Energy Transformation: Electrical Generator Q&A

[ViolentCorpse]

Hello everyone,

I've been thinking about energy transformations and an electrical generator came to my mind which basically transforms mechanical energy into electrical energy. What confused me as I was thinking about it is whether the electrical energy that results due to the rotation of the turbine is at the expense of the mechanical energy of the turbine, since energy must always remain conserved? In other words, does the turbine rotate slower when its energy is being converted into electrical energy (i.e it is lending its motion to electricity) than it would be rotating if no such energy transformation was taking place?

Thank you!

 

[tiny-tim]

Hello ViolentCorpse!

…does the turbine rotate slower when its energy is being converted into electrical energy (i.e it is lending its motion to electricity) than it would be rotating if no such energy transformation was taking place?

Yes …

just like a water-wheel, which actually slows the river down if it's doing any work (grinding corn etc).

 

[Dale]

In other words, does the turbine rotate slower when its energy is being converted into electrical energy (i.e it is lending its motion to electricity) than it would be rotating if no such energy transformation was taking place?

Yes, in fact, this is the principle behind regenerative braking in hybrid cars.

 

[ViolentCorpse]

Oh. Very nice. Now I want to understand this from a purely mechanical point of view without taking energy considerations into account. I presume that the magnetic field associated with the induced current produce a counteracting torque on the turbine which slows it down? I'm sorry if it feels like I'm trying to answer my own question, but the truth is, I'm not very sure if my presumptions are correct. So I very much need your help to correct my misunderstanding please..

Thank you so much, tiny-tim and DaleSpam! Helpful as always!

 

[russ_watters]

You have it correct.

 

[ViolentCorpse]

Oh? That's a first, hehe!

Thank you so much all of you! I owe you people a lot for all the help I've received since joining this forum.

 

[Naty1]

In other words, does the turbine rotate slower when its energy is being converted into electrical energy...

It would slow down were the rotation speed uncontrolled...and you might get 53 cycles per second in the output instead of the 60 cycles required [in the US]. In fact power companies must match in real time the demand [electrical load] with the energy produced.

If you have ever run a portable gasoline generator, as during a commercial power outage, you probably noted the change in sound as additional load is added...the engine speed is controlled by opening the gas supply a bit more so the engine can provide more power and keep the RPM about constant...analogous to going up a hill in car and slowing down unless you provide more fuel via the gas pedal.

There is often a centrifugal control in the flywheel and a linkage arm to the carburetor in a generator. All this attempts to match the input to output power via the rotational mechanical energy you described.

 

[ViolentCorpse]

Thank you Naty1. That was very informative.
Incidentally, I am familiar with portable gasoline generators, since I live in a third world country where there's power load-shedding due to energy shortage.

By the way, I had a follow-up question: It seems to me that conservation of energy might be violated at the moment the turbine induces a current in the wires, because I think the opposing torque that is to be provided by the magnetic field of the current to slow the turbine down won't be applied to the turbine instantaneously. I think electric/magnetic field effects travel at the speed of light (not sure), so there will be a moment when the turbine is rotating full-swing as well as there's a new form of energy produced (electricity), without one form of energy borrowing any energy from the other.

I highly doubt this should happen, even for an infinitesimally small interval of time, but my extremely limited knowledge of physics misleads me. :(

 

[Naty1]

the opposing torque that is to be provided by the magnetic field of the current to slow the turbine down won't be applied to the turbine instantaneously.

I don't know exactly what controls are used on large scale industrial turbines, but in general if you have a power source with a voltmeter or ammeter you'll see the voltage and current gradually increase as the rpm of the power source revs up if there is a load present.

On the other hand, one usually starts generators without any load, that is an open circuit, so no current flows, and so there is no power consumed. One usually adds load gradually. edit: Once the voltage is at the rated value, you can start adding load.

[Even nuclear power plants convert steam [heat] to rotational mechanical energy and then to electric power and I know they have to monitor and match power consumption and heat production closely.] Check out 'back emf' for some ideas on all this.

I see wiki calls it 'counter emf' here...http://en.wikipedia.org/wiki/Back_emf

 

[Dale]

By the way, I had a follow-up question: It seems to me that conservation of energy might be violated at the moment the turbine induces a current in the wires, because I think the opposing torque that is to be provided by the magnetic field of the current to slow the turbine down won't be applied to the turbine instantaneously. I think electric/magnetic field effects travel at the speed of light (not sure), so there will be a moment when the turbine is rotating full-swing as well as there's a new form of energy produced (electricity), without one form of energy borrowing any energy from the other.

You are correct that there is a time when the electrical power output is less than the mechanical power input (even for an ideal generator with no losses). During this time the fields inside the generator are increasing and the energy is stored in them.

When you shut the generator off the reverse happens. The generator continues to produce electrical power for a brief moment while the mechanical power is stopped. This extracts the energy that was added to the internal fields and leaves the generator in it's original state.

 

[Naty1]

It seems to me that conservation of energy might be violated...

just to be clear: No.

 

[ViolentCorpse]

So, I suppose that initially there's some mechanical power in the generator and zero electrical power. The instant when electrical power becomes non-zero, does the mechanical power go down by the same amount that the electrical power goes up during the same interval of time? It's the instantaneousness of the process that I'm concerned about, since as I said, I don't think the counteraction of the magnetic torque acting on the turbine would appear instantaneously with the production of electrical power, so there would be a time when electrical power + mechanical power would exceed the mechanical power that we started with...

Sorry for repeating the same questions over and over again, I'm not very quick on the uptake, but I'm getting there thanks to you people!

 

[Naty1]

The instant when electrical power becomes non-zero, does the mechanical power go down by the same amount that the electrical power goes up during the same interval of time?

If that happened the generator would shut down.

The idea is no different in concept than a capacitor charging or an inductance blocking a varying voltage...those don't happen 'instantaneously' either...nothing much macro happens 'instantaneously'...When you run trying to launch a kite, does the kite move with you 'instantaneously'..

 

[CWatters]

It seems to me that conservation of energy might be violated at the moment the turbine induces a current in the wires, because I think the opposing torque that is to be provided by the magnetic field of the current to slow the turbine down won't be applied to the turbine instantaneously

If the load does not find it's way back to the turbine instantly then the generator will start to slow down. The apparent excess energy you mention comes from the energy stored in the rotating mass of the electrical generator. That energy originally came from the turbine when it was first started up. If the generator does slow down then eventually that energy will have to be replenished by the turbine.

 

[Dale]

It's the instantaneousness of the process that I'm concerned about

At every instant there is mechanical power in and electrical power out. There is also internal kinetic energy stored in the rotating rotor and internal electromagnetic energy stored in the fields. Any instant where the electrical power out does not match the mechanical power in the difference goes to increasing the internal energy (mechanical and electromagnetic) of the generator.

 

[ViolentCorpse]

I think I'm too used to thinking things in terms of the simplistic conversion of K.E into P.E when a ball is hurled upwards where every gain in P.E is compensated by an equal decrement in the K.E. I was applying a similar reasoning here.

Thanks a lot to all of you people!