Basic AC Principles Flashcards
Inductance
Property of a device or circuit which opposes a change in current flow
Inductors oppose changes in current flow and therefore offer opposition to alternating current flow
Inductance characteristics can be increased by
Increasing the number of turns on the coil
Inserting a piece of soft iron into the coil
Capacitance
Property of an electrical component which is able to store energy
Capacitor charging
When connected to a DC circuit the capacitor charges to the max value and remains charged until the energy is released by connecting it to a complete circuit
Factors determining capacitance
Area of the plates - a
Distance between the plates - d
Types of dielectric - k
C = ka/d
Advantages of AC
Can be efficiently charged from one voltage to another using a transformer
Simpler and lighter in construction than DC machines
Can be efficiently converted to DC with a rectifier
RMS =
0.707 x peak
Average value = 0.637 x peak
The RMS value of AC is that which has the same heating effect as the same value as DC
Frequency of AC generators
=NP/60
N = speed in RPM P= number of pairs of poles (per phase)
Periodicity = 1/freq
Purely resistive AC circuit
Voltage and current are in phase
All to do with power - consumed in the form heat like a DC circuit
Purely capacitive AC circuit
Voltage and current are out of phase by 90* - current leads voltage
Would not produce any useable power as it is out of phase
Capacitive reactance
In an AC circuit the capacitor is constantly charging and discharging - because of the time lag - capacitor is in constant opposition to the supply voltage
Opposition to the current flow = capacitive reactance
Inversely proportional to freq
Purely inductive AC circuit
Current lags behind the voltage by 90* - voltage leads
Inductor opposes changes in current flow - opposes the flow of AC
Directly proportional to freq
Impedance (Z)
Total opposition to current flow in an AC circuit
Resonant freq
XL = Xc - cancel each other out
If you control the freq to the correct level you can make it purely resistive
Power in an RLC AC circuit
Generally given in kilo volt amps
Power = true power/ apparent power
100% = voltage and current are in phase - true and apparent are equal
