# Lower Bound for Generator Mass

#### Jstriet9312

Joined Jun 16, 2022
1
Is there a rule of thumb to quickly calculate a lower bound for the mass an electric machine must have in order to source or sink a given number of watts? Such a thing ought to be possible, especially if accuracy only needs to be within a factor of 2, etc.

For example, “given that neodymium is used, and shaft turn at X hz, mass must be at least Y.”

#### Papabravo

Joined Feb 24, 2006
20,603
AFAIK there is no relationship between mass and the ability to deliver power unless you propose to convert the mass into energy.

#### Papabravo

Joined Feb 24, 2006
20,603
Is the frame size related to the dimensions of the rotor or the exterior dimensions of the housing?

#### LowQCab

Joined Nov 6, 2012
3,580
"Frame-Size" refers only to Exterior-Dimensions and Mounting-Bolt-Patterns.
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#### Papabravo

Joined Feb 24, 2006
20,603
"Frame-Size" refers only to Exterior-Dimensions and Mounting-Bolt-Patterns.
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So, the total mass would depend on the internal construction and the density of the materials used. There would presumably be some variation in those factors.

#### LowQCab

Joined Nov 6, 2012
3,580
So, the total mass would depend on the internal construction and the density of the materials used. There would presumably be some variation in those factors.
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True,
but the manufacturers want their Motors to be compatible with other Motors of similar Frame-Sizes,
so the end result is that if You replace a Motor of a given Frame-Size,
with almost any new Motor of the same Frame-Size,
it will probably perform adequately, if not close to exactly the same, as the Motor it is replacing.
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#### Irving

Joined Jan 30, 2016
3,548
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True,
but the manufacturers want their Motors to be compatible with other Motors of similar Frame-Sizes,
so the end result is that if You replace a Motor of a given Frame-Size,
with almost any new Motor of the same Frame-Size,
it will probably perform adequately, if not close to exactly the same, as the Motor it is replacing.
.
.
.
Exactly so, and though mass of rotor isn't directly stated, its inertia is (either directly, or implied through no-load speed/torque/current). Certainly good enough, IMHO, to get within the tolerance the TS asked about.

#### Papabravo

Joined Feb 24, 2006
20,603
Then developing an empirical estimator of total mass based on frame size should be possible with a reasonable sample size. That data could then be further analyzed to see if there is an empirical formula that can be developed.

#### Irving

Joined Jan 30, 2016
3,548
Then developing an empirical estimator of total mass based on frame size should be possible with a reasonable sample size. That data could then be further analyzed to see if there is an empirical formula that can be developed.
It wouldn't surprise me if someone hasn't done it already, though I can't recall seeing anything like that. Overall machine mass however I think will be fairly consistent between suppliers because they are all governed by the same laws of physics and economics.

#### Papabravo

Joined Feb 24, 2006
20,603
It wouldn't surprise me if someone hasn't done it already, though I can't recall seeing anything like that. Overall machine mass however I think will be fairly consistent between suppliers because they are all governed by the same laws of physics and economics.
I would tend to agree with that unless the data suggest a large variance in the sample population. We are looking for a lower bound so I would suggest the mean minus 3σ would be a good starting point. You could revise that if no examples show up that are that far away from the mean.