Reactive energy/power and AC generators in power plants - Page 1
Forum section: electronics / general

author: nForce
author_rank: Frequent Contributor
timestamp: December 05, 2017, 09:41:05 pm
content: Hello,I don't understand how can we transfer reactive power from power plants to our loads in power system. If I correctly understand with turbines rotating which are connected with AC generators we generate active power, and with DC excitation we generate reactive power. But how is this newly created energy flow from one point to other. For example reactive power is needed for magnetization. But how does energy know what to do, how does active power differ from reactive as a whole in one system?Because reactive power is useless, we want to be close to zero, why is then needed?Thanks for any explanation.

author: Vtile
author_rank: Super Contributor
timestamp: December 05, 2017, 09:50:29 pm
content: There is no spoon eh. reactive power, it is just a mathematical concept. It is said to be created when the power factor is something else than unity, in other words capacitance and inductance (energy storage devices) aren't cancelling each other out. This whole concept of generating something as reactive or active power is teached (universally?) totally wrong as there is nothing to generate about.PS. Here is doodling about the reactive and active power. I would have failed in my math exam with those wobbly sinusoidal curvesPPS. The reactive power is not useless, but often unwanted feature of nature. Another thing in this concept that is often (universally?) taught in odd way.

author: VK5RC
author_rank: Unknown
timestamp: December 05, 2017, 11:09:37 pm
content: Perhaps one way that may help is to look at the RF situation, an AC generator, a transmission line and a reactive load.Impedance mismatches and a standing wave on the 'wrong length of transmission line' can impede transfer of energy.

author: nForce
author_rank: Frequent Contributor
timestamp: December 06, 2017, 05:56:14 pm
content: Quote from: Vtile on December 05, 2017, 09:50:29 pmWhy is reactive power just the negative of sinusoidal? Do you know why induction motor can't operate as a standalone generator? Because it needs reactive power. So reactive power is something that feels induction motor.

author: Vtile
author_rank: Super Contributor
timestamp: December 06, 2017, 07:19:31 pm
content: Quote from: nForce on December 06, 2017, 05:56:14 pmI really would need to refresh the theory in my head to answer to you precisely, but.. Induction motor can be used as standalone generator the reguirements are approximately as follows. The motor is turned near or above nominal speed, there is small amount remanence (magnetism in rotor and/or stator) and there is some capacitance attached to the motor to correct the phase shift and storage some energy to reform the rotating magnetic field in the motor. The remanence is tricky part with todays high efficiency motors, but it can be force started with a DC pulse applied to the turning motor (generator) coils or mechanically modified to contain a small permanent magnet in the cage.The negative in that picture is proposing that direction is opposite, it still is just a power as any other power, the rules of ohms law apply to it as it do "consume" in resistor ie. heating the transmission lines (wires). ..And there is no free energy, the reactive power is loan from active power that capacitor (electric) or inductor (magnetic) do take. [look what potential and current generates] We handle it as reactive power as it is typically easier to chop something to smaller sub-concepts. To really dive in towhy, you need to dig deep to the transmission theory, wave lengths and speed of light (which isn't constant outside the vacuum where we define it).PS. What I try to say is that you are not creating any form of "new energy" or "freshly created energy" nor power as you indicated in the first post. After that fact is sorted out you can start to investigate what reactive power represents and how the reactive power do work in our (simplified) circuit theory.Quote from: nForce on December 05, 2017, 09:41:05 pm

author: nForce
author_rank: Frequent Contributor
timestamp: December 07, 2017, 04:29:05 pm
content: Yes, you are the best Vtile.So let me rethink: We need capacitors for the standalone induction motor because the motor is a coil of wire, and for the compensation we add capacitors. If the motor was a capacitor (which of course is not) then we should add inductors.And last thing: So if the power is negative, then it's flowing in the opposite direction. It resists the flow of active power.Does this make sense?

author: Vtile
author_rank: Super Contributor
timestamp: December 08, 2017, 12:12:05 pm
content: Quote from: nForce on December 07, 2017, 04:29:05 pmThx. I also learn something every time I try to explain something like this. It took me awfully lot of time to sort by myself some of these "This device generates our new form of power" style of concepts that is used solely by most lecturers while they force spoon-feed the huge amounts of knowledge in a short time to the random mass of students, whose face are like. What I have noticed also is that there is awfully lot of similar concepts that are explained with different terms depending which sub-discipline the lecturer do have his or her background. As an UAS automation background myself all our lecturers were from different backgrounds (electronics, RF, power, control, computer science, chemistry, instrumentation, technical physics etc.) everyone did eventually describe the same phenomenons with different terms and concepts, what a mess.Quote from: nForce on December 07, 2017, 04:29:05 pmErr. Yes that is what my current understanding says. It have to do with the how the magnetic field collapses (and releases the stored energy) when the power outside of the motor* is going down to zero. If there is no storage device** attached to the network the motor do not get the needed power to reform the magnetic field as it doesn't produce any power itself when the magnetic field is zero. So the capacitor is the device with needed properties to store the energy and again release it to the motor when the magnetic field starts to reform again. Oscillation it is.* Motor used as Generator in above paragraph.** Capacitor and Inductor are in the end storage devices, one likes more current and one likes potential. Hence the whole phase shifting and compensation thing.This aspect of motors starts to be on point where I have been satisfied (read edge of understanding) for now and just plugged the numbers to equation and hit solve.I suggest you take a look also in parallel LC oscillators and draw your own conclusions.Quote from: nForce on December 07, 2017, 04:29:05 pmI don't know if you can say that it does resist the active power (kind of yes), but it is released back to network when so called active power goes to zero.Quote from: nForce on December 07, 2017, 04:29:05 pmYes....And yes in the end the energy we call reactive power do travel in the transmission lines in form of wave (at 50Hz aprox. 6000km long wave) until it reach something that is ready to consume or store it.

author: Vtile
author_rank: Super Contributor
timestamp: December 09, 2017, 01:58:53 pm
content: By the way. Tesla were obsessed with anything oscillating and the induction motor happens to be Tesla's invention.

author: nForce
author_rank: Frequent Contributor
timestamp: December 09, 2017, 03:53:53 pm
content: Do you maybe know, why is the equivalent circuit of induction motor equal to the transformer's equivalent circuit?I don't understand why is the equivalent circuit that way. For the synhronous motor it's just an inductor in series with a voltage source. And that's it.

author: Vtile
author_rank: Super Contributor
timestamp: December 09, 2017, 04:00:07 pm
content: Unfortunately no I don't know why the model is like it is. The exact reason is something that is poorly explained most of the sources, mostly the model is just given as is, with a short description that stalled induction motor behaves electrically as transformer and then mathematical formulas. At least my mind yells "why and how?" unfortunately the lecturers I have faced what comes to these electrical subjects behave more like a book/paper reading machines than explaining and directing teachers (helping students to think and understand) with these key concepts. The bitterness against the quality of the paper reading biomass aside, the reason for the transformer model for induction motor is that when the rotor is stalled it acts as a secondary of transformer, I assume that this is also the reason of the excessive heating as all the energy is consumed in resistive rotor material instead of transformed to mechanical movement. Unfortunately I have not managed to form a solid visual model of it in my head so it is hard to me to explain it to concise manner. Maybe the pieces start to go in right places some day.Hopefully this helps something.Random pieces of information below.You can get plenty of scientific papers with search phrase "Steinmetz equivalent circuit" about the equivalent model.PS.. It is similar model that is used as simplified transmission line 'T-model' in power line transmissions.Edit. A re-write.

author: Jeroen3
author_rank: Super Contributor
timestamp: December 11, 2017, 08:27:21 pm
content: This video has an excellent explanation of inductions motors. Though not explaining the equivalent model. That is seriously complicated stuff.It does touch the key subject of your question only briefly at 3:50.Quote from: nForce on December 09, 2017, 03:53:53 pmBecause it is a transformer. It just so happens to move because of it.

author: nForce
author_rank: Frequent Contributor
timestamp: December 16, 2017, 06:27:44 pm
content: Does anyone know the answer to this question:https://www.chegg.com/homework-help/questions-and-answers/thanks-formula-referring-slip-r2-x2--derived-q15049959?There is so little information about this on the web.