All About Induction Motor Facts, Tutorials and Important Information
Advantages of a three phase induction motor:
This is simple in construction. It is robust. It si almost unbreakable.
- It requires minimum care and maintenance.
- For a given horse power cost of induction motor is less.
- It has high efficiency
- It has a reasonably good power factor.
- It is self starting
Disadvantages of a three phase induction motor:
- Its speed can not be varied without loss of efficiency.
- Its speed decrease with the increase of load.
- It has inferior starting torque
Principle of Induction Motor:
When a three phase supply is given to the stator, a rotating field produces induced e.m.f. in rotor windings which cause induced currents tend to propose the action, producing them and therefore they circulate in such a manner that a torque is produced in the rotor tending it to cause it to flow the rotating field and thus reduce the relative motion which is producing the induced currents.
Key facts on Induction Motor:
- Squirrel cage induction motor has very small starting torque.
- Just like a D.C. shunt motor, the speed of an induction motor operating in stable region decreases with increase in load.
- Magnetic flux produced due to the stator winding with three phase current of frequency and which is of constant value, rotates at synchonous speed. If the number of pole of the motor. the synchronous speed is given by Ns = 120f/P.
- An induction motor rotor runs at a speed which is always less than the speed of the stator field.
- In a three phase induction motor the torque increases with the increase in supply voltage.
- Improvement in the power factor in an induction motor increase the torque and decrease the current due to increased impedance.
- Two of the power supply terminals to a three phase induction motor got interchanged during recondition after maintenance of the motor, when put back into service, the motor will rotate in the reverse direction to that prior to maintenance.
- If the air gap of the induction motor is increased the magnetizing current will increse.
- In a slip ring induction motor resistance is connected in rotor phases to increase the starting current.
- For a three phase wound rotor induction motor, and increase in rotor resistance effects the motor performance in the starting current decrease.
- It is advisable to avoid line starting of induction motor and use starter because motor takes five to seven times its full load current.
- A change of five percent in the supply voltage of a three phase induction motor will produce to its torque a change of approximately ten percent.
A cable is a length of single conductor, generally with several wires stranded together, or two or more conductors, each provided with its own insulation and laid up together. The insulated conductors may or may not be provided with an over all protective converging. The conductor with its insulation but without mechanical protection is known as the core of the cable.
A cable consist of the following three parts.
1. Conductor: It provides conducting path of electric current.
2. Insulation: An electrical insulating medium that prevents direct contact or unsafe proximity between the conductor and other objects.
3. External protection: Protects against mechanical damage, chemical or electromechanical attact, free or any other deleterious external influences. Copper used on overhead lines is generally hard drawn for providing adequate mechanical strength. However in case of cables, copper in annealed condition is used. Aluminum being lighter is extensively used in cables, However, joining of aluminum conductors calls for the use of special techniques and expertise.The size of conductor depends on the current rating of the conductor.Classification of Cables based on Insulation:On the basis of insulation, the cables are classified as under: Paper insulated cables: In these cables paper is wound on the conductor in successive layers to achieve the needed dielectric strength. Such cables are manufactured in various sizes from 5 mm squared to 500 mm squared cross-section of conductor and voltage ratings up to 33kv.The cable are provided with steel armour made up of galvanised steel wires or shielding tap. The outer sheath on the cable provides protection against external forces on the cable. The metallic shielding around the conductor is used to control the electrostatic voltage stress, reduce corona effect and to decrease the thermal resistance. Metallic shielding in only 3 to 5 mm thick. Lesser thickness reduces circulating currents and power loss.
The Flow Of Electricity and How Grounding Is Important In the Systems Tutorials
By: IIEE
The electric power utility provides a ground somewhere in its local distribution system; therefore, there is a ground wire in addition to the hot wires within the service drop. This ground can be seen at the power pole that contains the step down transformer.In addition to the ground connection provided by the electric utility, every building is required to have an independent ground, callead a"system ground." The system ground provides for limiting the voltage upon the circuit, which might otherwise occur through exposure to lighting, or for limiting the maximum potential to ground due to normal voltage. Therefore, the system ground's main purpose is to protect the electric system itself limited protection to the user.The system ground serves the same purpose as the power company's ground, however, being closer to the building it has a lower resistance. The equipment ground on the other hand, protects man from potential harm during the use of certain electrical equipment. The system ground should be a continuous wire of low resistance and of sufficient size to conduct current safely from lightning and overloads.
RESIDENTIAL ELECTRICAL SYSTEMS
ELECTRICAL FUNDAMENTALS - Think of a pipe or a garden hose. When you turn on the water, it moves through the pipe and out the faucet or hose end. In a similar way , you can think of electricity as a current of very tiny particles (electrons) flowing inside a wire and through a light bulb or an appliance that’s been switched on.
How fast does electricity travel?
Electric current is defined as the ordered movement of charge. The charge that moves in
electricity could be ionic charges in a liquid or a gas (right). However most of us usually
think of electrons as the charge carrier in electricity. So we will refine the question
further by saying “How fast is an electric current comprising of moving electrons?”.
Measuring Electricity
Electricity makes our lives easier, but it can seem like a mysterious force. Measuring electricity is confusing because we cannot see it. We are familiar with terms such as watt, volt, and amp, but we do not have a clear understanding of these terms. We buy a 60-watt light bulb, a tool that requires 120 volts, or an appliance that uses 8.8 amps, but we don’t think about what those units mean.
The Flow Of Electricity On Transmission Lines Engineering Tutorials
By: IIEE
Electricity is usually generated by a generator that converts mechanical energy into electrical energy. The electricity is then run through a transformer where voltage is increased to several hundred thousand volts and in some instance to a million or more volts. This high voltage is necessary in order to increase the efficiency of power transmission over long distances.This high-transmission voltage is then stepped down reduced to normal 110 volt/220 volt household current by a transformer located near the point of use residence. The electricty is then transmitted to the house by a series of wires call a service drop. In areas where the electric wiring is underground, the wires leading to the building are burried in the ground. In order for electric current to flow, it must travel from a higher to a lower potential voltage. In an electrical system the hot wires black or red are at a higher potential that the neutral or ground wire white or green. Therefore, current will flow between the hot wires and the neutral or ground wires. The voltage is a measure of the force at which electricity is delivered. It is similar to pressure in a water supply system. Current is the quantity of flow of electricity. It is similar to measuring water in gallons per second. A watt is a measure of how much power is flowing. Electrity is sold in quantities of watt-hour. The earth, by virtue of moisture contained within the soil, serves as a very effective conductor. Therefore, in power transmission, instead of having both the hot and neutral wires carried by the transmission poles, one lead of the generator is connected to the ground, which serves as a conductor. Only hot wire are carried by the transmission towers. At the house or point where the electricity is to be used the circuit is completed by another connection to ground.
Tracing The Flow Of Electricity
The mesh Structure of high voltage transmission networks provides a large number of possible routes by which electrical power can flow from the sources generators to the sinks grid supply points.
Circuits and the Flow of Electricity
To understand current electricity, many vocabulary words must be introduced. The first
part of this lesson uses a hands-on, problem-solving activity that helps students define the
vocabulary terms and demonstrate the terms’ relationships. After gaining foundational
understanding, students create their own circuits.
