How Does Generator Size Affect Weight at Different Speeds?

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Discussion Overview

The discussion centers around the relationship between generator size and weight when generating a fixed power output at different rotational speeds. Participants explore the implications of operating at lower RPMs and the associated weight changes of generators, considering various mechanical and electrical principles.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant suggests that a generator operating at half the speed would weigh four times as much to produce the same power output.
  • Another participant estimates that the weight increase would be a factor of two.
  • A different participant proposes a weight increase of approximately 1.66 times.
  • One contributor mentions that logical arguments indicate a potential weight factor of eight, but practical experience suggests it is less than that.
  • Another participant notes that in small DC motors, power output can be adjusted by changing the winding, which affects RPM and efficiency, implying that the weight increase would not be as significant as four times.
  • One participant argues that using a 2:1 drive ratio could mitigate the need for a significant weight change, suggesting that practical solutions exist to match speeds without drastically altering generator weight.
  • Another participant highlights that in automobile alternators, increasing the field current at lower RPMs can compensate for power generation without necessitating a larger generator.

Areas of Agreement / Disagreement

Participants express differing views on how much the weight of the generator would increase when operating at lower speeds, with no consensus reached on a specific factor. Multiple competing estimates and approaches are presented, indicating an unresolved debate.

Contextual Notes

Participants acknowledge that the discussion lacks concrete information regarding specific generator types and configurations, which may influence the weight-to-power relationship. There is also an assumption that both RPMs discussed do not require a gearbox.

blturner
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Over at the Airborne Wind Energy forums we are having a debate about the physics of a generator.

The question was posed that if you could generate 1kw of power in the form of rotational torque at 2 different speeds, one begin half as fast as the other how much more would the generator weigh to convert that slower RPM.

One poster is maintaining that it would weigh 4 times as much.
Another is guessing that it would weigh twice as much.
I guessed that it would weigh about 1.66 times as much.

I believe we are presuming that both the RPMs fall in a range where a gearbox is not needed.

I find myself unable to explain this well enough to convince even myself.

We thought that we should be able to "look it up" but I have been unable to do so. A link would be great.

Brian
 
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I don't know much about electrical engines. I work with mechanical, but I assume many of the same principles apply. I've seen logical arguments that it would be a factor of eight. But in practice it is less than that. Sometimes not much less.
 
Yes, like you, I have experience in various applications but not generators specifically. In small DC electric motors the ultimate power is limited by cooling. By changing the winding you can get different RPMs out of the same motor. Even as much as double. You can also adjust the windings until you have the same power output at a different RPM. But there is more to it than that. Efficiency would be better at one RPM vs the other. So to get the efficiency back to where it started the motor would have to get larger at the slower RPM. I am like 90% positive that it would not get 4 or more times larger.
 
In practice I don't see why you would need to change the weight by much. Just use a 2:1 drive ratio (either gears or a belt) to match the speed to whichever generator is lightest.

To get a rough idea of how big the system would be, a car alternator generates about 1kW, and it only needs a cheap toothed belt to handle the power to drive it.
 
It depends on the generator. In an automobile alternator we can just increase the field current at lower RPM. What you have proposed is not really enough information to give a concrete answer.
 

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