3.14 RLC Circuits Flashcards
What is the difference between resistance and reactance in electrical circuits?
Resistance dissipates energy as heat, while reactance stores energy and returns it to the circuit
Reactance is measured in ohms but is not the same as resistance.
What are the two types of reactance?
- Capacitive reactance
- Inductive reactance
How do AC voltage and current behave in circuits with reactance?
They are out of phase with each other.
What does LCR stand for in electrical circuits?
Inductors, Capacitors, and Resistors.
True or False: In a DC circuit, both AC and DC light bulbs glow with the same brightness.
True
What is a ‘real load’ in electrical circuits?
A load that produces a measurable effect, converting electrical energy into another form of energy.
What is the phase relationship between current and voltage in a real load?
Current and voltage are in phase.
In an AC circuit, how can resistance be calculated?
Using the RMS values of voltage and current by applying Ohm’s law.
What is the effect of inductive reactance on AC current?
It causes the AC current to be smaller than the DC current.
What causes the phase shift in a purely inductive circuit?
The self-induced voltage opposes the applied voltage.
What is the phase angle between current and voltage in a purely inductive circuit?
90° or π/2 radians.
What is a phasor diagram?
An alternative way of representing sinusoidal variables using vectors that rotate at a steady angular speed.
How is inductive reactance affected by frequency?
It increases with higher frequency.
What happens to AC current in a capacitor compared to a DC circuit?
In a DC circuit, no current flows after initial charging, while in an AC circuit, the current repeatedly rises and falls.
What is capacitive reactance?
The opposition a capacitor presents to the flow of AC current.
How does the phase relationship between current and voltage behave in a circuit with only capacitive reactance?
The current leads the applied voltage by 90°.
What happens at the peak values of voltage in a capacitive circuit?
Voltage is at its peak value when current is zero.
What is the formula for calculating capacitive reactance?
Capacitive reactance = 1 / (2πfC)
What is the effect of increasing capacitance on capacitive reactance?
It decreases capacitive reactance.
How does frequency affect capacitive reactance?
It decreases as frequency increases.
What are the two types of reactance in AC circuits?
Capacitive reactance and inductive reactance
What is reactive energy?
Energy stored in the electric field of a capacitor or in the magnetic field of an inductor during a cycle of AC
What is the formula for true power in AC circuits?
P = V * I
Where P is power in watts (W), V is voltage in volts (V), and I is resistive current in amps (A)
What is true power also known as?
Real power or effective power
What is the unit for true power?
Watt (W)
In a pure inductor, how does the current relate to the voltage?
Current lags the voltage by 90°
What does positive power represent in a purely inductive load?
Power supplied to the inductor by the circuit
What is inductive reactive power?
Power that is not dissipated as heat, light, or mechanical energy in a purely inductive load
What is the symbol for inductive reactive power?
Q_L
What is the unit for inductive reactive power?
Volt-ampere reactive (VAr)
In a purely capacitive load, how does the current relate to the voltage?
Current leads the voltage by 90°
What is capacitive reactive power?
Power that is not dissipated as heat, light, or mechanical energy in a purely capacitive load
What is the symbol for capacitive reactive power?
Q_C
What is the unit for capacitive reactive power?
Volt-ampere reactive (VAr)
How can a real coil be represented for analysis?
As a pure inductor in series with a pure resistor
What is apparent power?
The product of RMS voltage and RMS current measured in an AC circuit
What is the formula for apparent power?
S = V * I
Where S is apparent power in volt-ampere (VA), V is RMS voltage in volts (V), and I is RMS current in amps (A)
What is the unit for apparent power?
Volt-ampere (VA)
True or False: The true power is always greater than the apparent power.
False
What is the opposition to current flow in a purely resistive circuit called?
Resistance R
In an AC circuit, what is the term used for the opposition to current flow caused by inductors and capacitors?
Reactance
What symbols represent inductive and capacitive reactance?
Inductive reactance: XL, Capacitive reactance: XC
What is the overall measure of opposition to AC in a circuit called?
Impedance Z
How is impedance Z calculated in relation to resistance R and reactance?
Vector addition (geometric addition) must be used
What theorem is applied to determine unknown values of power or current in AC circuits?
Pythagoras’s theorem
What is the relationship between inductive reactance and frequency?
Inductive reactance is directly proportional to frequency
What happens to capacitive reactance as frequency increases?
Capacitive reactance decreases
What effect does high frequency have on the resistance of a conductor?
Resistance can increase due to the skin effect
What is the effective resistance range below which it is considered constant?
0 Hz to approximately 1000 Hz
What is the formula that relates RMS voltage, RMS current, and impedance in AC circuits?
Z = V / I
In a series connection of a resistor and inductor, how does the current relate to the voltage across the resistor?
The voltage VR is in phase with the current I
In a coil, how does the current relate to the voltage across the inductive reactance?
The current I lags the voltage VL by 90°
What is the shape of the triangle formed in the phasor diagram for a series connection of resistance and inductance?
Right-angled triangle
What is the phase relationship between the voltage phasor VL and the current phasor I?
Voltage phasor VL is perpendicular to current phasor I
What geometric method is used to determine the resultant voltage across a coil?
Geometric addition of the phasors VR and VL
In a series connection of a resistor and capacitor, how does the current relate to the voltage across the resistor?
The voltage VR is in phase with the current I
How does the current relate to the voltage across the capacitive reactance in a series connection?
The current I leads the voltage VC by 90°
What is the term for the resistance present in real capacitors?
Loss resistance
What is the phase angle between the total current I and the applied voltage V in a parallel circuit?
Between 0° and 90°
What is the formula for calculating total current in a parallel resistor and capacitor circuit?
I = IR + IC
What happens to the phase angle in real capacitors due to loss resistance?
The phase angle is always less than 90°
In a series connection of a resistor, coil, and capacitor, what must be performed to find the total voltage?
Geometric addition of the voltages across VR, VL, and VC
What is the relationship between the voltages VL and VC in a series connection?
They act in opposite directions and can be subtracted from one another
What is the relationship between voltage and current in a coil?
For the coil, the voltage leads the current
This indicates that the voltage reaches its maximum value before the current does.
What is the relationship between voltage and current in a capacitor?
For the capacitor, the voltage lags the current
This means that the current reaches its maximum value before the voltage does.
In a series circuit with a resistor, coil, and capacitor, what is the phase angle between the voltage phasors VL and VC?
180°
This indicates that VL and VC act in opposite directions.
How is the supplied RMS voltage calculated in a series circuit with L, C, and R?
It is equal to the geometric sum of phasors for the three component voltages.
What theorem is applied to calculate unknown values in LCR series circuits?
Pythagoras’s theorem
What are the three types of reactance in LCR circuits?
- Capacitive reactance (XC)
- Inductive reactance (XL)
- Resistance (R)
When inductive reactance XL is greater than capacitive reactance XC, how is the circuit characterized?
The circuit is mainly inductive.
When capacitive reactance XC is greater than inductive reactance XL, how is the circuit characterized?
The circuit is mainly capacitive.
What occurs when inductive reactance XL and capacitive reactance XC are equal?
The circuit operates like a purely resistive load.
In a parallel circuit with a resistor, inductor, and capacitor, what is the common factor?
The applied RMS voltage V
What happens to the supplied current I in a parallel circuit with L, C, and R?
It is equal to the geometric sum of the three current phasors.
What is the phase relationship between the current phasors IL and IC in a parallel circuit?
They have a phase angle of 180°.
What happens if the current through the inductive reactance is higher than that through the capacitive reactance?
The circuit is inductive.
What happens if the current through the capacitive reactance is higher than that through the inductive reactance?
The circuit is capacitive.
What does the power factor indicate?
The fraction of apparent power S that is converted into true power P.
What is the ideal value for the power factor to maximize true power?
As close to 1 as possible.
Fill in the blank: The power factor is affected by the presence of _______ in a device or circuit.
inductive reactance
What does a power factor of less than 1 typically indicate?
The presence of reactive power in the circuit.
What is real power?
Real power is transferred to the environment by the circuit or device.
What is reactive power?
Reactive power is stored and returned to the circuit later in the AC cycle.
Why is reactive power important in AC circuits?
It increases the supply voltage and power, making it an important consideration in design, operation, and maintenance.
How must power be treated when designing AC electrical systems?
Power must be treated as a vector quantity.
What mathematical theorem is applied in power calculations for AC systems?
Pythagoras’s theorem.
What is the relationship between voltage, current, and power in AC circuits?
One can multiply each voltage by the current to determine the power associated with each.
What is the formula for apparent power?
Apparent power is the result of the geometric addition of true and reactive power.
What units are used to rate AC generators and transformers?
Volt-Ampere (VA).
What units are used to indicate the mechanical output power of engines?
Watts (W).
Fill in the blank: Apparent power is the result of the geometric addition of _______ and _______.
true power, reactive power.
True or False: True power is always equal to apparent power in AC circuits.
False.
What do the information plates of engines indicate?
The mechanical output power delivered to the shaft in units of Watts.
What must electrical systems and transmission lines be designed for?
Apparent power.
What is the main consideration when analyzing voltages across inductance and resistance in series?
Vector analysis.
What type of functions can be applied to calculate power in AC systems?
Trigonometric functions.
