Questions and Answers
What effect does an inductor have on a change in current?
An inductor opposes a change in current.
What is the phase relationship between current and voltage in an inductor?
Current lags voltage by 90º (ELI).
What is the term for the opposition an inductor presents to AC?
Inductive reactance.
What is the formula used to compute the value of this opposition?
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What happens to the value of XL as frequency increases?
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What happens to the value of XL as inductance decreases?
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What effect does the capacitor have on a changing voltage?
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What is the phase relationship between current and voltage in a capacitor?
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What is the term for the opposition that a capacitor presents to AC?
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What is the formula used to compute this opposition?
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What happens to the value of XC as frequency decreases?
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What happens to the value of XC as capacitance increases?
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What is the formula for determining total reactance in a series circuit where the values of XC and XL are known?
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What is the total amount of reactance (X) in a series circuit which contains an XL of 20 ohms and an XC of 50 ohms? (Indicate whether X is capacitive or inductive)
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What term is given to total opposition to AC in a circuit?
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What formula is used to calculate the amount of this opposition in a series circuit?
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What is the value of Z in a series AC circuit where XL = 6 ohms, XC = 3 ohms, and R = 4 ohms?
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What are the Ohm's law formulas used in an AC circuit to determine voltage and current?
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What is the true power in an AC circuit?
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What is the unit of measurement of true power?
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What is the formula for calculating true power?
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What is the reactive power in an AC circuit?
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What is the unit of measurement for reactive power?
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What is the formula for computing reactive power?
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What is apparent power?
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What is the unit of measurement for apparent power?
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What is the formula for apparent power?
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What is the power factor of a circuit?
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What is a general formula used to calculate the power factor of a circuit?
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An AC circuit has a total reactance of 10 ohms inductive and a total resistance of 20 ohms. The power factor is 0.89. What would be necessary to correct the power factor to unity?
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What is the difference between calculating impedance in a series AC circuit and in a parallel AC circuit?
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An inductor in an AC circuit opposes any change in current flow just as it does in a DC circuit.
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Current lags the voltage by 90º in an inductor (ELI).
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The opposition an inductor offers to AC is called capacitive reactance.
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A capacitor in an AC circuit opposes any change in voltage just as it does in a DC circuit.
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Current leads the voltage by 90º in a capacitor (ICE).
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The total reactance of a series AC circuit can be determined by the formula X = XC − XL.
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Study Notes
Inductive Reactance
- An inductor opposes changes in current flow.
- Voltage leads current by 90º in inductors (known as ELI).
- Opposition to AC from an inductor is termed inductive reactance (XL).
- The formula for inductive reactance: XL = 2πfL.
- Inductive reactance increases with rising frequency.
- Inductive reactance decreases if inductance is reduced.
Capacitive Reactance
- A capacitor opposes changes in voltage.
- Current leads voltage by 90º in capacitors (termed ICE).
- Opposition to AC from a capacitor is known as capacitive reactance (XC).
- The formula for capacitive reactance: XC = 1 ÷ 2πfC.
- Capacitive reactance increases as frequency decreases.
- Capacitive reactance decreases with increasing capacitance.
Total Reactance in AC Circuits
- Total reactance in a series circuit can be computed using: X = XL - XC or X = XC - XL.
- Example calculation: if XL is 20 ohms and XC is 50 ohms, total reactance is 30 Ω (capacitive).
Impedance and Power in AC Circuits
- The total opposition to AC in a circuit is termed impedance (Z).
- The formula to calculate impedance in a series circuit is: Z = √(R² + X²).
- In series AC circuits with XL = 6 ohms, XC = 3 ohms, and R = 4 ohms, Z is 5Ω.
Power Concepts
- True power is the energy consumed by the circuit, measured in watts.
- Formula for true power: True Power = (IR)²R.
- Reactive power is the power returned to the source, measured in volt-amperes reactive (var).
- The formula for calculating reactive power: Reactive Power = (IX)²X.
- Apparent power represents total power perceived by the circuit, measured in volt-amperes (VA).
- Apparent power can also be calculated as: √(True Power² + Reactive Power²).
Power Factor
- Power factor (PF) calculates the efficiency of the circuit, expressed as: PF = True Power ÷ Apparent Power or PF = Cosθ.
- Desired power factor correction is close to unity (1 or 100%) to minimize losses.
- If total reactance is inductive, add capacitive reactance; if capacitive, add inductive reactance.
Phase Relationship and Calculations
- The phase angle (θ) indicates the degree to which current leads or lags voltage.
- For AC circuits, Ohm's law is represented as: E = IZ or I = E/Z.
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Test your knowledge on the concepts of inductive and capacitive reactance with these flashcards. Learn about the effects of inductors on current changes, phase relationships, and the terminology associated with reactance in AC circuits.