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As all we know, there are different ways of generating electricity such as solar cells, wind turbines, dams, etc... I'm gonna pick wind turbines as a metaphor to tell what is on my mind.

Let the wind turbine is generating electricity, and let disconnected it from the network where representative the demand side. This means the wind turbine generator is running still with no loads connected to its terminals. (Assume that there are no controlling units on wind turbines such as brakes, tilting wings, etc... nothing to stop him whilst the wind blows ). No-load connected but it still generates a voltage...

Here is my question came upon this point:

Where does this voltage go? Does it turn back into a stator in the opposite direction? Or does it make this generator into being a motor by not using generated electricity due to no-load and make the wind turbine turn faster and faster? Eventually, does it burn itself due to not a load consumed the generated electricity?

Basically, I'm wondering about the effect of the generated voltage on the generator. And what happens to that voltage?

I'm expecting theoretical answers, explain as much as you can please. Equations are welcome...

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    \$\begingroup\$ Nothing to control the blades so it overspeeds and may fail. And once the blades fail, no rotation so voltage = 0. \$\endgroup\$
    – Solar Mike
    Dec 20 '20 at 19:40
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    \$\begingroup\$ The voltage doesn't have to go anywhere, nor does it continue to rise indefinitely just because you are drawing no current. Voltage also doesn't flow so it doesn't "go in the opposite direction back into the generator". Only current flows. No current means no load which means no resistive voltage drop and the generator is easier to turn, so voltage will rise a bit but not accumulate ad infinitum. \$\endgroup\$
    – DKNguyen
    Dec 20 '20 at 19:44
  • \$\begingroup\$ The speedup comes from removal of the torque load. At some speed not so very much higher than the loaded speed, the angle of attack of the wind turbine blades relative to the wind will be 0, and they won't accelerate further. So accelerating to the point of self destruction would be (and is) highly unusual. \$\endgroup\$ Dec 20 '20 at 22:46
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    \$\begingroup\$ Thought experiment: When you disconnect the load on a battery where does the voltage go? Nowhere. Voltage doesn't go anywhere. Voltage causes current to flow. "I'm expecting theoretical answers ..." as opposed to what - practical answers? \$\endgroup\$
    – Transistor
    Dec 20 '20 at 23:32
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Voltage doesn't go anywhere. It's something you measure between two terminals. If you don't connect the generator to anything, the voltage is still there.

Voltage is can be thought of as the pressure that pushes electrons around a circuit. if you don't connect any circuit, the pressure is still there, but there's nowhere for the electrons to go. More technically, the voltage is a measure of the electric field that persuades charged particles (including electrons) to move.

As to what happens to the generator, that entirely depends on what sort of generator it is. In many cases, the generator is something mechanical driving an alternator. That could be a steam turbine, a diesel engine or a wind turbine. The mechanical load that an alternator puts on the thing driving it depends on the current you're taking from the alternator. If there is no current, then the alternator spins more freely, with only a bit of friction. Real-world generators will have some governer to stop the whole engine spinning faster and faster until it explodes. The easiest way to slow down a wind turbine is to "feather" the blades - turning them edge-on to the wind.

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  • \$\begingroup\$ Let assume that wind speed is ideal-constant. The wind turbine running at a nominal speed, and no-load connected initially. Will it still accelerate to infinity? Think as there are no controlling units, brakes, or blades, and wind speed is constant. I'm trying to understand that is this acceleration due to a voltage that the generator generates or due to removing the load which was connected to the generator. \$\endgroup\$ Dec 20 '20 at 20:06
  • \$\begingroup\$ @automatickebab A wind turbine with no blades isn't a wind turbine! A theoretical alternator with no electrical load on it will put no mechanical load on whatever is driving it. It doesn't push the thing driving it any faster (that would risk turning into a perpetual motion machine). In the case of the wind turbine, the speed would be limited by the wind speed and the shape of the blades. If the blades started spinning too fast, they would become a fan pushing the air along, rather than being pushed by the wind. \$\endgroup\$
    – Simon B
    Dec 20 '20 at 20:18
  • \$\begingroup\$ moving blades* I mean... \$\endgroup\$ Dec 20 '20 at 20:22
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Voltage potential can exist without any load. Example: any unconnected battery or power outlet. The voltage exists across the space between terminals, similar to any capacitor.

A generator without any load or control mechanism can speed up until friction limits it (e.g. any wind power goes into heating the bearings and airflow turbulence). e.g. a toy fan in the wind usually won't self destruct, but will self-limit. The generator itself will have some heat losses due to magnetic eddy currents in various metallic portions.

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  • \$\begingroup\$ Not seen a real wind turbine destruct? Some blades have more mass than medium or large cars so are not “toys” especially when bits are flying. \$\endgroup\$
    – Solar Mike
    Dec 20 '20 at 20:01
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A generator in normal operation has a back emf acting through it produced by the current flow in its coils. This produces a torque on the rotor. In steady state, this back torque is equal to the input power torque (in this case, that from the wind turbine blades).

When the load is removed, the circuit is opened. The current flow stops and the back emf drops to zero. There is now no counter-torque, so the rotor accelerates.

In real systems this can result in overspeed, fires, and destruction of the generator.

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  • \$\begingroup\$ Can we conclude to this acceleration is due to generated voltage? If it is, how affect the solar cells the same situation when the load is removed? \$\endgroup\$ Dec 20 '20 at 19:54
  • \$\begingroup\$ @automatickebab No, the acceleration is simply because the load has been removed. Like an engine designed to move a truck weighing 1 ton unloaded and runs at max safe RPM at that weight because the bearings were not selected to handle higher RPM that would result if the truck was any lighter. It depends on that minimum expected load to hold back the RPM to a safe level. For both generator and solar cell, voltage would rise a bit as a result of no load. \$\endgroup\$
    – DKNguyen
    Dec 20 '20 at 20:04
  • \$\begingroup\$ because the "pressure of electrons" (ie voltage) getting knocked loose inside the cell have nowhere to go so build until an equilibrium is reached in the electric field which opposes more electrons getting knocked loose, thus preventing further voltage rise \$\endgroup\$
    – DKNguyen
    Dec 20 '20 at 20:10
  • \$\begingroup\$ @DKNguyen Basically, can we infer that any generator that works with no-load has no harm to the generator? including solar cells, dynamos, etc... \$\endgroup\$ Dec 20 '20 at 20:13
  • \$\begingroup\$ @automatickebab Electrically, no issue unless you chose an insulation voltage rating that was super borderline and could not handle no-load voltage rise. Potential mechanical issues if the bearings or mechanical strength of the rotor (mass, balance, etc.) were chosen to only be sufficient to support the expected RPM and forces under a minimum expected load. \$\endgroup\$
    – DKNguyen
    Dec 20 '20 at 20:24
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You can consider a solar panel and a wind power generator to be powered by current sources where the maximum voltage is dependent upon the design of the component.

In the case of a solar panel, the maximum voltage is called Voc for open circuit and the maximum power transfer occurs when the impedance is matched to the load and the source which occurs around 82% of Voc depending on solar input and drops with useful intensity to 70%.

In the case of wind turbine the blade pitch allows the blade tip to go much faster than the wind, due the eddy current effects just as a sailboat goes faster into the wind. Thus with no load the blade angle /diameter ratios determine the upper speed limit and safety brakes are critical.

In all cases the MPT max power transfer is always when the load impedance matches the source impedance. This is also true of batteries but due to the heat rise and electrode warp that will occur, you never match impedance of near true voltage sources, only current sources.

A car AC/DC alternator is considered a field current controlled (mutually coupled) current source which at constant RPM can regulate constant voltage for dynamic loads with proportional field currents. If there was no voltage feedback with no load , the voltage might rise according to its kV/RPM (?) for any given field current. The 3 phase AC is also rectified by. 3ph diode bridge.

So in every case given above these are all current sources. This is not the case with permanent magnet motor/generators which are more constant kV per RPM with no load. Then max power transfer again occurs at matched impedance.

Anecdotal

In my 1st job at Bristol Aerospace 1975, our team produced a 1Hp 10m tall window power generator at 5 MPH stall speed using Darrious rotor and Sevonious blades for the Arctic, on a floating automatic weather station transmitting to GOES 1. Speed brakes were tested with guy wire resonances avoided prior to shipping.

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