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In 1831, Michael Faraday, a British physicist, invented the electromagnetic generator, a device that converts different types of energy like mechanical energy into electrical energy. Various mechanical energy sources, such as hand cranks, internal combustion engines, water turbines, and gas and steam turbines can be used as power generators. On the other hand, an electric motor can perform the opposite function of a generator, converting electrical energy into mechanical energy. Both generators and motors share many common properties. Electrical generators are saviours when the electricity from the local power grid is not available. These generators work as backups for the supply of electricity. From powering our homes to fueling complex machinery, a steady flow of electrical current is essential for modern life. But how is this electricity generated? Know about what is DC generator along with its parts and working principle.
Table of Contents
What is a DC Generator?
A DC generator is also known as a direct current generator. It is an electrical machinery which functions by converting mechanical energy into direct electricity. It operates based on the fundamental principle of electromagnetic induction, which states that a voltage is induced in a conductor whenever there is a relative motion between the conductor and a magnetic field.
In simpler terms, a DC generator utilizes a rotating mechanism to create a changing magnetic field around a set of conductors. This changing field induces a voltage or electromotive force (EMF), within the conductors which results in the flow of direct current.
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Parts of a DC Generator
The parts of a DC generator consists of –
- Stator, Rotor, Armature Core and Armature Winding
- Magnetic Field System
- Commutator
- DC Generator Commutator Function
- Yoke
- Shaft
- Bearings
- Pole Shoe
- Poles
They can be discussed below – A DC generator can be fundamentally divided into two main sections: the stator and the rotor.
- Stator: The stator is the stationary part that comprises a hollow cylindrical frame made of steel that houses the magnetic field system. This system can involve electromagnets created by coils of wire wrapped around iron cores or permanent magnets which are strategically placed within the frame, depending on the generator’s design.
- Rotor: The rotor is the rotating component that comprises the armature core and armature windings. It consists of a cylindrical core made of laminated steel to minimize energy losses due to eddy currents. Embedded within the core are slots that have the armature windings.
- Armature Core: A DC generator has an armature core on a shaft that pivots between field poles. This armature core has slots on its outer surface where the conductors of the generator are installed. Armature cores are composed of soft iron laminated laminations which are isolated from each other and securely attached together. In large machines, the laminated cores are keyed to the shaft, but in small machines, the keyed cores are directly attached to the shaft of the generator. Laminated armature cores help to reduce the loss of eddy current.
- Armature winding: In a DC generator armature winding is the process of inserting insulated conductors into slots of an armature core and connecting the wires in a linked arrangement. Armature windings can be divided into two types: wave winding, and lap winding.
- Yoke: The outer structure of a DC generator is a cylindrical component which is either made by casting or rolling steel and is called a yoke. This yoke has two main purposes.
- It holds the field pole core and acts as a shield for the apparatus.
- It provides a pathway for the magnetic flux generated by the field coil.
- Magnetic Field System: The magnetic field system in a DC generator is also the stationary part of the generator. The primary magnetic flux is generated in the generator by the magnetic field system. There are many pole cores in the field arrangement of the DC generator. The pole cores are fastened to yoke and field winding coils around the pole cores. The pole arrangement of the field arrangement of the DC generator has poles, such as poles which thrust inwards. Each pole core is equipped with a post shoe, which is a curved surface on which the shaft is mounted. The purpose of the shaft shoe is twofold:
- It supports the field coils
- It reduces the reluctance of the magnetic circuit by extending its cross-section
To avoid eddy current loss the pole cores are made of thin, shielded sheet steel laminate. The field coils are connected in series to form alternating north poles and south poles in rotation as the current passes through them.
- Commutator: Commutators are mechanical rectifiers that transform the alternating magnetic field generated by the armature winding through the load terminals into a direct voltage. A commutator is composed of wedge-shaped, copper segments that are separated from each other and from the shaft by a layer of mica. Each segment of a commutator is attached to one of the ends of an armature coil.
- DC Generator Commutator Function: The primary function of the commutator is to transform direct current (DC) into alternating current (AC). It acts as a switch that reverses the current flow and its role in the generator is further explained below.
Inside the generator’s armature coil, the electromagnetic field switches back and forth, causing an alternating current to flow through the coil. This current can be switched back using the commutator whenever the coil passes through a magnetic field without any influence. Consequently, the load receives a DC or unidirectional current.
The commutator guarantees that the current from the generator flows consistently in the same direction without interruption. As it moves along the commutator, the brushes establish strong electrical connections between the generator and the load.
- Shaft: The bracket serves as a critical element in a DC motor as it generates torque, leading to the rotation it facilitates. Made from mild steel, it boasts the most significant breaking strength among its materials. Within a DC generator, the bracket plays a crucial role in enhancing the generator’s capacity to convert mechanical energy into electrical energy. It connects with the commutator, the cooling fan, the armature center, and various rotating parts.
- Bearings: In a DC machine, bearings are the moving parts in a system. They are used to reduce the friction between the spinning and stationary elements of the machine. By reducing the friction, the components of the system are greased less often and last longer. Types of bearings are – Roller bearings and Ball bearings
- Poles: The poles also help to keep the field winding in good condition. Field windings are usually coiled on the poles and connected in a specific sequence to the armature windings. Because of this, the posts hold the welding process in place using screws on the yoke.
- Pole Shoes: It is a disc made of iron or steel that distributes magnetic energy and stops magnetic coils from collapsing.
Also Read – Balanced Force: Definition, Example and Characteristics
E.M.F Equation of DC generator
The emf equation of the DC generator is given by the equation:
Eg = (Φ * P * Z * N) / (60 * A)
where,
- Eg – EMF generated by the DC generator (Volts)
- Φ – Magnetic flux per pole (Webers)
- P – Number of poles in the generator
- Z – Total number of armature conductors
- A – Number of parallel paths in the armature winding
- N – Speed of the armature in revolutions per minute (rpm)
- N/60 is the number of turns per second
- The time for one turn will be dt=60/N sec.
Working Principle of DC Generator
The direct current (DC) generator operates on the well-established principle of electromagnetic induction. This principle dictates that a voltage, or electromotive force (EMF), is induced in a conductor whenever there’s relative motion between the conductor and a magnetic field. Here’s a breakdown of the core working principle:
A direct current (DC) generator has two main parts:
- Stationary Magnetic Field System: This creates a constant magnetic field within the generator.
- Rotating Conductor System (Rotor): This houses a set of wires called armature windings. As the rotor spins, these wires move through the magnetic field.
Generating Electricity:
- The movement between the wires and the magnetic field induces a voltage (EMF) in the wires, following Faraday’s Law. Stronger magnetic fields and faster rotation speed lead to higher voltage.
- This induced voltage creates a potential difference across the ends of the wires.
- Connecting the wires to an external circuit allows current to flow.
Maintaining Direct Current:
- Unlike AC generators, DC generators use a commutator and brushes.
- The commutator is a rotating component with segmented contacts that reverses the direction of the induced voltage at the right moments.
- Brushes make contact with the commutator, collecting the now-constant direct current and delivering it to the external circuit.
Also Read – What is the Difference Between Active and Reactive Power?
Types of DC Generators
There are three types of DC Generators –
- Separately Excited DC Generators: Separately excited DC generators have field coils that are excited by an external power source. This separate power source energizes coils within the generator, creating the magnetic field it uses.
- Self-Excited DC Generators: Self-excited DC generators have field coils that are energized by the generator’s output. These generators are more self-reliant. They take a portion of the electricity they produce and use it to create their own magnetic field. The generated current is sent back through the generator’s internal coils, setting up the magnetic field that keeps it running. This generator can be further divided into –
- Series Wound DC Generators – Here, the magnetic field strength is linked to the amount of electricity the generator is producing. As the demand for electricity goes up, the magnetic field gets stronger and the generator produces more electricity. This is useful for situations where one needs a bigger boost at the start.
- Stunt Wound Generators – This type acts like having a separate switch for the magnetic field. It uses a constant current for the field, leading to a steady voltage output.
- Compound Generators – This type combines the features of both shunt and series, offering a balance between stable voltage and adjusting to changing demands.
- Permanent Magnet DC Generators: The field coils are energized by permanent magnets instead of electromagnets or field windings in permanent magnet DC generators. These generators keep things simple by using permanent magnets to create the magnetic field. They do not need any external power or rely on their own current for this purpose. Permanent magnet DC generators are known for being easy to use and requiring minimal maintenance.
Also Read – What is the Difference Between Current and Static Electricity?
Uses of DC Generator
The applications of DC Generators are –
1. Separately Excited DC Generators:
- Laboratory Testing: These generators are ideal for testing electronic circuits, components and power supplies in laboratories.
- Electroplating: They are also suitable for electroplating, a process where a metal coating is applied to an object using an electric current.
2. Self-Excited DC Generators:
a) Shunt Wound DC Generators:
- Battery Charging: These generators are well-suited for charging batteries, which ensures proper and safe charging for various devices.
- Lighting Systems (Limited): In some standalone lighting systems, particularly in remote locations, shunt wound generators can provide a reliable DC source for powering lights.
b) Series Wound DC Generators:
- Starting Motors: They are ideal for starting motors in vehicles. The initial high current surge helps crank the engine.
c) Compound Wound DC Generators:
- Transportation (Streetcars & Trolleys): These generators allow for both stable voltage operation and some degree of response to load changes, making them suitable for powering streetcars and trolleys.
- Industrial Applications: In industrial settings where machinery might have fluctuating power demands, compound wound generators can provide a more stable and adaptable DC power source.
3. Permanent Magnet DC Generators:
- Portable Electronics: Due to their compact size, low maintenance requirements, and ability to generate DC power from mechanical sources like wind or water turbines, permanent magnet DC generators are often used in portable electronics like flashlights or bicycle lights.
- Uninterruptible Power Supply (UPS): Some UPS systems utilize small permanent magnet DC generators to provide temporary backup power in case of a power outage.
Thus these were some examples of how DC generators are applicable in our daily lives and how without electricity these generators would come to our use.
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