What happens to the generator if we don't consume the generated electricity from generator? Only theoretical explanations, please

Question

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...

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.