Class VII September
Making a simple circuit of electromagnet by using soft iron and copper wire:
You Need:
1. A large iron nail (about 3 inches)
2. About 3 feet of THIN COATED copper wire
3. A fresh D size battery
4. Some paper clips or other small magnetic objects
What to do
1. Leave about 8 inches of wire loose at one end and wrap most of the rest of the wire around the nail. Try not to overlap the wires.
2. Cut the wire (if needed) so that there is about another 8 inches loose at the other end too.
3. Now remove about an inch of the plastic coating from both ends of the wire and attach the one wire to one end of a battery and the other wire to the other end of the battery. See picture below. (It is best to tape the wires to the battery - be careful though, the wire could get very hot!)
4. Now you have an ELECTROMAGNET! Put the point of the nail near a few paper clips and it should pick them up!
NOTE: Making an electromagnet uses up the battery somewhat quickly which is why the battery may get warm, so disconnect the wires when you are done exploring.
How does it works
Most magnets, like the ones on many refrigerators, cannot be turned off, they are called permanent magnets. Magnets like the one you made that can be turned on and off, are called ELECTROMAGNETS. They run on electricity and are only magnetic when the electricity is flowing. The electricity flowing through the wire arranges the molecules in the nail so that they are attracted to certain metals. NEVER get the wires of the electromagnet near at household outlet! Be safe - have fun!
VIII class September
Types of Resisters
1. CARBON FILM
The most popular resistor type. This resistor made by depositing a carbon film onto a small ceramic cylinder. A small spiral groove cut into the film controls the amount of carbon between the leads, hence setting the resistance. Such resistors show excellent reliability, excellent solder ability, noise stability, moisture stability, and heat stability. Typical power ratings range from 1/4 to 2 W. Resistances range from about 10 Ohm to 1 M Ohm, with tolerances around 5 percent.
Carbon Composition
This type is also popular. Its made from a mixture of carbon powder and glue like binder. To increase the resistance, less carbon is added. These resistors show predictable performance, low inductance, and low capacitance. Power ratings range from about 1/4 to 2 W. Resistances range from 1 Ohm to about 100 M Ohm, with tolerances around +/- 5 percent.
Metal Oxide Film
This type is general purpose resistor. It uses a ceramic core coated with a metal oxide film. These resistors are mechanically and electrically stable and readable during high temperature operation. They contain a special paint on their outer surfaces making them resistant to flames, solvents, heat, and humidity. Typical resistances range from 1 Ohm to 200 k Ohm, with typical tolerances of +/- 5 percent.
Precision Metal Film
This type is very accurate, ultra-low noise resistor. It uses a ceramic substrate coated with a metal film, all encased in an epoxy shell. These resistors are used in precision devices, such as test instruments, digital and analog devices, and audio and video devices. Resistances range from about 10 Ohm to 2 M Ohm, with power rating from 1/4 to about 1/2 W, and tolerances of +/- 1 percent.
Foil Resistors
Foil resistors are similar in characteristics to metal film resistors. Their main advantages are better stability and lower temperature coefficient of resistance (TCR). They have excellent frequency response, low TCR, good stability, and are very accurate. They are manufactured by rolling the same wire materials as used in precision wire wound resistors to make thin strips of foil. This foil is then bonded to a ceramic substrate and etched to produce the value required. They can be trimmed further by abrasive processes, chemical machining, or heat treating to achieve the desired tolerance. Their main disadvantage is that the maximum value is less than metal film resistors. The accuracy is about the same as metal film resistors, the TCR and stability approaches precision wire wounds but are somewhat less because the rolling and packaging processes produce stresses in the foil. The resistive materials used in precision wire wound resistors is very sensitive to stresses, which result in instability and higher TCS. Any stresses on these materials will result in a change in the resistance value and TCR, the greater the stresses, the larger the change. This type can be used as strain gauges, strain being measured as a change in the resistance. When used as a strain gauge, the foil is bonded to a flexible substrate that can be mounted on a part where the stress is to be measured.
Filament Resistors
Filament resistors are similar to bathtub or boat resistors except that they are not packaged in a ceramic shell (boat). The individual resistive element with the leads already crimped is coated with an insulating material, generally a high temperature varnish. They are used in applications where tolerance, TCR, and stability are not important but the cost is the governing consideration. The cost of this type is slightly higher that of carbon composition and the electrical characteristics are better.
Precision Wire Wound
The precision wire wound resistor is a highly accurate resistor (within 0.005%) with a very low TCR. A TCR of as little as 3ppm/o C can be achieved. However these components are too expensive for general use and are normally used in highly accurate dc applications.
High Powered Wire Wound
These resistors are used for high power applications. Types include vitreous enamel coated, cement, and aluminum housed wire wound resistors. Resistive elements are made from a resistive wire that is coiled around a ceramic cylinder. These are the most durable of the resistors, with high heat dissipation and high temperature stability. Resistances range from 0.1 Ohm to about 150 k Ohm, with power ratings from around 2 W to as high as 500 W, or more.
Photo Resistors and Thermistors
These are special types of resistors that change resistance when heat or light is applied. Photo-resistors are made from semi-conductive materials, such as cadmium sulfide. Increasing the light level will decreases the resistance. This type also called LDR (Light Dependent Resistor). Thermistors are temperature sensitive resistors. Increasing the temperature will decreases the resistance (in most cases). This type also called Thermistor NTC (Negative Temperature Coefficient). The reciprocal type is Thermistor PTC (Positive Temperature Coefficient). Increasing the temperature will increases its resistance.
Variable Resistors
Variable resistors provide varying degrees of resistance that can be set with the turn of a knob. Special kinds of variable resistors include potentiometers, rheostats, and trimmers. Potentiometers and rheostats are essentially the same thing, but rheostats are used specially for high power AC electricity, whereas potentiometers typically are used with lower level DC electricity. Both potentiometers and rheostats are designed for frequent adjustment. Trimmers, on the other hand, are miniature potentiometers that are adjusted infrequently and usually come with pins that can be inserted into pcb. They are used for fine tuning circuits (e.g.: fine tuning a circuit that goes astray as it ages), and they are usually hidden within a circuit’s enclosure box. Variable resistors come with 2 or 3 terminals. There are 2 kinds of taper, ie. : linear tapered and nonlinear tapered (logarithmic). The 'taper' describes the way in which the resistance changes as the control knob is twisted. Linear taper usually has coded as 'A' while nonlinear tapes has coded as 'B'.
Measuring Instruments
Voltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to digital converter.
Ammeter
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes (A), hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. Early ammeters were laboratory instruments which relied on the Earth's magnetic field for operation. By the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems.
Ohmmeter
An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. Micro-ohmmeters (microhmmeter or microohmmeter) make low resistance measurements. Megohmmeters (aka megaohmmeter or in the case of a trademarked device Megger) measure large values of resistance. The unit of measurement for resistance is ohms (Ω).
Energy Meter
An electricity meter or energy meter is a device that measures the amount of electric energy consumed by a residence, business, or an electrically powered device.
Electricity meters are typically calibrated in billing units, the most common one being the kilowatt hour. Periodic readings of electric meters establishes billing cycles and energy used during a cycle.
In settings when energy savings during certain periods are desired, meters may measure demand, the maximum use of power in some interval. In some areas the electric rates are higher during certain times of day, reflecting the higher cost of power resources during peak demand time periods. Also, in some areas meters have relays to turn off nonessential equipment.[1]
September Class IX
Motor Fundamentals
Before we can examine the function of a drive, we must understand the basic operation of the motor. It is used to convert the electrical energy, supplied by the controller, to mechanical energy to move the load. There are really two types of motors, AC and DC. The basic principles are alike for both. Magnetism is the basis for all electric motor operation. It produces the force required to run the motor. There are two types of magnets the permanent magnet and the electro magnet. Electro magnets have the advantage over permanent magnet in that the magnetic field can be made stronger. Also the polarity of the electro magnet can easily be reversed. The construction of an electro magnet is simple. When a current passes through a coil of wire, a magnetic field is produced.
This magnetic field can be made stronger by winding the coil of wire on an iron core.
One end of the electro magnet is a north pole and the other end is a south pole The poles can be reversed by reversing the direction of the current in the coil of wire. Likewise, if you pass a coil of wire through a magnetic field, a voltage will be induced into the coil And, if the coil is in a closed circuit, a current will flow.
Types of AC Motors
Classification Based On Principle Of Operation:
(a) Synchronous Motors.
1. Plain
2. Super
(b) Asynchronous Motors.
1. Induction Motors:
(a) Squirrel Cage
(b) Slip-Ring (external resistance).
2. Commutator Motors:
(a) Series
(b) Compensated
(c) Shunt
(d) Repulsion
(e) Repulsion-start induction
(f) Repulsion induction
Classification Based On Type Of Current:
1. Single Phase
2. Three Phase
Classification Based On Speed Of Operation:
1. Constant Speed.
2. Variable Speed.
3. Adjustable Speed.
Classification Based On Structural Features:
1. Open
2. Enclosed
3. Semi-enclosed
4. Ventilated
5. Pipe-ventilated
6. Riveted frame-eye etc..
Uses:
A.C. Motors are playing a very vital role in everyday life, right from pumping water to overhead tank to modern robot's maneuvering arm. The main factor which lead to the adoption & wide usage in various fields is its flexibility and its huge variety which can be matched with almost any kind of demand. To know what are the different types of A.C. Motors available, to match it perfectly with the demand, it is highly essential to know about the different classifications of A.C. Motors.
Dc Motor
DC Motors are classified in to 3 types
i) Shunt-wound motor
ii) Series-wound motor
iii) Compound-wound motor
DC. shunt motor _ lathes, fans, pumps disc and band saw drive requiring moderate torques.
DC . series motor_ Electric traction, high speed tools
DC . compound motor _Rolling mills and other loads requiring large momentary toques.
Read more: http://www.brighthub.com/engineering/electrical/articles/41740.aspx#ixzz1bvjOKgFu
Generator:
Introduction:
A generator is a machine by which mechanical energy is transformed into electrical energy. Generators can be sub-divided into two major categories depending on whether the electric current produced is alternating current (AC) or direct current (DC). The basic principle on which both types of generator works is the same.
Generator Principle:
The scientific principle on which generators operate was discovered almost simultaneously in about 1831 by the English chemist and physicist, Michael Faraday, and the American physicist, Joseph Henry. Imagine that a coil of wire is placed within a magnetic field, with the ends of the coil attached to some electrical device, such as a galvanometer. If the coil is rotated within the magnetic field,
Uses for Generators
In all seasons, from winter to summer, you may need a back up power supply. Rain, snow, flood, blackouts, anything can affect your electricity supply. Having a backup source like a generator is a must.
You may be without power for hours or days. A generator can power appliances that you absolutely need. Like a refrigerator, fans, lights, and possibly heaters.
Generators for Camping
Camping can be an even more fun experience with a generator. You can run lights, a radio, CD player, even television and of course your laptop!
It can even be a backup electrical source for your RV or camper. If you are out in the wilderness, you never know when you will need backup power.
Generators take all parties outdoors!
Even if you just have a part in your backyard, a generator will avoid extension cords cluttering up your yard.
Your family reunion, birthday party, or other family or group celebration will know no bounds with a generator. You will need music, lights, even a PA system.
Contractors need backup electrical power.
Contractors and construction workers use power tools. Power tools take electricity. Many times you are away from a power supply, or need to shut the main power down. A generator could save you time and may well save your business. Some of your tools may run on batteries, but it will never hurt to be able to recharge them when needed. Many power tools run off of an air compressor. That too needs electricity. Time is money in the construction trade, and a generator will save both.
Other uses for a generator. Tailgate parties use electricity. Even if you have a camper or RV, you will need a backup power source. You may need a hot plate for food warming, music, as well as big screen TV. And of course after dark, some lights. Why run your vehicle power supply down? Get a generator.
You will find so many uses for a generator, you will wonder why you ever did without one. Don't think it will just sit in the garage unused until a power outage. If you are into outdoor activities and work, your generator will see a lot of use.
