AET Study Group Notes:
Concepts of AC –
You will need to understand the parts of an AC waveform which are:
1.
2.
3.
4.
5.
6.
7.
Amplitude (voltage)
Frequency (how many times a second does the waveform repeat itself)
Interval (time of one complete cycle)
Peak Voltage (voltage between the baseline and the top or bottom of the waveform)
Peak-to-Peak Voltage
Effective Voltage (peak voltage times .707)
Root Mean Square (RMS) (the square root of the average of the sum of the squared voltages
along the waveform)
Be able to calculate the frequency of a waveform after determining its time interval using the formula F
= 1/t
Oersted’s Law – a steady electric current through a conductor always creates a magnetic field around
the conductor (states that there is a magnetic field)
From Oersted’s Law, the Left-Hand Rule for conductors was created – wrap your left hand around a
conductor with your thumb pointed in the direction of electron flow and your fingers will indicate the
direction of the magnetic field (states the direction of the magnetic field)
Faraday’s Law – the induced electromotive force in a conductor is directly proportional to the rate of
change of the magnetic flux in the conductor (used to calculate the amount of magnetism or strength of
the magnetic field)
Parallel wires carrying current in the same direction produce magnetic fields that combine and
strengthen to produce a larger magnetic field
Parallel wires carrying current in the opposite direction produce magnetic fields that oppose each other
or work to cancel each other out
Multiple waveforms with aligned peaks are said to be ‘in-phase’ – if the peaks are not aligned, a ‘phaseshift’ and the amount of phase shift is measured in degrees (0-360)
Inductance –
Inductance is the characteristic of a circuit that opposes the starting, stopping, or change in current
Designation for inductance is capital letter ‘L’
Measured in Henries (H)
One Henry equals the inductance required to induce 1V in an inductor by a change in current of 1A per
second
Inductance is basically inertia – coils in a circuit help stabilize the current by resisting changes in current
amplitude – they keep electrons flowing
Factor that affect the inductance of a coil:
1.
2.
3.
4.
5.
Number of turns in the coil
Diameter of the coil
Length of the coil
Material used in the core
Number of layers of windings
Calculating total inductance in series and parallel circuits is identical to calculating total resistance
Troubleshooting Inductors –
Normal resistance is 2-3 ohms
Infinite resistance indicates an open coil
Short in the coil would result in lower than normal resistance but that is sometimes to proves since the
normal resistance is only 2-3 ohms
A special meter can be used to measure the inductance of a coil
A short can be caused by windings melting together as a result of overheating – too much current
running through the coil
Windings can short to the core also – normally there will be infinite resistance between core and coil –
low resistance would indicate a short
Capacitance –
Capacitors store energy in an electrostatic field
Capacitors are essentially two metal plates separated by a material called a dielectric
Capacitance is represented with the capital letter ‘C’
Measured in Farads (F)
A 1 Farad capacitor stores one coulomb of electrons when a potential of one volts is applied across the
terminals
Factors that affect the value of a capacitor:
1. Plate area
2. Distance between the plates
3. Dielectric constant
Voltage rating of a capacitor should be 50% higher than the effective voltage applied to the capacitor
Exceeding the voltage rating will cause the dielectric to break down and results in arcing across the
plates
Calculating total capacitance in a series circuit is accomplished using the reciprocal of the sum of the
reciprocals of the individual capacitances formula
Troubleshooting Capacitors –
Capacitors can be tested with a special capacitance tester
To test with an ohmmeter, first discharge the capacitor and isolate it from the circuit, then connect the
ohmmeter and you should initially observe a meter movement toward zero ohms as the capacitor
charges (from the internal battery of the ohmmeter sending voltage through the capacitor), then the
meter movement will slowly move back toward infinite resistance as the capacitor becomes fully
charged
Continuous low ohms indicates a shorted capacitor – dielectric has broken down from overvoltage or
overheating
Continuous high ohms indicates an open capacitor
Inductive Reactance –
The opposition to current flow offered by an inductor
Represented with the Letters ‘XL’
Measured in ohms
Formula is XL =2πfL where:
1. XL= Inductive Reactance measured in ohms
2. f = AC Frequency
3. L = Inductance in Henries
Inductive reactance causes the current to lag the voltage
Inductive Capacitance –
The opposition to current flow offered by a capacitor
Represented with the Letters ‘XC’
Measured in ohms
Formula is XC = 1/(2πfC) where:
1. XC= Inductive Capacitance measured in ohms
2. f = AC Frequency
3. C = Capacitance in Farads
Capacitive reactance causes the current to lead the voltage
If inductive reactance causes current to lag voltage and capacitive reactance causes current to lead
voltage, they basically counter each other
If a circuit has 50 ohms of inductive reactance and 25 ohms of capacitive reactance, the inductive
reactance wins and the circuit will be more inductive than capacitive
Impedance is the total combined resistance (opposition to current flow) to the flow of alternating
current in an electrical circuit – this can be a combination of resistance from resistors, inductive
reactance, and capacitive reactance
Impedance is represented by the letter Z