Lecture 1

Question 1

Where is the energy source for power plants usually from?

Answer 1

1. Gravitational Energy
2. Thermal Energy

Question 2

Energy obtained from either source is used to do what to a turbine? What is the turbine connected to?

Answer 2

Rotate the turbine; electrical generator

Question 3

What is the usual rpm of a turbine? What is the frequency?

Answer 3

3000rpm; 50Hz

Question 4

What is the normal electrical energy supply in Jamaica? What is the voltage variation?

Answer 4

50Hz at 220V; 6%

Question 5

What system does the generator use for the production of electromotive force (emf)?

Answer 5

Three phase system

Question 6

In a three phase system:
1. How many degree are the three symmetrical coins mounted apart?
2. Thus, how many times will each coil peak in one rotation?
3. How many peaks occur in total?
4. What are the peaks separated by?
5. What voltage would each phase then supply?

Answer 6

1. 120 degrees
2. Twice
3. Six
4. The time taken for the coil to rotate 120
5. 220V

Question 7

Give 3 advantages of the three phase system

Answer 7

1. More efficient use of the generator
2. Smoother electrical supply in comparison to that of single sinusoidal output
3. More economical use of conductors

Question 8

When the electric current is produced at the power plant, it is then conducted to what?

Answer 8

Step up transformer

Question 9

At the consumer end, what happens to the voltage?

Answer 9

The voltage is stepped down at a sub-station by a transformer to 33kV, then down to 11kV by a transformer for local distribution, and finally by another transformer to 240V(UK), (220V) for domestic use.

Question 10

Equipment requiring a 3 phase supply is usually connected by a ___ connection?

Answer 10

Delta

Question 11

1.Where does the hospital receive its supply?
2.In case of power failure, where are services supplied from?

Answer 11

1. Sub-station
2. Local petrol or diesel driven generator

Question 12

What is Direct current? (DC)

Answer 12

DC current is electric current flowing in one direction

Question 13

Name 2 sources that produce DC current

Answer 13

Battery, inverters (convert AC to DC)

Question 14

What is Alternating Current? (AC)

Answer 14

Alternating current is electric current that flows in multiple directions. This is due to the electrons moving back & forth in the conducting wire.

Question 15

Compare AC & DC with respect to:
1. Amt of energy that can be carried
2. Cause of direction of flow of electrons
3. Frequency
4. Direction
5. Current
6. Flow of electrons

Answer 15

1. AC carries energy far distances; DC cannot travel far until it begins to lose energy
2. AC: rotating magnet along wire; DC: steady magnetism along wire
3. AC: 50Hz, DC: 0
4. AC: reverses direction, DC: unidirectional
5. AC: current of magnitude varying with time; DC: current of constant magnitude
6. AC: backwards & forwards; DC: forward

Question 16

1. Why must all current carrying conductors be insulated?
2. What is the amount of insulation dependent on?

Answer 16

1. To protect the user against shock
2. The potential difference of the conductor to the earth or other conductors at different potential

Question 17

What material is most often used as the conductor?

Answer 17

Copper

Question 18

The greater the current carried by the conductor, what must be done to minimize voltage drop?

Answer 18

The larger the size of the conductor must be.

Question 19

What colour is the live cable, neutral cable and the earth cable?

Answer 19

Live: Brown
Neutral: Blue
Earth: Green w/ yellow stripes

Question 20

Each cable is rated in terms of the (maximum or minimum?) ____ it can safely carry and (maximum or minimum?) _____ its insulation can withstand to earth.

Answer 20

maximum current it can safely carry; maximum voltage its insulation can withstand to earth.

Question 21

What prevents damage to the high tension cable in the x-ray unit?

Answer 21

The high tension cable is surrounded by a thick rubber insulator which is enclosed by a metallic braid. At points where it needs to be bent, it is equipped with a metal snake which prevents it from bending too much and preventing damage to the conductors and/or the insulation

Question 22

1. What is earthing?
2. What is its purpose?

Answer 22

Earthing is conducting all metal parts of electrical equipment to the earth, to ensure that in the event of breakdown in the insulators around the conductors, no serious hazard is present to the user.