%. In motorcycles, the alternators use permanent magnet as stators. The permanent magnets used are from rare earth magnets which are smaller and lighter. Hybrid vehicles are also using this technology to make cars efficient and lighter.
Bicycle headlights are powered mostly by smallest generators, about one-half ampere, with permanent magnet alternator that provides power of 3 to 6 watts at 6 or 12 volts. Since a bicycle is powered by the rider, the efficiency is premium. With these consideration, the design and make of the bicycle must be optimum and with great precision, and may used rare-earth magnets. The maximum efficiency is about 80% for the best generators, 60% typically for most due to the losses caused by rolling friction at the tires, and some factors may add up like poor wheel alignment, small generator, and losses in the bearing. Hub generators may provide remedy for the drawback of falling efficiency at high speeds.
Engine-generator
An engine-generator is an electrical generator combined with an engine or prime mover. Both are mounted to make a single piece of an equipment that is self-contained. Piston engines are used as engines while gas turbines may also be used. They vary in sizes, from small or portable ones powered by petroleum to large turbines.
Human powered electrical generators
Electric generators may also be driven by human power like a field radio station equipment.
Most Do-It-Yourself (DIY) projects display how human powered direct current generators can be done. An example is by using pedal power like in a bicycle, or a foot pump in order to recharge batteries. An estimated 125 to 200 watts of power could be generated using pedal power generator by an adult human being. At a 200-watt power, a healthy adult person would be completely exhausted after 1.3 hours. Portable radio receivers with manual power generator are produced to reduce the need for batteries. They are sometimes called clockwise radio.
Linear electric generator
A linear generator functions by sliding a magnet to and fro through a solenoid or a spool of copper wire. In the wire loops, AC current is induced every time the magnet is slid to and fro. This is explained under the Faraday’s law of induction. A typical example is a Faraday flashlight. In wave power schemes, large linear electricity generators are employed.
Tachogenerator
Tachogenerators power tachometers. Tachometers measure speed of electric motors or electric engines. Generators produce voltages which are proportional to the shaft speed. With accurate make and design, generators can be used to draw precise voltages for some range of shaft speed.
ORIGINAL ARTICLE: 1018 WORDS
DC GENERATOR
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 electric 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 (dynamos) require a commutator on the rotating shaft to convert the alternating current produced by the armature to direct current, 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 requires a current 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.
Smaller generators are sometimes self-excited, which means the field coils are powered by the current produced by the generator itself. The field coils are connected in series or parallel with the armature winding. When the generator first starts to turn, the small amount of remanent magnetism present in the iron core provides a magnetic field to get it started, generating a small current in the armature. This flows through the field coils, creating a larger magnetic field which generates a larger armature current. This “bootstrap” process continues until the magnetic field in the core levels off due to saturation and the generator reaches a steady state power output.
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.
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.[4] 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 tyre–generator interface from poor alignment, the small size of the generator, bearing losses and cheap design. The use of permanent magnets means that efficiency falls even further at high speeds because the magnetic field strength cannot be controlled in any way. Hub generators remedy many of these flaws since they are internal to the bicycle hub and do not require an interface between the generator and tyre.
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.
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, but at a power of 200 W, a typical healthy human will reach complete exhaustion and fail to produce any more power after approximately 1.3 hours.Portable radio receivers with a crank are made to reduce battery purchase requirements, see clockwork radio.
Linear electric generator
In the simplest form of linear electric generator, a sliding magnet moves back and forth through a solenoid – a spool of copper wire. An alternating current is induced in the loops of wire by Faraday’s law of induction each time the magnet slides through. This type of generator is used in the Faraday flashlight. Larger linear electricity generators are used in wave power schemes.
Tachogenerator
Tachogenerators are frequently used to power tachometers to measure the speeds of electric motors, engines, and the equipment they power. Generators generate voltage roughly proportional to shaft speed. With precise construction and design, generators can be built to produce very precise voltages for certain ranges of shaft speeds