1.3. Alternating current fundamentals
Principle of generation of alternating voltages and currents and their equations and waveforms, average, peak and rms values
1. What is the principle behind the generation of alternating voltages and currents?
A. Faraday's Law of Electromagnetic Induction
B. Ohm's Law
C. Coulomb's Law
D. Kirchhoff's Law
Answer: A. Faraday's Law of Electromagnetic Induction
Explanation:
Alternating voltages and currents are generated based on Faraday's Law of Electromagnetic Induction, which states that a changing magnetic field induces an electromotive force (EMF) in a conductor. This principle forms the basis for the generation of alternating currents and voltages in electrical systems.
2. What is the equation that describes the relationship between the induced EMF and the rate of change of magnetic flux?
A. V = IR
B. V = IZ
C. EMF = dΦ/dt
D. P = VI
Answer: C. EMF = dΦ/dt
Explanation:
The equation that describes the relationship between the induced electromotive force (EMF) and the rate of change of magnetic flux (Φ) is given by Faraday's Law as EMF = dΦ/dt, where dΦ/dt represents the rate of change of magnetic flux.
3. What is the waveform of an ideal sinusoidal alternating voltage or current?
A. Square wave
B. Triangle wave
C. Sine wave
D. Sawtooth wave
Answer: C. Sine wave
Explanation:
The ideal waveform for sinusoidal alternating voltages and currents is a sine wave. It is characterized by a smooth, periodic oscillation and is commonly used in AC power systems.
4. What is the formula for calculating the average value of an alternating voltage or current over one complete cycle?
A. V_avg = V_peak/2
B. V_avg = V_peak/√2
C. V_avg = V_peak
D. V_avg = 2V_peak/π
Answer: B. V_avg = V_peak/√2
Explanation:
The average value (V_avg) of an alternating voltage or current over one complete cycle of a sine wave is calculated using the formula V_avg = V_peak/√2.
5. How is the RMS (Root Mean Square) value related to the peak value of an alternating voltage or current?
A. RMS = Peak
B. RMS = 2 * Peak
C. RMS = Peak/√2
D. RMS = Peak * √2
Answer: C. RMS = Peak/√2
Explanation:
The relationship between the RMS value and the peak value of an alternating voltage or current is given by RMS = Peak/√2. This relationship is essential for calculating the effective or equivalent value of AC quantities.
6. In the context of AC waveforms, what does the term "frequency" represent?
A. The rate of change of magnetic flux
B. The number of cycles per unit time
C. The amplitude of the waveform
D. The total energy of the waveform
Answer: B. The number of cycles per unit time
Explanation:
Frequency in AC waveforms refers to the number of cycles (complete oscillations) that occur in one second. It is measured in Hertz (Hz) and represents the rate at which the waveform repeats.
7. What is the equation for calculating the peak value of an alternating voltage or current if the RMS value is known?
A. Peak = RMS
B. Peak = RMS/√2
C. Peak = RMS * √2
D. Peak = 2 * RMS
Answer: C. Peak = RMS * √2
Explanation:
The relationship between the peak value (V_peak or I_peak) and the RMS value (V_RMS or I_RMS) is given by Peak = RMS * √2. This relationship is crucial for converting between RMS and peak values.
8. What is the term used to describe the phase difference between two AC waveforms with the same frequency?
A. Amplitude
B. Frequency
C. Phase angle
D. Wavelength
Answer: C. Phase angle
Explanation:
Phase angle represents the angular displacement between two AC waveforms with the same frequency. It is measured in degrees or radians and indicates the relative timing of the waveforms.
9. How is the power factor of an AC circuit defined?
A. The ratio of active power to apparent power
B. The ratio of active power to reactive power
C. The ratio of apparent power to reactive power
D. The ratio of apparent power to active power
Answer: A. The ratio of active power to apparent power
Explanation:
The power factor of an AC circuit is defined as the ratio of active power (real power) to apparent power. It is a measure of how effectively electrical power is being converted into useful work.
10. What is the purpose of a transformer in the context of AC power distribution?
A. To convert AC to DC
B. To change the frequency of AC
C. To step up or step down AC voltage
D. To regulate the power factor of AC circuits
Answer: C. To step up or step down AC voltage
Explanation:
Transformers are used in AC power distribution to step up (increase) or step down (decrease) the voltage levels, facilitating efficient transmission and distribution of electrical energy.
THREE PHASE SYSTEM
1. What is the primary advantage of a three-phase power system over a single-phase system in electrical distribution?
A. Higher voltage levels
B. Greater power efficiency
C. Simplicity in design
D. Lower current requirements
Answer: B. Greater power efficiency
Explanation:
The primary advantage of a three-phase power system is its greater power efficiency compared to a single-phase system. Three-phase systems provide a more balanced and continuous power delivery, resulting in improved efficiency for the transmission and distribution of electrical energy.
2. In a balanced three-phase system, what is the relationship between the line voltage (VL) and the phase voltage (VP)?
A. VL = VP
B. VL = VP/√3
C. VL = VP * √3
D. VL = 3 * VP
Answer: C. VL = VP * √3
Explanation:
In a balanced three-phase system, the relationship between the line voltage (VL) and the phase voltage (VP) is given by VL = VP * √3. This factor accounts for the geometric arrangement of three-phase voltages in a system.
3. What is the purpose of a neutral wire in a three-phase system?
A. To provide a ground reference
B. To balance the system
C. To carry the highest current
D. To act as a safety feature
Answer: B. To balance the system
Explanation:
The neutral wire in a three-phase system is used to balance the system by providing a return path for unbalanced currents. It ensures that the currents in the three phases are equal, preventing undesirable effects such as overheating of equipment.
4. What is the angular displacement between the voltages of two consecutive phases in a balanced three-phase system?
A. 60 degrees
B. 90 degrees
C. 120 degrees
D. 180 degrees
Answer: C. 120 degrees
Explanation:
In a balanced three-phase system, the angular displacement between the voltages of two consecutive phases is 120 degrees. This arrangement ensures a continuous and smooth power delivery.
5. How is the total power (kVA) in a three-phase system related to the individual power in each phase?
A. Total power = Power in one phase
B. Total power = Power in one phase * 3
C. Total power = Power in one phase / √3
D. Total power = Power in one phase * √3
Answer: B. Total power = Power in one phase * 3
Explanation:
The total power (kVA) in a three-phase system is calculated by multiplying the power in one phase by 3. This accounts for the presence of three phases in the system.
6. In a three-phase system, what is the purpose of a delta (Δ) connection for the load?
A. To provide a common neutral point
B. To balance the system
C. To simplify wiring
D. To increase voltage stability
Answer: C. To simplify wiring
Explanation:
A delta (Δ) connection in a three-phase system is used to simplify wiring and reduce the number of conductors required for connecting the load. It is often chosen for its practical advantages in certain applications.
7. What is the significance of the term "power factor" in a three-phase system?
A. It indicates the efficiency of the system
B. It measures the total power consumption
C. It quantifies the balance between active and reactive power
D. It determines the voltage levels in the system
Answer: C. It quantifies the balance between active and reactive power
Explanation:
The power factor in a three-phase system quantifies the balance between active (real) power and reactive power. It is a crucial parameter for assessing the efficiency of the system and ensuring optimal power utilization.
8. How does a wye (Y) connection differ from a delta (Δ) connection in a three-phase system?
A. Wye connection has a higher voltage
B. Delta connection requires more conductors
C. Wye connection provides a neutral point
D. Delta connection simplifies phase balancing
Answer: C. Wye connection provides a neutral point
Explanation:
A wye (Y) connection in a three-phase system provides a neutral point, allowing the connection of single-phase loads and facilitating easier grounding. In contrast, a delta (Δ) connection does not have a neutral point.
9. What is the relationship between the line current (IL) and the phase current (IP) in a balanced three-phase system?
A. IL = IP
B. IL = IP/√3
C. IL = IP * √3
D. IL = 3 * IP
Answer: A. IL = IP
Explanation:
In a balanced three-phase system, the line current (IL) is equal to the phase current (IP). This equality holds true when the system is in balance.
10. Why is a three-phase system preferred in industrial applications over a single-phase system?
A. Higher voltage levels
B. Greater power efficiency
C. Simplicity in design
D. Lower transmission losses
Answer: B. Greater power efficiency
Explanation:
A three-phase system is preferred in industrial applications due to its greater power efficiency. The balanced and continuous power delivery of three-phase systems results in improved performance and efficiency in industrial settings.