About the moment of inertia

[zimou888]

Could anybody tell me what is the exact meaning of load moment of inertia? And what is the weighting factor?

 

[btrueblood]

http://en.wikipedia.org/wiki/Moment_of_inertia

 

[AutoXer]

Moment of inertia is basically rotational inertia. It's the resistance to rotational acceleration/deceleration of a mass. Load moment of inertia is just the moment of inertia of the connected load. If you're running through any mechanical advantage, it must be factored in.

 

[cloving]

On a similar note, I have a question regarding generator inertia. I know that for islanded (or backup) generation, generators must be larger than the largest motor loads which they must power. Obviously a 600hp generator cannot start a 500hp motor. When large loads come online, the power to supply them, before governor action, comes from the inertia of the generator's rotor. This is basically saying that the instantaneous power consumed must equal the generation, and before the prime mover can supply the power it must come from the rotating stored energy, slowing down the system. Is there any way to translate the mass or inertia of a generators rotor to how large of a motor load that can be started from it? Suppose a 5,000 kg cylindrical rotor?

 

[electricpete]

Voltage drop considerations would typically play a big role in determining how large a motor can be started from a given generator. I don't think generator inertia typically plays a big role.

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[itsmoked]

I think it does play a large role just as cloving is suggesting. Indeed a mechanical engine requires time to adjust to a new large load. That time will translate to frequency loss due to speed loss before the prime mover can recover. The magnitude of the frequency drop is going to be directly tied to the kinetic energy present in the rotor.

electricpete; What about those UPSs that hold up the load on this kinetic energy alone while the prime mover is started?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricpete]

Here's my view (right or wrong). It depends primarly on associating reactive power with voltage and real power with frequency.

1A- The motor power factor during start is very low. It demands a lot of reactive power. Reactive power depends primarily on voltage.

1B - The inertia assists the generator to supply real power. This real power helps to keep the frequency up.

2 - The presence of larger inertia means that the frequency will not droop as much during start. It doesn't tell us anything about the voltage.

3A - The torque developed in the motor during start depends primarily on the voltage (varies roughly as square of voltage).

3B - The torque developed in motor during start depends to lesser extent on frequency.

For a typical system, I believe the fractional variation in voltage will be much higher than the fractional variation in frequency. And the motor torque is more sensitive to voltage variation than frequency variation to begin with (item 3).

So the challenge during start is to keep the voltage up to help the motor start fast. The inertia does not particularly help with this aspect.

Again, just my view. I don't get involved in design activities or operating generators in island mode. If my logic is incorrect, I'd be interested to hear.

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[electricpete]

The frequency droop during motor start will certainly be less severe if the generator has higher inertia. It's my impression that frequency droop is not typically a factor considering that the motor is demanding primarily reactive (vs real) power.

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[cloving]

Thanks electricpete and itsmoked for your thoughts. That makes sense. I just wondered because voltage drop certainly plays a big role when starting loads under "normal" conditions. I wondered if that was any different in a less rigid, smaller system.