3.13: AC Theory Flashcards
- When a loop of wire is rotated through 360º in a magnetic field, the induced
voltage will be zero at which of the following points?
(a) 90º
(b) 180º
(c) 270º
(b) 180º
- The instantaneous voltage of an AC waveform is given as:
Vi = 115Sin 800Πt (volts).
What is the signal’s frequency?
(a) 400 Hz
(b) 115 Hz
(c) 128Hz
(a) 400 Hz
Note 800 should be equal to the angular velocity 2pif
Therefore, 800 = 2pif
f = 800 / (2*pi)
f = 128Hz
- During each cycle, a sine wave reaches a peak value;
(a) twice
(b) four times
(c) six times
(a) twice
An alternating voltage is represented by v = 20 sin 157.1t volts. Find maximum value and frequency
Maximum value = 20 volts
Frequency = 25 Hz
157.1 = 2pif
f = 157.1/(2*pi), f = 25 Hz
- A sine wave with a frequency of 15 kHz is changing at a faster rate than a sine
wave with a frequency of
(a) 20 kHz
(b) 10,000 Hz
(c) 1.25 MHz.
(b) 10,000 Hz
(a) A lags C by 90º
First ensure both have the same period, i.e both complete one cycle in 360^ check
Second, note at 0 degrees, C is at a maximum
At 90 degrees, A reaches its maximum
Therefore, A lags C by 90 degrees
- An alternating current completes 40 cycles in 0.1 seconds. Its frequency is :
(a) 400Hz
(b) 40Hz
(c) 20Hz
(a) 400Hz
Frequency is the number of cycles completed in 1 second and unit (Hz Hertz)
- If the peak value of a sine wave is 10 V, the peak-to-peak value is,
(a) 20V
(b) 5V
(c) 10V
(a) 20V
The voltage between two peaks is called the peak to peak voltage and is 2 X Peak value
- If an AC waveform has a frequency of 1kHz, what is its periodic time?
(a) 1 S
(b) 1 mS
(c) 10 mS
(b) 1 mS
Time period (in seconds) = 1/Frequency in Hertz
- An ac voltage of 250 hertz has a period Of
(a) 0.004 second
(b) 0.025 second
(c) 0.4 second
(a) 0.004 second
Time period = 1 / Frequency in Hertz
Period = 1 / 250
Period = 1 / 0.25x10^3
Period = 4x10^-3 or 0.004 second
- Which of the following mathematical formulas is used to find the average value of
voltage for an ac voltage?
(a) Eavg = 0.707 × Emax
(b) Eavg = 1.414 × Eeff
(c) Eavg = 0.636 × Emax
(c) Eavg = 0.636 × Emax
- If the RMS value of the voltage of an ac waveform is 12.4 volt what is its average
value?
(a) 8 volts
(b) 11 volts
(c) 15 volts
(b) 11 volts
- If an AC sinewave has an RMS value of 200V. What is its Peak Value?
(a) 282.8 V
(b) 141.4 V
(c) 200 V
(a) 282.8 V
RMS*1.41 = Peak
- In a series RC circuit, the voltage across the resistance is
(a) in phase with the source voltage
(b) lagging the source voltage by 90°
(c) in phase with the current
(c) in phase with the current
Note: RC circuit is a Resistor/Capacitor circuit
The voltage drop across a capacitor is proportional to its charge, and it is uncharged at the beginning; whereas the voltage across the resistor is proportional to the curent and there is a current at the start. But charge starts to build up on the capacitor, so some voltage is dropped across the capacitor now.
- In a series inductive and resistive circuit ,when XL = R the phase angle is;
(a) 0 degrees
(b) 45 degrees
(c) 90 degrees
(b) 45 degrees
Remember: XL is Inductive Reactance (XC is capacitor Reactance) and is measured in ohms (form of resistance)
Note: If inductance L > Resistance R, the angle between these two will be greater than 45 degrees and more close to 90 degrees
If Resistance R > Inductance L, the angle will be less than 45 degrees and closer to 0 degrees
- In a parallel RC circuit, there is 1A RMS through the resistive branch and 1A RMS
through capacitive branch. The total RMS current is
(a) 1 A
(b) 2 A
(c) 1.414 A
(c) 1.414 A
RC circuit is a resistor capacitor circuit
1 amp through each branch, total current in circuit 2 amps
2 Amps * 0.707 (RMS) = 1.414 A
- For a Sine Wave
(a) Vrms = Vp X square root of 2
(b) Vrms = Vp / square root of 2
(c) Vrms = -Vp
(b) Vrms = Vp / square root of 2
Square root of 2 = 1.14 1/sq root 2 = 1/1.41 = X.707
i.e 1/root2= 0.707
Vrms = Vp * 0.707 or 1/square root of 2
- What is the phase relationship between voltage and current in an inductor?
(a) the current is in phase with the voltage
(b) the current is 90 degrees before the voltage
(c) the voltage is 90 degrees before the current.
(c) the voltage is 90 degrees before the current.
CIVIL
- In a series RC circuit, the voltage across the capacitor is
(a) in phase with the source voltage.
(b) lagging the source current by 90°.
(c) leading the source current by 90°.
(b) lagging the source current by 90°.
- When the AC frequency increases in an resistive circuit, the current flow will:
(a) increase
(b) decrease
(c) remain the same
(c) remain the same
- When the AC frequency increases in an inductive circuit, the current flow will:
(a) increase
(b) decrease
(c) remain the same
(b) decrease
(inductors don’t like/oppose changes in current
- When the AC frequency increases in an capacitive circuit, the current flow will:
(a) increase
(b) decrease
(c) remain the same
(a) increase
- In a Balanced Delta System, if the current in each winding is equal to 5 Amps, what
is the line current equal to?
(a) 5 A
(b) 7.07 A
(c) 8.66 A
(c) 8.66 A
Note sq root 3 = 1.73
- A 3 phase AC circuit is Star Connected. The phase to phase voltage is 1700 Volts.
What is the voltage between phase and Neutral?
(a) 100 Volts
(b) Root 3 times 1700 volts.
(c) 1700 volts multiplied by one over root 3
(c) 1700 volts multiplied by one over root 3
- An alternating current completes 40 cycles in 0.2 seconds. Its frequency is :
(a) 400Hz
(b) 40Hz
(c) 200Hz
(c) 200Hz
What is the square root of 2 and 3 each?
Sq root 2 = 1.414
Sq root 3 = 1.732
- Unless otherwise specified, any values given for current and voltage in an AC
circuit are assumed to be:
a. average values
b. instantaneous values
c. effective values.
c. effective values.
'’The AC value required to produce the same amount of power or heating as its DC equivalent is known as its effective or RMS value’’
800 Hz
Average = Peak * ?
RMS = Peak * ?
Peak = RMS * ?
Average = Peak * 0.637
RMS = Peak * 0.707
Peak = RMS * 1.414
What is the calculation for RMS to average?
RMS to Average = RMS * 0.9
Series RC circuit. Voltage across the resistance is
Series RC circuit. Voltage across the resistance is in Phase with current
Series inductive and resistive circuit. What is the phase angle?
Series inductive and resistive circuit. Phase angle is 45^
Phase relationship between Voltage and Current in an inductor?
Phase relationship between V & I in a conductor V is 90^ before I
Series RC circuit. Are Voltage and current in phase with each other?
Series RC circuit. Voltage across capacitor lagging the source current by 90^
AC frequency increase.
Resistive circuit, current flow = ?
Capacitive circuit, current flow = ?
Inductive circuit, current flow = ?
AC frequency increase.
Resistive circuit, current flow = remains the same
Capacitive circuit, current flow = increases
Inductive circuit, current flow = decreases
What is the formula for line current in a balanced Delta system?
What is the formula for phase to phase voltage in a 3 phase AC star system?
Question Number. 1. Convention requires that in a symmetrical 3 phase system, the.
Option A. red voltage is taken as the reference phase.
Option B. yellow voltage phase leads the red phase by 120 degrees.
Option C. red voltage phase leads the blue phase by 120 degrees.
Option A. red voltage is taken as the reference phase.
