CSLB C-10 Electrical ContractorElectrical Theory and CalculationsHard
A three-phase motor operates on a 480V supply and draws 30 amps. If the motor has a power factor of 0.85, what is the approximate reactive power consumed by the motor?
- A29.8 kVAR
- B24.9 kVAR
- C22.3 kVAR
- D14.4 kVAR
Show answer & explanationAnswer & explanation
Correct answer: D. 14.4 kVAR
Reactive power (Q) in a three-phase system is calculated using the formula Q = √3 * V * I * sin(θ). First, find the angle θ using the power factor (cos(θ) = 0.85), then calculate sin(θ) and apply the formula.
Why the other options are wrong
- A. This value is too high, indicating a misapplication of the power formula components.
- B. This is likely a calculation error, possibly confusing real or apparent power.
- C. This might be a miscalculation of the power factor angle or an incorrect formula.
Three-Phase Reactive Power
Reactive power in a three-phase system is the portion of apparent power that does not perform useful work; it is measured in volt-amperes reactive (VAR).
- Calculated as Q = √3 * V * I * sin(θ).
- Power factor (PF) = cos(θ).
- Represents energy stored and returned to the source.
Memory trick: Power Triangle: Real is horizontal, Reactive is vertical, Apparent is the hypotenuse.