# A question in regards to how generators work...

Asked by JMCSD (243) June 16th, 2010

Hi everyone. My question is this:

Lets say you have a shaft. This shaft rotates at a relatively constant speed and does so with a large amount of torque. Let’s say I were to connect this to a generator that harnesses this rotation and converts it to electricity which is then stored in a battery. Could I then attach 10 more generators to this same shaft and create 10 times the electric output? Sorry if this is a silly question, but I have an idea and it heavily relies on using multiple generators to actually be worth anything. Please let me know if you need any supporting details to make an informed answer.

Thanks

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You may get close to 10 times the output (electrical engineers, correct me if I am wrong here), but you will certainly not get 10 times the output, because making electrons in the wire move will slow down the shaft. Not by much, but it will, otherwise it would be a perpetuum mobile and thus in violation of the laws of physics.

ragingloli (48214)

That makes sense. I suppose I shouldn’t ask if I get exactly 10X, but I would have assumed that (with what is making the shaft turn out of the picture) that more generators would equal more output….. once again I am unfortunately not all the edumacated in this field :)

JMCSD (243)

Is the torque supply unlimited? As you add generators the torque required to spin the shaft will increase. Two geneators will take twoce the torque.

I work with wind turbines and have to select the generator very carefully.

LuckyGuy (39100)

@worriedguy Let’s say…. yes. May I ask for a bit of an explanation as to what gets taken into consideration when selecting different generators? I find this very interesting…

JMCSD (243)

The generators run off the energy of the shaft’s rotation.

So, your assumption here is that this hypothetical shaft is being powered by a force that can accommodate whatever number of generators you want to attach without slowing down the rotation. (And also that the shaft is sufficiently strong to withstand the torque load.)

All things being equal, yes, your theory should work… but I think you’ll find it far more difficult to employ in real life.

robmandu (21293)

@robmandu That helps clear things up. Of course I am not planning on using magic to spin a shaft which is made out of diamond :P, but I just wanted to remove that variable from the equation.

JMCSD (243)

This is too big a discussion to be done here but basically I look at the site and determine the average effective wind speed. From that, I calculate the total power density in watts/ square meter . I then work with the customer to determine the total annual energy output they expect to capture. That sizes the unit and determines the cost envelope.
With that info I start on the generator. I look at max generator output based upon the max wind speed and size. Different generator designs have different operating speed “sweet spots” depending upon the number of poles and type of magnet structure. My goal for the design is to eliminate the need for a gearbox so I pick all the components such that their best operating points line up. (This is also my signature design style) Once I know the power output I then calculate the torque on the shaft using power = k x torque x rpm. That tells me the loading on each blade and the size of the bolts and other mountings needed to hold it all together.
Depending upon the fluid there is a different theoretical limit to how much energy is contained in the fluid and how much you can extract from it. For air this number is 0.59 known as the Betz limit.
It’s all physics.

LuckyGuy (39100)

It depends on how much torque is available. If there are no electrical loads on those generators, then those additional generators won’t add an appreciable resistance to the shaft. But as you add electrical loads to the generators, those generators will put up greater and greater mechanical resistance and so require more and more torque from the shaft. If that shaft is being turned by, say, a hydroelectric turbine, no problem (though there will come a point at which the force of the water through the turbine will be insufficient to maintain the rotation of the shaft). But if that shaft were being turned by a diesel engine, say, then that engine will have to work harder as the generators put a greater and greater load on the shaft.

HungryGuy (16014)
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