Chapter 7: DC generators, AC generators and Transformers
Chapter 7: DC generators, AC generators and Transformers
Chapter Overview
Chapter 7 will cover DC generators, AC generators and transformers. Most electrical systems contain both AC and DC voltages, so it is important to understand how both types are generated and utilized in the field. Worksheet information link.
Key points to remember:
Transformers are also covered in this section. Because of the extensive use of transformers throughout the industry, it is important to understand their operating principles and usages, especially when designing, installing, or troubleshooting an electrical system.
- Electromagnetism is the basic property behind the operation of DC/AC generators and transformers.
- In the &#;War of Currents&#; Tesla&#;s AC system was proven to be the best because AC systems can be transformed and distributed over longer distances.
- The cores of transformers are laminated for the same reason the magnetic assembly in a solenoid is: To confine the eddy currents in the individual laminations.
DC and AC Generator Basics
A generator converts mechanical energy into electrical energy using electromagnetic induction. Generators are designed to either produce DC or AC power, and both use electromagnetism in slightly different ways to accomplish the same goal of electrical power generation. Electricity produced from generators is widely used in modern-day power distribution. The modern-day reliance on electricity has continued to increase the need for qualified electricians to build and maintain these systems integral to society. As an electrician it is important to understand how they work and the critical role they play in modern electrical systems.
DC generators operate through the rotation of a coil of wire spinning in a magnetic field, which induces a voltage into the coils as it cuts through the magnetic force lines of the associated magnetic field. The amount of voltage produced depends on the rate at which the coil is rotated and the strength of the associated magnetic field. The alternating current generated from this process is converted to direct current (DC) by a segmented commutator that establishes the negative and positive polarity. AC generators operate very similarly to DC generators with the main difference being that the generated current is delivered to the outside circuit using slip rings, which produces the alternating wave form that defines AC systems.
Transformers Basics
Transformers also rely on electromagnetic induction but differ from generators as they do not produce electricity but simply step up or down voltage and current levels respectively. All transformers contain a primary and secondary winding which is comprised of many turns of wire surrounding a steel core. The primary and secondary windings are in close proximity but are isolated from each other. The ratio of the primary to secondary windings will determine the amount of reduction or increase of the associated voltage and current levels, known as &#;buck&#; or boost&#; transformers.
Transformer losses are always present due to inherent inefficiencies within the system. Not all the energy that is delivered to the primary coil is effectively transferred to the secondary or load circuit. Most of this loss is materialized as heat, and is a sign of the inefficiencies inherent to the transformers&#; operation. These inefficiencies can be described as resistive, eddy current, and hysteresis type losses. Resistive losses are directly affected by whatever material the transformer is constructed from. Eddy currents that form perpendicular to the natural flow of current also contribute to this inefficiency as they effectively impede current flow. Hysteresis is the final transformer type loss that can be described as magnetic friction. At the associated AC magnetic field fluctuates, this causes the affected atoms to realign constantly thereby causing even further losses in the system.
Looking Deeper
The prevalence of AC electrical distribution systems is largely due to the &#;War of Currents&#; waged between Nikola Tesla&#;s alternating current (AC) system and Thomas Edison&#;s direct current (DC) system. Tesla&#;s AC systems were simply better as AC can easily be stepped up and down for transmission over longer distances.
Keep in mind
Generators and transformers are essential components of modern electrical generation and distribution systems, and they will likely be a constant presence in many if not all, electrical systems encountered throughout your career.
DC Generator – Definition, Parts and Working Principle
Machines that convert mechanical energy into electrical energy are called Electric Generators. The electrical energy generated is further transmitted and distributed through power lines for domestic, commercial use. There are two types of generators,
AC Generator
DC Generator
A DC generator is the type of electrical generator that converts mechanical energy into direct current electricity. However, a generator that converts mechanical energy into alternating current electricity is an AC generator.
Do you know why we study generators in their working principle? On this page, we will get to resolve all our queries on the DC generator's parts,working principle and how we describe it in mathematical terms.
What About DC Generators?
In DC generators, the energy conversion is based on the principle of dynamically induced EMF production. These generators are most suitable for off-grid applications. DC generators supply continuous power to electric storage instruments and power grids (DC).
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DC Generator consists of the following parts -
Stator - A stator is a set of two magnets placed in such a way that opposite polarity faces each other. The purpose of the stator is to provide a magnetic field in the region where the coil spins.
Rotor - A rotor is a cylindrical laminated armature core with slots.
Armature Core - The armature core is cylindrical in shape and has grooves on the outer surface. These slots accommodate armature winding in it.
Armature Winding - These are the insulated conductors placed in the armature core. Because of them, the actual conversion of power takes place.
Field Coils - To produce the magnetic field, field coils are placed over the pole core. The field coils of all the poles are connected in series. When current flows through them, adjacent poles acquire opposite polarity.
Yoke - The outer hollow cylindrical structure is known as Yoke. It provides support to main poles and inter poles and gives a low reluctance path for the magnetic flux.
Poles - The main function of the poles is to support the field coils. It increases the cross-sectional area of the magnetic circuit, which results in a uniform spread of magnetic flux.
Pole Shoe - To protect the field coil from falling and to enhance the uniform spread of magnetic flux pole shoe is used. The pole shoe is fixed to Yoke.
Commutator - The commutator is cylindrical in shape. Several wedge-shaped, hard drawn copper segments form a commutator. The functions of a commutator:
To connect stationary external circuits to the rotating armature conductors through brushes and
To convert induced alternating current into direct current.
Working Principle of a DC Generator
A DC generator operates on the principle of Faraday&#;s laws of electromagnetic induction. According to Faraday&#;s law, whenever a conductor is placed in a fluctuating magnetic field (or when a conductor is moved in a magnetic field) an EMF is induced in the conductor.
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If the conductor is guided with a closed path, the current will get induced. The direction of the induced current (given by Fleming&#;s right-hand rule) changes as the direction of movement of the conductor changes.
For example, consider the case, an armature rotating in clockwise direction and conductor at the left moving in an upward direction. As the armature completes its half rotation the direction of movement of the conductor will get reversed downward. The direction of the current will be alternating. As the connections of armature conductors get reversed, a current reversal takes place. Thus, we get unidirectional current at the terminals.
EMF Equation of a DC Generator
The EMF equation for DC generator is expressed as:
Eg = (PØNZ)/60A
Where,
Eg - Generated EMF across any parallel path
P - Total number of poles in the field
N - Rotational speed of armature(rpm)
Z - Total number of armature conductors in the field.
Ø- Magnetic flux produced per pole.
A - number of parallel paths in the armature.
Losses in DC Generators
While converting the mechanical energy into electrical energy, there are losses of energy i.e. whole input isn&#;t converted into output. These losses are classified into mainly three types:
Copper Loss- These losses occur while current flows through windings and are of three types: armature copper loss, field copper winding loss and losses because of brush resistances.
Iron Losses- Due to the induction of current in the armature, eddy current losses and hysteresis loss occur. These losses are also called Core losses or Magnetic losses.
Mechanical Losses- Losses which occur because of friction between the parts of the generator are called mechanical losses.
Types of DC Generators
The three types of self-excited DC generators are:
Series Wound Generators.
Shunt Wound Generators.
Compound Wound Generators.
Applications of DC Generators
Applications of DC generators are as follows:
The separately excited type DC generator is used for power and lighting purposes using the field regulators.
The series DC generator is used in arc lamps for stable current generator, lighting and booster.
Level compound DC generators are used to supply power to hostels, offices, lodges.
Compound DC generators are used for supplying power to DC welding machines.
A DC generator is used to compensate for the voltage drop in the feeders.
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