Principles of AC Circuits Flashcards
How does inductance affect current flow in an AC circuit?
As the current in a coil changes, a constantly expanding and contracting magnetic field is created. This produces a constant counter EMF, which limits the total current which can flow in the circuit.
How does capacitance affect current flow in an AC circuit?
As the voltage changes, the polarity across the capacitor is constantly reversed. The capacitor’s stored voltage, will therefore constantly be opposing the source voltage, which results in a lower net voltage. This means less current will be allowed to flow.
What five values contribute to AC (effective) resistance?
DC ohmic value, Eddy current losses, Hysterisis losses, Dielectric losses, Skin effect.
How do eddy currents affect AC resistance, and how do we limit this effect?
Heat losses due to induced circulating currents in an iron core require energy from the circuit to establish themselves.
We reduce this effect by laminating the core with thin electrically isolated sheets of metal.
How do hysterisis losses affect AC resistance, and how do we limit this effect?
Heat losses due to molecular friction (produced by constant reversal of the magnetic domains within) steal energy from the circuit.
We limit this effect by using a silicon steel alloy in the core so that it’s more “slippery”.
How do Dielectric losses affect AC resistance?
When the electrons within the insulation are distorted by a force due to the voltage, it takes energy from the circuit. This results in an increase of the power requirements, thereby increasing effective resistance.
What is skin effect?
The changing magnetic field produced by an AC conductor induces a counter EMF along the core of the conductor which causes electrons to flow better along the surface. This results in an effective loss of conductor area, increasing resistance.
What is one common real-world application to counter skin effect?
In high voltage transmission lines, the core of the wire is made of steel to give high tensile strength. Since current will be limited in the core anyway we gain an advantage by using steel there.
What is the SI unit for capacitance?
Farads (F).
What is the SI unit for inductance?
Henrys (H).
How is inductive reactance represented?
X_L
How is capacitive reactance represented?
X_C.
