Question about the efficiency of a generator

[Dong-oh Lee]

Using a certain kinetic energy, does a generator produce the same amount of electric energy even if I change the rpm by gearing?
Also, does a generator produce energy in proportion to the number of rotation? I mean, does a generator create the same amount of energy between when it's rotated 10 times slowly and when it's rotated 10 times quickly?
Is there a certain RPM where a generator is most efficient so that it needs gearing to match the most efficient RPM? If so, how do I know the most efficient RPM?
I would be so thankful if you can help me out

 

[russ_watters]

Using a certain kinetic energy, does a generator produce the same amount of electric energy even if I change the rpm by gearing?

Yes. Note that the equations for gear torque vs rpm are conservation of energy statements. ,Rpm goes up, torque goes down.

Also, does a generator produce energy in proportion to the number of rotation? I mean, does a generator create the same amount of energy between when it's rotated 10 times slowly and when it's rotated 10 times quickly?

Not necessarily. Generators don't produce energy in isolation, they make energy available for a load to use. The type of generator dictates what happens when you increase rpm, but the load's response dictates how much more power is drawn...if the prime mover can keep up.

For example, for a DC generator increasing rpm increases voltage. If the load is a simple resistor, then it will draw more power.

Is there a certain RPM where a generator is most efficient so that it needs gearing to match the most efficient RPM? If so, how do I know the most efficient RPM?

Yes; the manufacturer of the generator will tell you how best to run it.

 

[anorlunda]

Using a certain kinetic energy, does a generator produce the same amount of electric energy even if I change the rpm by gearing?

Remember, a generator conserves energy, and power is rate of delivery of energy. So when the RPM is steady (not changing) then mechanical power in matches electrical power out. Neither number of windings nor RPM changes that. If you go beyond the capabilities of the generator is speeds up or slows down or melts.

Also, does a generator produce energy in proportion to the number of rotation? I mean, does a generator create the same amount of energy between when it's rotated 10 times slowly and when it's rotated 10 times quickly?

See above. The power out depends on the power in and also on what is connected as the electric load. If there is no load connected, then the generator doesn't make any power no matter what the RPM.

Is there a certain RPM where a generator is most efficient so that it needs gearing to match the most efficient RPM? If so, how do I know the most efficient RPM?

There are many kinds of generators. Synchronous generators always spin at the same speed, no mater how much power. Induction generators (or motors) change their speed depending on power.

Where the AC frequency is 60 Hz, the nominal speed of a generator is 3600/N RPM. N is the number of pole pairs. For example, if there are 2 pairs of poles, 3600/2 = 1800 RPM.

There are also electronic ways to vary the speed of a generator or motor. We call them VFDs. You can also use gears or belts to change the speed. Think of the belt and pulleys in your car that spin the alternator.

Most electric generators are very efficient, so we seldom have to worry about that.

If you describe your application, people here at PF might help you select the right equipment.

Edit: i ses that @russ_waters beat me to the answer.

 

[Dong-oh Lee]

Thanks for your replies,
BTW, would the difference be significant between the most efficient RPM and not the most effecient RPM?

 

[russ_watters]

Thanks for your replies,
BTW, would the difference be significant between the most efficient RPM and not the most effecient RPM?

Usually no, as long as it isn't significantly off it's most efficient operating point (say, less than 50%).