AC Theory Flashcards
Convention requires that in a symmetrical 3 phase system, the
red voltage is taken as the reference phase.
What is the phase difference in a circuit with 100V, drawing 0.5 amps, consuming 50 Watts?
0°
When power is in Watts, it is all true power - there is no apparent power.
The power factor of a circuit containing an imbalance of inductive and capacitive reactance is
less than unity.
If you apply this waveform to this circuit, what is the output across the capacitor?
Pulsed positive.
With the output taken across the capacitor, this is a basic integrator - so the output is.
What value is the same as the equivalent D.C. heating effect?
Root Mean Square.
What shape is the waveform when the input pulse and the time base are unequal?
Rectangular.
A sine wave has 5 amps RMS value. What is the peak value?
7.07 amps.
Peak = 1.414 * RMS (also remember that 5 is half of 10).
What is the wave shape of the x-axis input of a traversing oscilloscope image?
Triangular.
In a purely resistive AC circuit, the current vector is
in phase with the voltage vector.
When comparing the average values of an AC generator output, to the peak values
average voltage = 0.63 * peak voltage.
A parallel circuit at resonance has
maximum impedance.
A 10 ohm resistor has a 14.14 V peak drop across it. What power is dissipated?
10 W.
Since it mentions peak, it is AC. Always use RMS voltage when calculating power. RMS voltage = peak voltage / 1.414 (more than coincidence that 14.14 / 1.414 = 10). Power = V2/ R.
If a 1 ohm circuit produces a 50 W output, the phase angle is
0°.
What does a rectifier do?
Changes alternating current into direct current.
The ratio between apparent power and true power is the
power factor.
One advantage of using AC electrical power in aircraft is
AC electrical motors can be reversed while DC motors cannot.
Unless otherwise specified, any values given for current or voltage in an AC circuit are assumed to be
effective values.
Which aircraft circuit would be most likely to use frequency wild 200V AC?
Windscreen heating.
In an AC circuit how is the value of true power calculated?
By volts multiplied by amps multiplied by power factor.
One of the chief advantages of alternating current is that it can be transmitted at a high voltage with a low power loss; the voltage can then be changed to any desired value of
AC by means of transformers.
How many amperes will a 28-volt generator be required to supply to a circuit containing five lamps in parallel, three of which have a resistance of 6 ohms each and two of which have a resistance of 5 ohms each?
25.23 amperes.
Frequency (Hz) is the number of cycles per
second.
The frequency of a power wave in an AC resistance circuit is
twice the frequency for voltage and current.
In an AC circuit, the effective voltage is
less than the maximum instantaneous voltage.
Which wave form is stepped positive and negative with unequal length sides?
Trapezoidal wave.
The value of an AC sine wave that will give an equivalent heating effect in a DC resistor is the
RMS value.
The peak factor for a sine wave is
1.414.
An integrated step input at unequal time base produces
a saw tooth wave.
The mark-to-space ratio of the waveform shown is
A to D / D to E.
Form Factor for a sine wave AC output is
1.1.
A sine wave of RMS value 7.07 volts has a peak to peak value of
20.0 volts.
The true power in an AC circuit is given by
PF * volts * amps.
The average value of 100 volts peak AC is
63.7 volts.
The RMS value of 200 volts peak to peak is
141.4 volts.
The power factor of an AC circuit is the
cosine of the phase angle.
The Form Factor of an AC waveform can be found by
RMS value divided by the average value.
A differentiator has a time constant which is
short.
If the frequency is increased in an AC circuit of pure resistance, the effect is
nothing.
What does a differentiator provide?
A voltage proportional to the rate of change of the input.
What happens if the frequency is increased in an AC circuit of pure resistance?
Nothing.
What sort of wave is used in a CR oscilloscope to control sweep?
Sawtooth.
If an AC sine wave has an RMS value of 5V, what is its peak value?
7.07 V
Peak value = 1.414 * RMS = 1.414 * 5 = 7.07 V.
How is apparent power in an AC circuit found?
V * I.
If a sine wave has a periodic time of 0.5 milliseconds, what is its frequency?
2 kHz
Frequency is 1/time period. f = 1/T = 1/0.0005 = 10,000/5 = 2000 Hz = 2 kHz.
What is phase angle?
The difference in degrees of rotation between 2 alternating voltages or currents or a voltage and a current.
In what unit is the impedance of an AC circuit measured?
Ohms.
What happens to the current in a series resistive resonant circuit?
It is maximum.
What is the power dissipated in a circuit with 115 V RMS, 2.5A at 60 degrees?
143 W
Apparent Power = 115 * 2.5 = 287.5. PF = cos 60 = 0.5. True Power = Apparent Power * PF.
In a resonant parallel circuit, what is the current?
Minimum.
What is power factor?
cos theta.
What is the peak factor in a standard AC circuit?
1.414 times max. value of applied voltage.
What is the controlling wave in a CRT?
Sawtooth.
What is a wave form having equal amplitude and time base?
Square waveform.
How are sine wave signals of a circular time base phased?
Phase separated by 90°.
What type of pulse does a differentiator have?
A short pulse.
What is the power factor in a purely resistive circuit?
Equal to 1.
