Three Phase Theory Flashcards
When are single-phase systems used?
For small consumers i.e. residential households, only side-lines aka laterals are used for small customer loads at the lowest voltage levels, 415 V.
Why is the phasor representation used for 3-phase voltage?
- Easier to draw than sinusoids.
- The phasors rotate at a constant frequency.
- Horizontal projection give instantaneous value.
What are balanced three phases?
A set of three electical values that: have the same magnitude and are 120 degrees apart from each other.
Neutral wire in a Three-Phase System
- Since all currents are balanced, they can be combined into a single return wire known as the neutral wire, rather than six wires.
- In a balanced system, the neutral wire is unnecessary.
When is a neutral wire needed?
Near Loads where the three phases are seriously imbalanced. To carry the current and maintain voltage.
Advantages of 3-phase power systems over single-phase systems.
- Equivalent power can be transmitted with less wires.
- Generator torque is constant.
- Reduced Cost of transformers:
- Smaller core, shared by each phase.
- Can be represented on a single line diagram.
Star and Delta Connections
- Star: Phases are connected between the phase and a neutral point.
- Delta: Phases are connected across the windings.
Define: line voltage
(line-to-line voltage) is the RMS voltage of each phase to phase.
Define: Phase Voltage
The RMS voltage of each line voltage to neutral.
For a time series Energy =
integral of P dt
What is a kWh?
The amount of energy expended when using 1 kW load for an hour.
1 kWh = 3.6 e6
Difference Between Star and Delta Conection
- Star
- Equations
- Line current = Phase current
- Line voltage = √3 * Phase voltage
- Supplies 2 voltages and connects a single-phase load
- Equations
- Delta
- Equations
- Line voltage = Phase voltage
- Line current = √3 * Phase current
- Used for equal voltage situations e.g. motors
- Equations
