Lecture 1 Flashcards
Where is the energy source for power plants usually from?
- Gravitational Energy
- Thermal Energy
Energy obtained from either source is used to do what to a turbine? What is the turbine connected to?
Rotate the turbine; electrical generator
What is the usual rpm of a turbine? What is the frequency?
3000rpm; 50Hz
What is the normal electrical energy supply in Jamaica? What is the voltage variation?
50Hz at 220V; 6%
What system does the generator use for the production of electromotive force (emf)?
Three phase system
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?
- 120 degrees
- Twice
- Six
- The time taken for the coil to rotate 120
- 220V
Give 3 advantages of the three phase system
- More efficient use of the generator
- Smoother electrical supply in comparison to that of single sinusoidal output
- More economical use of conductors
When the electric current is produced at the power plant, it is then conducted to what?
Step up transformer
At the consumer end, what happens to the voltage?
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.
Equipment requiring a 3 phase supply is usually connected by a ___ connection?
Delta
1.Where does the hospital receive its supply?
2.In case of power failure, where are services supplied from?
- Sub-station
- Local petrol or diesel driven generator
What is Direct current? (DC)
DC current is electric current flowing in one direction
Name 2 sources that produce DC current
Battery, inverters (convert AC to DC)
What is Alternating Current? (AC)
Alternating current is electric current that flows in multiple directions. This is due to the electrons moving back & forth in the conducting wire.
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
- AC carries energy far distances; DC cannot travel far until it begins to lose energy
- AC: rotating magnet along wire; DC: steady magnetism along wire
- AC: 50Hz, DC: 0
- AC: reverses direction, DC: unidirectional
- AC: current of magnitude varying with time; DC: current of constant magnitude
- AC: backwards & forwards; DC: forward
- Why must all current carrying conductors be insulated?
- What is the amount of insulation dependent on?
- To protect the user against shock
- The potential difference of the conductor to the earth or other conductors at different potential
What material is most often used as the conductor?
Copper
The greater the current carried by the conductor, what must be done to minimize voltage drop?
The larger the size of the conductor must be.
What colour is the live cable, neutral cable and the earth cable?
Live: Brown
Neutral: Blue
Earth: Green w/ yellow stripes
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.
maximum current it can safely carry; maximum voltage its insulation can withstand to earth.
What prevents damage to the high tension cable in the x-ray unit?
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
- What is earthing?
- What is its purpose?
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.
