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Question 30 Are the transformers connecting power plant generators to high-voltage power lines considered step-up or step-down ? Explain your answer. file 02214 Question 31 In each of these circuits, calculate the amount of load impedance ”seen” by the voltage sources given the turns ratio of each transformer: 30 Ω 1:1 30 Ω 120 VAC 240 VAC 1:2 Z load = ??? Z load = ??? Hint: ”impedance” ( Z ) is defined mathematically as the ratio of voltage ( E ) to current ( I ). file 00659 16
Answers Answer 1 Only the resistor actually dissipates power. The capacitor only absorbs and releases power, so its ”4 mW” figure does not actually represent power in the same sense as the resistor. To make this sensible, we must think of all the non-resistive ”powers” as something other than actual work being done over time: Q = 4 mVAR P = 3 mW S = 5 mVA Correct power triangle Follow-up question: when making the leap from DC circuit analysis to AC circuit analysis, we needed to expand on our understanding of ”opposition” from just resistance ( R ) to include reactance ( X ) and (ultimately) impedance ( Z ). Comment on how this expansion of terms and quantities is similar when dealing with ”power” in an AC circuit. Answer 2 Fluorescent lamp: I = 0.674 A ; Actual power = 60 W Incandescent lamp: I = 0.5 A ; Actual power = 60 W Induction motor: I = 0.465 A ; Actual power = 52.0 W In every load except for the incandescent lamp, more current is drawn from the source than is ”necessary” for the amount of power actually dissipated by the load. Answer 3 Power factor is the ratio between true power (Watts) and apparent power (Volt-Amps), ranging between 0 and 1 inclusive. Answer 4 ”Apparent” power is apparently the total circuit power when volts and amps are multiplied together. ”Reactive” power is due to reactive components ( L and C ) only, and ”True” power is the only type that actually accounts for energy leaving the circuit through a load component. Answer 5 P.F. = 0.872 Answer 6 A leading power factor is one created by a predominantly capacitive load, whereas a lagging power factor is one created by a predominantly inductive load. 17
Answer 7 Fluorescent lamp: P = 60 W ; Q = 54.3 VAR ; S = 80.9 VA ; PF = 0.74, leading Incandescent lamp: P = 60 W ; Q = 0 VAR ; S = 60 VA ; PF = 1.0 Induction motor: P = 52.0 W ; Q = 20.4 VAR ; S = 55.8 VA ; PF = 0.93, lagging Answer 8 The beer itself is ”true” power ( P , measured in Watts). Good beer, good. Ideally, we’d like to have a full mug of beer (true power). Unfortunately, we also have foam in the mug, representing ”reactive” power ( Q , measured in Volt-Amps-Reactive), which does nothing but occupy space in the mug. Bad foam, bad. Together, their combined volume constitutes the ”apparent” power in the system ( S , measured in Volt-Amps). Follow-up question: can you think of any potential safety hazards that low power factor may present in a high-power circuit? We’re talking AC power circuits here, not beer!
- Three '19
- Alternating Current, Power factor