AC Fundamentals Flashcards
3 main ways to measure amplitude?
- Peak value
- Peak to peak value
- RMS value (DC equivalent)
3 basic measurements used when measuring an AC waveform?
- Amplitude (Voltage, current and/or power)
- Frequency (Time related / number of cycles in a given waveform that occurs in 1 second)
- Phase angle (used to compare 2 sine waves of identical frequency)
Define peak?
- Measured from the horizontal axis to either the maximum positive value or maximum negative value
- 1 positive or negative half cycle
Define peak to peak?
- Measurement between the maximum positive and the maximum negative portion of the waveform
- Equal to 2 x peak value
Define RMS?
- Root mean square
- Average (0.707 of the peak value)
- We see the RMS
Formula for VRMS?
VRMS = Vpk x 0.707
VRMS = V pk-pk divided by 2 x 0.707
Define cycle with regards to waveforms?
- 1 full positive and 1 full negative in a horizontal fashion across the axis
Define frequency?
- Represented by the letter f
- Measure in Hz
- Refers to the number of cycles completed in 1 second
Define period?
- Represented by the letter t (time)
- Measured in seconds (s)
- The time it takes for a waveform to complete 1 cycle
Formula for frequency?
f = 1/t OR f = t to the power of -1
Formula for period?
t = 1/f OR t = f to the power of -1
Define phase angle?
- The degree of separation between 2 waveforms of the same frequency
What does a Oscilloscope allow us to test?
Allows us to measure circuit values, such as:
- Voltage (pk=pk, pk and RMS)
- Frequency
- Period
- Phase angle
Oscilloscope construction?
- Liquid crystal display (LCD)
- Menu soft buttons
- Menu option buttons
- Channel 1, 2 and Maths settings
- Horizontal settings
- Miscellaneous buttons
- Trigger settings
- Probe connections
Probe construction?
- Earth alligator clip
- Probe tip (with witches hat extension)
- Variable compensation capacitor
- BNC connector
How to check calibration of oscilloscope?
Check the label is:
- In tact
- In date
- Signed
- Legible
Serviceability check for oscilloscope?
- Physically check for damage and ensure the power lead has a ‘test-n-tag’ label attached (in-tact, in date, signed and legible)
- Allow ‘power-on built-in-test’ to complete. Once this is finished, ensure all tests are displayed as passed
- Probe calibration
Oscilloscope safety?
- Ensure probes are connected to the testing equipment before connecting to the circuit under test
- Ensure alligator clip is only ever connected to the earthing point
- Ensure the maximum voltage is not exceeded
- Ensure the maximum voltage of probe is not exceeded
- Take care when disconnecting and connecting the oscilloscope
Phase angle voltmeter construction?
- On / Off switch
- Liquid crystal display (LCD)
- Test port
- Menu navigation buttons
- Reference input
- Signal input
- Charging indicator LED
- 5V DC input
- Charged indicator LED
How to check calibration of a phase angle voltmeter?
Check the label is:
- In tact
- In date
- Signed
- Legible
Serviceability test for phase angle voltmeter?
- Turn on
- Press the < button to start check and then connect the red and black leads into the reference ports and test ports
- Press the > button. Ensure the frequency reads 400Hz and the voltage reads 4.8V pk-pk
- Press the > button and then connect the red and black leads into the signal ports and test ports
- Press the > button. Ensure the frequency reads 400Hz and the voltage reads 4.8V pk-pk
3 different screen displays on the phase angle voltmeter?
- Phase angle info screen
- Signal info screen
- Reference info screen
3 types of transformer losess?
- Eddy current losses (Current that circulates in the core - this loss is given off as heat and can be extremely dangerous)
- Copper losses (Power lost due to resistance of the wire - this loss is given off as heat)
- Hysteresis losses (Reversing magnetism of a transformers core will release energy in the form of heat)
Construction of a transformer?
- Primary and secondary winding and the core
- Needs strong magnetic coupling (higher the magnetic coupling - the higher the induced voltage into the secondary coil, with as little loss as possible)
- Both coils are wound around a core (creates a physical structure and magnetic path for the flux line)
Formula for X L1
XL1 = 2 x pie x f x L
Formula for X C1
XC1 = 1 / 2 x pie x f x L
Formula for current total?
IT = VA / Z
Define reactive power?
Formula for reactive power?
- Power that does does not do work in the circuit - it is lazy
- Applies to inductor and capacitors
Q = IT squared x X
Define active power?
Formula for active power?
Power that is actually doing work in a circuit
P = I squared x R1
Define apparent power?
Formula for apparent power?
Combination of active and reactive power
Pol (x, y) OR S = I squared x Z
What makes up a power phasor diagram?
X axis - P (Active power)
Y axis - Q (Reactive power)
- S (Apparent power)
- Angle
What makes up a voltage phasor diagram?
X axis - VR1
Y axis - VC1 or VL1 or VEQ
What makes up an impedance phasor diagram?
X axis - R1
Y axis - XC1 or XL1 or XEQ
- Z (impedance)
- Angle
Define mutual induction?
Changing current in one coil, induces EMF into the other
The magnitude of the EMF is found to be directly proportional to 2 things. What are these?
- Strength of the magnetic field
- The speed that the magnetic field is expanding and collapsing
Define the basic operations of a transformer?
A transformer will operate when an AC voltage is applied to its primary winding, then through mutual induction, an AC voltage is induced into the transformers secondary winding
3 types of transformer cores?
- Open core
- Closed core
- Shell core
The amount of resistance in each winding depends on what 3 things?
- Number of turns (length of the wire)
- Thickness of the wire
- The material that the wire is made of
3 types of transformers?
- Step up (Larger amount of voltage on the secondary)
- Step down (Smaller amount of voltage in the secondary)
- Isolation (1:1 ratio)
What is the voltage / current relationship in a purely capacitive circuit?
Current leads voltage
What is the voltage / current relationship in a purely inductive circuit?
Voltage leads current
What is the voltage / current relationship in a purely resistive circuit?
In phase
In a purely resistive circuit why does the power waveform always sit above the x axis?
- Because all of the power is being used for a useful purpose
- No waste
In a purely reactive circuit, why does the power waveform continuously go positive to negative?
Power is being used both for a useful purpose and also being returned to the power supply
3 types of materials used for transformers?
- Iron
- Ferrite
- Air
How many degrees are in 1 full wave cycle?
360 degrees
Characteristics of an open core?
- Poor magnetic coupling
- Cheap
Characteristics of a closed core?
- Forms a loop independently wound - primary winding and secondary winding
- Closed core
- They do not touch
- More efficient than the open core but still has losses
Characteristics of a shell core?
- Wound around each other
- Tight magnetic coupling
- Highest induced voltage
- Best option for best performance and least loss
