Inductive and Capacitive Reactance
Q1. What effect does an inductor have on a change in current?
A1. An inductor opposes a change in current.
Q10. What is the formula used to compute this opposition?
A10. X = 1 ÷ 2πfc
Q11. What happens to the value of XC as frequency decreases?
A11. XC increases.
Q12. What happens to the value of XC as capacitance increases?
A12. XC decreases.
Q13. What is the formula for determining total reactance in a series circuit where the values of XC and XL are known?
A13. X = XL - XC or X = XC - XL
Q14. 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)
A14. 30 Ω (capacitive).
Q15. What term is given to total opposition to AC in a circuit?
A15. Impedance.
Q16. What formula is used to calculate the amount of this opposition in a series circuit?
A16. Z = √ R square = X square
Q17. What is the value of Z in a series AC circuit where XL = 6 ohms, XC = 3 ohms, and R = 4 ohms?
A17. Z = 5Ω.
Q18. What are the Ohm's law formulas used in an AC circuit to determine voltage and current?
A18. E = IZ ≥ I = E ÷ Z
Q19. What is the true power in an AC circuit?
A19. True power is the power dissipated in the resistance of the circuit or the power actually used in the circuit.
Q2. What is the phase relationship between current and voltage in an inductor?
A2. Current lags voltage by 90º (ELI).
Q20. What is the unit of measurement of true power?
A20. Watt.
Q21. What is the formula for calculating true power?
A21. True Power = (IR) 2R.
Q22. What is the reactive power in an AC circuit?
A22. Reactive power is the power returned to the source by the reactive components of the circuit.
Q23. What is the unit of measurement for reactive power?
A23. var.
Q24. What is the formula for computing reactive power?
A24. Reactive Power =
Q25. What is apparent power?
A25. The power that appears to the source because of circuit impedance, or the combination of true power and reactive power.
Q26. What is the unit of measurement for apparent power?
A26. VA (volt-amperes).
Q27. What is the formula for apparent power?
A27. Apparent Power = Iz square x Z or. √ true power square + reactive power square
Q28. What is the power factor of a circuit?
A28. PF is a number representing the portion of apparent power actually dissipated in a circuit.
Q29. What is a general formula used to calculate the power factor of a circuit?
A29. PF = TP ÷ AP or PF = Cosθ
Q3. What is the term for the opposition an inductor presents to AC?
A3. Inductive reactance.
Q30. An AC circuit has a total reactance of 10 ohms inductive and a total resistance of 20 ohms. The power factor is .89. What would be necessary to correct the power factor to unity?
A30. Add 10 ohms of capacitive reactance to the circuit.
Q31. What is the difference between calculating impedance in a series AC circuit and in a parallel AC circuit?
A31. In a series circuit impedance is calculated from the values of resistance and reactance. In a parallel circuit, the values of resistive current and reactive current must be used to calculate total current (impedance current) and this value must be divided into the source voltage to calculate the impedance.
Q4. What is the formula used to compute the value of this opposition?
A4. XL = 2πfL.
Q5. What happens to the value of XL as frequency increases?
A5. XL increases.
Q6. What happens to the value of XL as inductance decreases?
A6. XL decreases.
Q7. What effect does the capacitor have on a changing voltage?
A7. The capacitor opposes any change in voltage.
Q8. What is the phase relationship between current and voltage in a capacitor?
A8. Current leads voltage by 90º (ICE).
Q9. What is the term for the opposition that a capacitor presents to AC?
A9. Capacitive reactance.
The power that appears to the source because of circuit impedance is called apparent power. It is the combination of true power and reactive power and is measured in volt-amperes (VA).
APPARENT POWER
Capacitor in an AC circuit opposes any change in voltage just as it does in a dc circuit.
CAPACITANCE IN AC CIRCUITS
The opposition a capacitor offers to AC is called capacitive reactance. Capacitive reactance will decrease if there is an increase in frequency or an increase in capacitance.
CAPACITIVE REACTANCE
An inductor in an AC circuit opposes any change in current flow just as it does in a dc circuit.
INDUCTANCE IN AC CIRCUITS
Opposition an inductor offers to ac is called inductive reactance. It will increase if there is an increase in frequency or an increase in inductance. The symbol is XL, and the formula is XL = 2πfL.
INDUCTIVE REACTANCE
The number of degrees that current leads or lags voltage in an ac circuit is called the phase angle. The symbol is θ. OHM'S LAW FORMULAS FOR AC—The formulas derived for Ohm's law used in ac are: E = IZ and I = E/Z.
PHASE ANGLE
Current leads the voltage by 90º in a capacitor (ICE).
PHASE RELATIONSHIPS OF A CAPACITOR
Current lags the voltage by 90º in an inductor (ELI).
PHASE RELATIONSHIPS OF AN INDUCTOR
The portion of the apparent power dissipated in a circuit is called the power factor of the circuit. It can be expressed as a decimal or a percentage.
POWER FACTOR
To reduce losses in a circuit the power factor should be as close to unity or 100% as possible. This is done by adding capacitive reactance to a circuit when the total reactance is inductive. If the total reactance is capacitive, inductive reactance is added in the circuit.
POWER FACTOR CORRECTION
The total reactance of a series AC circuit is determined by the formula X = XL − XC or X = XC − X L. The total reactance in a series circuit is either capacitive or inductive depending upon the largest value of XC and XL.
TOTAL REACTANCE
The power dissipated across the resistance in an ac circuit is called true power. It is measured in watts and the formula is: True Power = (IR) 2R.
TRUE POWER
The power returned to the source by the reactive elements of the circuit is called reactive power. It is measured in volt-amperes reactive (var). The formula is: Reactive Power = (IX)2X.
UNCLASSIFIED REACTIVE POWER
