DC Generator Design
A Permanent Magnet DC Generator Design
The DC Generator is an electrical machine which converts mechanical energy in the form of motion, into electrical energy in the form of a dc voltage and current by using the principles of magnetic induction. The voltage and current output produced by a dc generator depends on its shaft speed (rpm) and the electrical load connected to it.
The shaft speed required to reach any particular output voltage is determined by the load. The lighter the load, the lower the rpm needed to reach the specified voltage. Then low rpm dc generators are a popular choice for use in wind power and hydro power battery charging systems.
The DC Generator gets its energy of motion from the wind or water turbine blades attached to its rotor shaft. Most AC generators are designed to run too fast to be coupled directly to these turbine blades so gearboxes or pulley systems are used to increase an AC generators speed. However, speed increasing gearboxes are complex mechanical items requiring good mechanical alignment and lubrication for reliable operation, so low rpm dc generators are ideal for this type of application.
The way to generate dc electricity is to spin a coil inside a magnetic field such that the magnetic lines of force generated by the magnetic field are cut by the spinning coil. We know from school that magnets have two poles, north and south, and that magnetic flux emerges from the north pole and flows back to the south pole.
In a dc generator we can make this magnet circuit in two ways. Firstly, feeding some of the generators output power back into its own field coils to make an electromagnet which can be precisely controlled or secondly to use permanent magnets to generate the magnetic flux rather than current in a coil of wire.
The advantage of permanent magnets is that no field supply is needed as the magnetic field is permanently excited reducing costs, and it also means that there are no I2R power loss in the magnetic field winding, which helps to increase the generators efficiency.
Magnetic theory teaches us that a voltage is induced into a coil of wire due to generator action. Generator action is based on Faraday’s law of electromagnetic induction in which an N-turn rectangular coil rotates within a uniform magnetic field. The magnets and coils within a dc generator are configured in such a way that the magnetic flux passes through the electrical coils of wire linking together the magnetic and electrical circuits.
All dc generators have two parts, one part called the “stator”, as it is stationary, and the other part which moves or rotates called the “rotor”. Generally for a dc generator design, the magnetic field is on the stator and the power generating coil winding is on the rotor.
DC generators work by rotating or passing the coils past the magnets (or magnets past the coils) with the electrical power generated being taken directly from the rotor, known commonly as the “armature” on a dc machine, via carbon brushes with the magnetic field, which controls the power, being supplied by either permanent magnets making what is generally called a Permanent Magnet DC Generator, or by an wound coils forming an electromagnet, making a Wound Field DC Generator.
The rotating armature coils pass through this stationary, or static magnetic field which in turn generates an electrical current in the coils. When the armature coil is adjacent to the direction of the stator magnetic flux, maximum voltage is induced into the coil since the most magnetic lines of force are being cut by the coil. As the armature moves, its coil now becomes perpendicular to the stator magnetic flux, and no magnetic lines of force are being cut, therefore the induced voltage is zero at this instant. Then as the armature of the generator rotates in an endless cycle, its coils constantly cut the lines of magnetic flux and an alternating dc voltage is induced in them. This process is known as “electromagnetic induction”.
In a DC generator as the armature rotates a full 360o every rotation, the generated current must pass through what is called a commutator which is constructed from a copper ring split into segments with insulating material between each segments. A carbon brush arrangement in contact with the commutator segments carry the electrical power to the output terminals as shown.
DC Generator Construction
The commutator segments in a dc generator replaces the continuous slip rings of the AC generator and is the main difference in their construction. The commutator mechanically reverses the armature coil connections to the external circuit producing a pulsating voltage. The output voltage is pulsating because it does turn “ON” or “OFF”, but it never reverses polarity neither unlike AC voltages and currents. Then since the polarity across the generators terminals remains constant, the output voltage is DC.
As well as permanent magnet generators, DC generators can also have a wound field coil to produce the required magnetic field. The names used to describe these types of dc generator depends upon the relationship and interconnection of each of the magnetic field coils with respect to the armature.
The two basic types of field winding excitation used for dc generators are called: self excitation and separate excitation, and depending upon which form of field excitation is used, the dc generator is classed as either a “self-excited generator” or a “separately-excited generator”.
Basically, for a separately excited dc generator, a separate external dc voltage supply is required to provide the excitation current through the field winding. Whereas in a self excited dc generator, the generated voltage itself is used to excite the field winding of the same dc generator as shown.
Classification of DC Generator
The two basic connections for a self-excited DC machine are the “Shunt Wound DC Generator”, were the field winding constructed with relatively many turns of small high resistance wire used to limit current flow through the field, is connected in parallel, or shunt with the armature. The “Series Wound DC Generator”, were the field winding is made with relatively few windings turns of very large wire of very low resistance is connected in series with the armature. Each type of DC generator construction has its own set of advantages and disadvantages and which one you use depends upon your application.
For battery charging applications the shunt wound self-excited dc generator or a permanent magnet dc generator are best suited as their output voltage remains fairly constant over a large range of rotational speeds.
When charging a battery with a dc generator, the generator rpm must first rise to the point where its output voltage increases above the battery terminal voltage for current to flow into the battery. The effort required to turn the generator (its input torque) increases and as long as the amount of input torque required can be supplied by the generators prime mover blades, it will continue to charge the battery.
The current or amperage of a dc generator at any rpm is governed only by the connected battery load, and not by its rpm speed. As soon as the battery becomes fully charged, no more charging current flows and the load effectively disappears. If the dc generator continues to be driven the generators terminal voltage will rise and no matter how high the terminal voltage, if there is no load connected to the generator there is no current flow. Also if you run a dc generator with no load, the current will always be zero no matter how high its rotational speed.
Then when charging batteries with a dc generator it is necessary to use a voltage regulator and dummy resistive load to protect the battery, or to disconnect the generator from the battery completely when the charging current drops to zero or the battery terminal voltage exceeds its rated value.
The dc generator is the one of the key components of a wind turbine or hydro turbine system, and as we have seen there are different options available which vary in their complexity and the type of output power they can supply. DC generators can be self excited or separately excited. Even a simple electric generator can be constructed using fixed permanent magnets to create a permanent magnet generator.
The invention of the DC generator has made our lives easy. But the fact that armatures, brushes, commutators and windings are complex and cost a lot of money, a lot of DC generators have been replaced by modern AC alternators and induction machines which are more economical and because DC or direct current voltages and currents, when required, can be produced by electronic rectifiers.
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