Transformers, AC Distribution & electronic systems/components Flashcards
What are the two main reasons for distributing electricity in A.C. over D.C. ?
A.c. can be easily transformed from one voltage to another
It produces less arcing and sparking on contact
Describe a transformer
Transfomers are four terminal devices that transform AC current into a higher or lower AC voltage
What is the transformer formulas?
VP/VS = NP/NS = IS/IP VP= Voltage primary VS= Voltage Secondary NP= No. of primary turns NS= No. of secondary turns IS= Secondary current IP= Primary Current
What is a current transformer?
these are typically used for metering purposes the high current is stepped down and results are displayed on the screen
What is an isolating transformer?
Isolating transformers work on the principle that any device that is connected to an AC supply is isolated from the power source. Used in medical scenarios hospitals etc.
What are the two main types of transformer cooling?
Dry type and oil immersed
What is an oil immersed transformer and a dry type transformer?
Oil immersed types have the transformer core immersed in oil. Dry type air is used to cool it
What is an Air natural type transformer?
Is a dry type, low power transformer. They use the natural circulation of air to cool it down by method of convection.
What is an air forced type transformer?
Is a dry type transformer, although can also be used in oil immersed transformer. High velocity cool air is provided to the core which ensures for quicker heat dissipation.
What is an ONAN transformer?
Oil natural air natural. Is an oil immersed transformer. The transformer core is submersed in oil, as the temperature rises the hot oil will move to the top which is replaced with colder oil at the bottom.
What is an ONAF transformer?
Oil Natural Air Forced type. Is an oil immersed transformer which uses temperature controlled power fans to provide quicker heat dissipation to the oil and assist convection.
What is OFAF?
Oil Forced air Forced. Typically used for high rated oil immersed transformers. Both oil and air are circulated in order to provide sufficient heat dissipation. Fans circulate the air and oil pumps circulate the oil at a higher rate.
What are oil directed air forced type transformers
is an updated version of OFAF. in this type hot oil follows a specific route for faster convection.
What are water forced type transformers?
Here the flow of hot oil is directed to a heat exchanger where a water shower is applied.
What are the two types of transformer cores called? and give a brief description of them.
Core type - Has one path for flux to circulate round, typically less efficient than shell type
Shell type has two paths for magnetic flux to follow, this is makes it more efficient.
What are the two types of transformer losses?
Copper losses and Iron Losses
Describe copper losses in a transformer
Copper losses are a result of electrical resistance of both windings. The resistance opposes the flow of magnetizing currents. this varies with the size of load current applied.
Describe hysteresis and how to reduce eddy current losses
Hysteresis is a result of the cores magnetization being contantly reversed and turns into heat
Eddy current losses are minimised by using insulated thin layers of steel lamination’s as the core. These are insulated by varnish or paper.
What are the 3 main types of resistors?
Carbon
Wire wound
Variable resistor (Potentiometer)
What is a carbon resistor?
A carbon resister is the cheapest an most unreliable. It consists of a tubular rod of carbon enclosed in an insulating ceramic tube.
What is a wire wound resistor?
A simple example consists of a length of resistance wire spiral wound onto a tubular ceramic former & covered with a heat proof ceramic or vitreous enamel coating.
What is a variable resistor?
Also known as a potentiometer. Consists of a circular carbon track fitted with a sliding metal contact attached to a rotary control shaft.
What is a capacitor?
Capacitors are devices made up of two conductive plates separated by an insulator known as a dielectric.
What is a diode?
Semi conductors which allow current to pass in one direction.
What is a Zener diode?
Zener diodes allow current to flow from the opposite direction once the ‘Zener’ voltage is reached
What is an LED
Light emitting diode. Use significantly smaller voltage and currents in comparison to filament bulbs.
What are the two types of transistor?
Are semi-conductor devices used to amplify and switch electronic signals and electronic power. NPN (Not pointing in) and PNP
What are photocells (LDR)?
They change resistance as light varies. Light up resistance down
What are fibre optics?
Light is reflected in a thin piece of glass. Usually LED or laser. Data is encoded into the light.
What are the two types of thermistors?
Negative temperature coefficient - Temp increases resistance decreases
Positive temperature coefficient - temperature increases so does resistance
What is a thermistor?
When two types of dissimilar are heated at the ‘hot junction’ a voltage is generated. This can be calibrated against a temperature scale.
What is a relay?
An electrically operated switch.
Used magnet to operate switching mechanism
Are used when a circuit is controlled by a low power signal
What is a DIAC
Are a form of solid state switch used to switch AC voltage. Known as a thyristor. like a junction transistor without a base.
What is a TRIAC.
TRIAC’s conduct current in both directions. Commonly used in lighting controllers, will conduct until a voltage applied reaches zero.
What is a thyristor?
Used to create a latching circuit, they are digital switches that respond to an input voltage. Will continue to flow after input voltage is removed.
What is an inverter?
Inverters are used to convert D.C. to A.C.
