Study Unit 1 Flashcards
What is the difference between J, W, Wh and Ah?
J - Energy
W - Power (Energy/Time)
Wh - Energy
Ah - Ampère * Hrs [C] (Charge)
How much chemical energy is there in a standard barrel of oil?
6GJ
How do you convert Ah to kWh?
kWh = V*Ah
DC Power
Unidirectional flow or movement of electrons from the negative pole to the positive pole. The polarity of the voltage does not change.
With what is DC power generated?
DC power is produced by batteries, fuel cells, rectifiers, and generators with commutators
How is DC power generated?
- A rotating armature is placed in a uniform magnetic field which causes a change in magnetic flux in the loop.
- According to Faraday’s Law this change will induce an emf that causes the electrons in the conductor to move, creating a current.
- Each arm of the armature is connected to a half of a split-ring that rotates with the armature. As the ring rotates, it slips against metal brushes, delivering current to the external circuit.
- The rotation of the commutator ensures that the brush touches a different side of the circuit with every rotation, which is essential for DC power generation.
Rectification
AC periodically reverses direction to DC, which then flows in only one direction. This reverse operation is performed by the inverter.
How is AC power generated?
- A rotating armature is placed in a uniform magnetic field which causes a change in magnetic flux in the loop.
- According to Faraday’s Law this change will induce an emf that causes the electrons in the conductor to move, creating a current in a certain direction.
- The loop will rotate 180 degrees and then the emf will reverse to the opposite direction, causing the current to change direction as well.
- The armature is connected to slip rings which ensure that the ends of the armature are always connected to the same side of the circuit. This ensures that the direction of the current changes with every half revolution.
Right-hand rule
Generators
Left-hand rule
Motors
Disadvantages of DC power
- DC voltage could not be transformed easily
- DC was generated at 110 V
- Very thick copper conductors required
- Generating plants had to be located within 1 to 2 km
- Expensive option
What frequency is used for electricity in South Africa?
South Africa operates on a 230V supply voltage and 50Hz.
What is meant by 50 Hz?
50 full sine waves per second (cycles per second)
What is the negative impact of a too low frequency?
A too low frequency (below 20 Hz) is useless for domestic lighting as the human eye records this as flicker
What is the negative impact of a too high frequency?
- Hysteresis losses in the transformer core
- Eddy current losses
- Capacitive reactance of cables and transmission lines
- Inductive reactance and related voltage drop
- Electromagnetic interference with radio traffic
Why was AC chosen over DC?
• DC system would have been too expensive
• AC became the standard
• Transformers easily transformed voltage from lower to
higher levels and vice versa
Single phase power
The distribution of AC where the voltage varies in unison. Requires two conductors (phase and neutral)
Eg. Lighting and heating
Wires in single phase power
Live
Neutral
Earth
Purpose of earth wire in single phase power
Safety feature in case the insulation or proper connection of the live or neutral conductors fail.
Three phase power
Most common method of AC electric power generation, transmission and distribution, consisting of three phases that are 120 degrees out of phase. Capacity to conductor material is doubled with three-phase.
Watt
Volt x Ampere
