Transformers, AC Distribution & electronic systems/components

Question 1

What are the two main reasons for distributing electricity in A.C. over D.C. ?

Answer 1

A.c. can be easily transformed from one voltage to another

It produces less arcing and sparking on contact

Question 2

Describe a transformer

Answer 2

Transfomers are four terminal devices that transform AC current into a higher or lower AC voltage

Question 3

What is the transformer formulas?

Answer 3

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

Question 4

What is a current transformer?

Answer 4

these are typically used for metering purposes the high current is stepped down and results are displayed on the screen

Question 5

What is an isolating transformer?

Answer 5

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.

Question 6

What are the two main types of transformer cooling?

Answer 6

Dry type and oil immersed

Question 7

What is an oil immersed transformer and a dry type transformer?

Answer 7

Oil immersed types have the transformer core immersed in oil. Dry type air is used to cool it

Question 8

What is an Air natural type transformer?

Answer 8

Is a dry type, low power transformer. They use the natural circulation of air to cool it down by method of convection.

Question 9

What is an air forced type transformer?

Answer 9

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.

Question 10

What is an ONAN transformer?

Answer 10

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.

Question 11

What is an ONAF transformer?

Answer 11

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.

Question 12

What is OFAF?

Answer 12

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.

Question 13

What are oil directed air forced type transformers

Answer 13

is an updated version of OFAF. in this type hot oil follows a specific route for faster convection.

Question 14

What are water forced type transformers?

Answer 14

Here the flow of hot oil is directed to a heat exchanger where a water shower is applied.

Question 15

What are the two types of transformer cores called? and give a brief description of them.

Answer 15

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.

Question 16

What are the two types of transformer losses?

Answer 16

Copper losses and Iron Losses

Question 17

Describe copper losses in a transformer

Answer 17

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.

Question 18

Describe hysteresis and how to reduce eddy current losses

Answer 18

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.

Question 19

What are the 3 main types of resistors?

Answer 19

Carbon
Wire wound
Variable resistor (Potentiometer)

Question 20

What is a carbon resistor?

Answer 20

A carbon resister is the cheapest an most unreliable. It consists of a tubular rod of carbon enclosed in an insulating ceramic tube.

Question 21

What is a wire wound resistor?

Answer 21

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.

Question 22

What is a variable resistor?

Answer 22

Also known as a potentiometer. Consists of a circular carbon track fitted with a sliding metal contact attached to a rotary control shaft.

Question 23

What is a capacitor?

Answer 23

Capacitors are devices made up of two conductive plates separated by an insulator known as a dielectric.

Question 24

What is a diode?

Answer 24

Semi conductors which allow current to pass in one direction.

Question 25

What is a Zener diode?

Answer 25

Zener diodes allow current to flow from the opposite direction once the 'Zener' voltage is reached

Question 26

What is an LED

Answer 26

Light emitting diode. Use significantly smaller voltage and currents in comparison to filament bulbs.

Question 27

What are the two types of transistor?

Answer 27

Are semi-conductor devices used to amplify and switch electronic signals and electronic power. NPN (Not pointing in) and PNP

Question 28

What are photocells (LDR)?

Answer 28

They change resistance as light varies. Light up resistance down

Question 29

What are fibre optics?

Answer 29

Light is reflected in a thin piece of glass. Usually LED or laser. Data is encoded into the light.

Question 30

What are the two types of thermistors?

Answer 30

Negative temperature coefficient - Temp increases resistance decreases Positive temperature coefficient - temperature increases so does resistance

Question 31

What is a thermistor?

Answer 31

When two types of dissimilar are heated at the 'hot junction' a voltage is generated. This can be calibrated against a temperature scale.

Question 32

What is a relay?

Answer 32

An electrically operated switch. Used magnet to operate switching mechanism Are used when a circuit is controlled by a low power signal

Question 33

What is a DIAC

Answer 33

Are a form of solid state switch used to switch AC voltage. Known as a thyristor. like a junction transistor without a base.

Question 34

What is a TRIAC.

Answer 34

TRIAC's conduct current in both directions. Commonly used in lighting controllers, will conduct until a voltage applied reaches zero.

Question 35

What is a thyristor?

Answer 35

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.

Question 36

What is an inverter?

Answer 36

Inverters are used to convert D.C. to A.C.