What is Reactance: Definition and 82 Discussions

In electric and electronic systems, reactance is the opposition of a circuit element to the flow of current due to that element's inductance or capacitance. Greater reactance leads to smaller currents for the same voltage applied. Reactance is similar to electric resistance in this respect, but differs in that reactance does not lead to dissipation of electrical energy as heat. Instead, energy is stored in the reactance, and a quarter-cycle later returned to the circuit, whereas a resistance continuously loses energy.
Reactance is used to compute amplitude and phase changes of sinusoidal alternating current (AC) going through a circuit element. Like resistance, reactance is measured in ohms, with positive values indicating inductive reactance and negative indicating capacitive reactance. It is denoted by the symbol


{\displaystyle \scriptstyle {X}}
. An ideal resistor has zero reactance, whereas ideal inductors and capacitors have zero resistance – that is, respond to current only by reactance. As frequency increases, inductive reactance also increases and capacitive reactance decreases.

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  1. Lotto

    B Why can't we calculate capacitive reactance like u/i?

    When I write ##X_C=\frac ui=\frac{U_\mathrm m \sin \omega t}{I_\mathrm m \sin \left(\omega t+\frac{\pi}{2}\right)}##, it should be ##\frac{U_\mathrm m}{I_\mathrm m}##, but it isn't. Why?
  2. A

    Engineering Short vs Open Circuit: Is That Correct?

    For the first circuit, Req = ZL + ZC = -j/(w*C) + j*w*L = 0 for short circuit, so w = 0? For the open circuit case, -j/(w*C) + j*w*L = infinity, so w = infinity? Is that correct?
  3. A

    Choosing a pulse capacitor, ESR vs reactance at frequency

    While going through the catalogues I started to wonder, typically lower ESR caps cost more, but if I need the cap for DC smoothing , to filter out unwanted AC ripple, then I put that cap across my DC rails +-. Now so far so good. It's ability to filter out the AC ripple will be directly related...
  4. J

    Find capacitive reactance from Inductive reactance

    I'm just confused about how to go about this problem. I don't have all variables. What does the current doubling with half plate separation have to do with the problem? Your help is much appreciated. Thank you.
  5. A

    Adjusting the Power Factor of an RLC circuit

    a) An inductor should be added because that would cancel out the lag of the voltage with the current so that P = IV is at a maximum since V is ahead of I in an inductor b) ##cos(\phi) = \frac{R}{Z}##, ##R = Zcos(\phi) = 60\times 0.5 = 30 ohms## ##X_l - X_c = \sqrt{Z^2 - R^2} = \sqrt{60^2 -...
  6. V

    Determining inductive reactance from a 3-phase AC circuit

    I am trying to figure something out, but I am in the first step and just trying to get through my "thought experiment". Let's say I have a 480V 60hz 3 phase supply feeding a circuit that comprises of nothing but the conductors themselves (effectively a shorted power supply). Consider the...
  7. J

    Engineering Calculating Capacitive Reactance

    the full question is: The following phase schematic diagram (FIGURE 4) shows an 11 kV, 50 Hz, 3-phase, short line feeding a load. By calculation or constructing the phasor diagram (use a scale of 1 mm = 2 A) for the load current with VR as reference, determine the capacitive current and •...
  8. E

    Calculation involving the reactance of a combination

    If just found out about reactances and impedances today and came across this little problem. I have worked it through with a sort of brute force approach (that I'm not totally sure is correct!) but wondered if it could be done slightly more quickly? I denoted the impedance of the top branch...
  9. MotoMike

    Home Brew RF dummy load showing reactance at higher frequencies

    I searched for dummy loads and found nothing on point. If there is and I missed it I apologize. If this is not correct section let me know and I will re-post. We need a high power dummy load in the 4kw range. for the amateur hf bands. The oil cooled "cantenna" type are at the most 1.5 kw...
  10. Mzzed

    Load matching/impedance matching

    Hi all, I am currently working on a project where the output/load consists of both a variable inductance and variable resistance, whilst the circuit driving this load is a class E radio frequency (RF) power amplifier with low resistance but relatively high/medium reactance (attempting to design...
  11. I

    Leakage Reactance of a three phase power transformer using Ansys Maxwell

    Hello all; I am trying to calculate leakage reactance of three phase power transformer . I am using Ansys Maxwell final element solution to analyse it. The software gives me total energy and linkages and self and mutual inductances.By using this quantities how can ı compute leankage reactances...
  12. GoodPost

    Short-circuited Resistance in Parallel with Reactance

    Dears, Please help me out with this. The attached figure is an equivalent circuit of an induction machine. I'm trying to find the current (Is). It's assumed that the core losses are neglected and hence Rc =0. If this is the case, then Rc (short-circuited) is in parallel with jXm. What's the...
  13. cnh1995

    Is synchronous reactance constant?

    In electrical machinery and power systems, the synchronous reactance is treated as a constant reactance in series with the generated emf and is independent of load condition. These threads were very helpful for me in understanding this concept (thanks to @jim hardy!)...
  14. E

    Synchronous generator reactance (Sub, transient, SS)

    Hi I know that when studying fault conditions for synchronous machines you have three reactances: Sub, transient and steady state reactance. Where the magnitude of the reactances are (from smallest to largest) in the same order as previously listed. However, I have tried searching - hoping to...
  15. Plat

    High-frequency transformers - how are they different?

    I apologize for being somewhat scatter-brained here, I just have too many questions on this topic. I know high-frequency transformers use ferrite cores instead of laminated silicon-steel, but what other differences are there? Does their reactance still follow the standard formula: 2*Pi*F*L ...
  16. F

    Inductive and capacitive reactance

    Why is it that capacitive reactance is subtracted from inductive reactance and not the other way around when determining the overall reactance?
  17. V

    Correcting power factor without affecting active power.......

    When correcting power factor without affecting active power, wouldn't the current and voltage of the system have to change due to Active power =VrmsIrmscos(theta), where cos(theta) is the power factor?
  18. O

    What is the Phase Angle of an LRC Circuit with Given Components and Frequency?

    Homework Statement A 35 mH inductor with 1.0 resistance is connected in series to a 20 µF capacitor and a 60 Hz, 40-V (rms) source. Calculate the phase angle. Homework Equations tan φ = (XL - XC) / R The Attempt at a Solution Solving for φ: φ = tan -1 [(XL - XC) / R] XL = 2πfL = 13.194 Ω...
  19. B

    I Capacitive Reactance: Opposing Voltage Changes & Impedance

    Hi Folks, I am reading some text about capacitive reactance that reads 'so we know that capacitors oppose changes in voltage' would this be the supply voltage or the voltage in the capacitor? I am struggling to see how the change is opposed surely the quoted statement would infer increasing...
  20. A

    Positive Sequence Resistance and Reactance calculation

    Hi, I have following two parameters with me a) Maximum three phase fault current in amperes (A) and phase angle in degrees (B). I want to calculate the positive sequence resistance (R) and reactance (X) from above two variables for a symmetric system. I couldn't figure out a way to find R and...
  21. Planobilly

    How to calculate reactance of this capasitor

    Hi, I am a little lost here. What is the reactance of C2 at the ripple frequency? Can someone show me the steps to make this calculation? Thanks, Billy
  22. S

    How do Capacitors and Inductors Add/Remove VARS in AC?

    To my understanding, capacitors cause the current to lead the voltage which adds VARS to the circuit and inductors cause the current to lag behind the voltage which removes VARS (in AC circuits). Also, it is my understanding that VARS increase voltage. I work for one of the largest utilities...
  23. KingDaniel

    How to get frequency, w (omega), when calculating reactance

    Hi, I came across a question from our weekly set of questions that involved finding the inductive reactance and capacitive reactance, ie: XL = jwL and XC = 1 / jwC. In the solution sheet, the professor wrote that the frequency, w (omega) = 10000, however, in the question sheet, it was nowhere...
  24. I

    Capacitive/inductive reactance in a series-parallel circuit

    I wired a capacitor in series with a 4 ohm tweeter speaker to serve as a high-pass filter. I used the capacitive reactance equation to figure my capacitor value to filter the speaker at 2.2khz. Xc=1/2PiFC 4=1/2Pi(2200)C Solve for C and you get .00001809 Farads or 18uF. And?It sounds like...
  25. Martin V.

    How is synchronous reactance for a cable defined?

    How is synchronous reactance for a cable defined?
  26. R

    Answer verification Series RLC, Reactance, Voltages, Current

    Homework Statement All relevant data and variables are included in the image. The questions are also included in it. Homework Equations My questsion is just verification. I have attempted all the asked questions on the paper. Its frustrating as the papers don't include answers to check them...
  27. C

    Finding Reactance Ratio in Electrical Resonance

    Homework Statement See the picture. If at electrical resonance ##U_c=60 V## and ##U_L=40 V##, find the ratio of reactances ##\frac{X_L}{X_c}##.Homework Equations [/B]##Z=\sqrt{R^2+(\omega L - \frac{1}{\omega C})^2}## , ## Y= \sqrt{\frac{1}{R^2} + (\omega C - \frac{1}{\omega L})^2}## , ##\phi =...
  28. T

    Engineering Find Frequency for Zero Reactance in AC Circuit

    The question is: The voltage source in the above circuit is a sinusoidal AC source with constant amplitude and constant phase shift but an adjustable frequency. Calculate the frequency ω at which the phasor current I...
  29. T

    Synchronous Machine Transent parameters from sudden 3phase Short Circuit

    Hi, So we're doing this Lab on Transient reactance of a synch generator and I'm reading Sarma's classic: Electric Machines Steady-state theory and Dynamic performance p. 474-479 But I can't see how to get the actual exponential time constants for the sub-transient and transient reactances? Thanks
  30. D

    Capacitive Reactance: Calculate Xc & V=I*Xc

    Hi, A pure capacitor C is in an a.c. circuit. Vc = V0 * sin(w*t) Q = C * Vc = C * V0 * sin(w*t) I = dQ/dt = wCV0 sin(wt) then I0 = wCV0 Xc = V0 / I0 = 1/(wC) So why people would say V = I * Xc? Is it a must to include complex number?
  31. arnab ghosh

    Why are synchronous motors grounded with reactance

    Why are synchronous motors reactance grounded and not resistance grounded like in alternators?
  32. P

    Finding Reactance from System Fault Level

    Hi, apologies if this is very simple but I am still learning the basics of power system analysis and would like it if someone would be able to clarify what I have been told regarding system fault levels and subsequent system reactance values (X) from this. Basically, I have been told that...
  33. G

    Where does inductive reactance go?

    Lenz law states the induced emf in any circuit is always in a direction to oppose the effect that produced it. So my question is, 1. Does this mean xl creates a difference in potential and actual electrons flowing in the opposite direction or just oppose the current direction that created it...
  34. Ahmad Kishki

    Understanding Reactive Power and Reactance: Exploring Complex Power Components

    Complex power has two components the "active" and the "reactive". I am comfused. What is the physical interpretation of the complex component? Also what is the reactance?
  35. EnchantedEggs

    Difference between inductive reactance and inductance?

    Hi All, I'm just trying to get my head around circuit theory at the moment (really, really basic stuff, like what a capacitor is etc) and I've run up against some difficulties in separating out some definitions. The main one I'm wrestling with right now is the difference between the inductance...
  36. ranju

    Engineering Load Reactance: Real or Reactive?

    f we are given a single phase load supplied by single phase voltage source..& a current 10<-1500 & the volge at load terminal is 100<600, then what type of power (real or reactive ) will the load absorb & supply?? Its clear here that the load is inductive one..so the power consumed should be...
  37. U

    Trend of resistance & reactance values in fault signal

    Hi, What I have been trying to achieve is learn about signal processing ,particularly filters. Newbie to DSP,so far! I have 2 signals with me.Voltage and current signals. Both are unfiltered. I have managed to filter out these two signals to remove most of the noise. Now,I want to find the...
  38. T

    Looking for capacitive reactance but missing "C"

    [b]1. Current and voltage graphs are shown for a capacitor circuit with ƒ=1000Hz. What is the capacitive reactance Xc? [b]2. I know the equation is Xc= 1/2∏ƒC to solve for capacitive reactance but where do I find C? All of our in class examples had C and ƒ given. The...
  39. H

    Induction motor and sequence reactance

    Hello! This is my second post about sequence reactances. This time- its in induction motor. To my understanding ( once again ) - In any network (symmetrical or unsymmetrical) , there would be 3 components- positive sequence component, negative sequence component and zero sequence...
  40. K

    Capacitor and reactance question

    A 9.08 microFarad capacitor is measured to have a reactance of 245.0 Ohm. At what frequency (in Hz) is it being driven? I converted 9.08microFaraad in farads- 9.08x10^-6 THe formula i used was 1/(2*pi*f*9.08X10^-6) -- f = 17528.07 Hz but i am not sure if this is the right answer.
  41. P

    Calculating Capacitive Reactance for 10V, 2.5kHz, Sine Wave

    Hello again, I am trying to knock out some challenge questions in my text (to prepare for my midterms) and I came across this question. A 10V, 2.5kHz, sine wave is applied to a series combination of a 0.01µF capacitor and a 10kΩ resistor. What would the total current be? The answer in the...
  42. skate_nerd

    RL circuit inductive reactance problem

    Homework Statement There's a coil, resistance R=7Ω, and inductance L=1mH, and its powered by a generator with variable frequency and Vmax=10V. At what frequency is inductive reactance equal to resistance? (when z=R√2) Homework Equations z=R√2, z=√(R2+(ωL)2) The Attempt at a...
  43. V

    Transformers - winding resistance and leakage reactance

    transformers -- winding resistance and leakage reactance Does anyone know what is meant by: 'determine the combined winding resistance and leakage reactance referred to the primary side'?
  44. R

    I understanding inductive reactance

    Hi guys, I have a small pet project at the moment and I'm trying to look at eddy current losses in transformers. Whilst researching my project I realized that my AC current, when applied to the coil (wrapped around the steel core), would be subject to an inductive reactance. Is it correct...
  45. V

    Calculating capacitive reactance

    Hi, Apologies if this has been posted/answered else where but I could not find it. I am trying to develop a model in MATLAB for a simple (short cylindrical coil) air cored transformer. So far I am capable of defining the coil such as core diameter, turns, length etc.., calculate the coil...
  46. P

    Calculating Impedance Using Capacitive Reactance

    Homework Statement A 29kΩ resistor is in series with a 62mH inductor and an ac source. Calculate the impedance of the circuit if the source frequency is 80Hz . Homework Equations XL = ωL Z = [(R^2 + (XL - XC)^2)] ^ 1/2 The Attempt at a Solution R = 29,000 Ω XL = 0.062 H...
  47. T

    Capacitors in DC Circuits, and How it Affects Reactance & Current

    I read that capacitors that are part of DC circuits have a reactance that is essentially infinite. When I look at equations, this seems to make sense: If the reactance X=1/ωC, ω is zero in a dc circuit and so this means that X approaches a very large value. Thus, if you use V=IX, this means...
  48. A

    Calculate Resonant Frequency & Capacitive Reactance

    Homework Statement Now demonstrate how to calculate resonant frequency. Calculate the capacitive reactance for a 300uF capacitor in series with a 800mH inductor. Homework Equations f = 1/2pi(LC)^1/2 Xc = -1/2pifC Hi I urgently need this solution checked out for my BTech...
  49. I_am_learning

    Does synchronous Reactance vary with frequency?

    Synchronous generator synchronous reactance Xs is composed of Xs = Xa + Xl where Xl is the armature leakage ractance can Xa is there to model the armature reactance. Although Xl is frequency dependent (Xl = Lw, where L is the leakage inductance which is constant), is Xa also frequency...