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Comment by femto | Hacker News Reader

Is Wikipedia's statement that hydro stations require an external source for field excitation correct? I thought it was possible,to start a hydro generator by "self-excitation". When a generator stops, there is typically a small residual permanent magnetic field left, due to hysteresis. This small field may be used to "bootstrap" the generator, whereby on initial spin up a small current is generated, which may then be fed back to the field coil, further exciting the generator's field and increasing the output, until full capacity is reached.

All that is then required is a method of opening the penstock, which can presumably be hand cranked?

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Some quick research on generator excitation:

* Each of Hoover Dam's generators appear to me, based on this photo [1], to have three stages of excitation, vertically stacked on the rotor shaft.

* A year 2010 upgrade to digital controls at Hoover Dam [2] shows:

___ o A total plant capacity of 2.074 GW from the 17 generators

___ o 20 seconds to go from reactive power control ("condense mode") to generator mode.

___ o Pictures of the original and replacement governor equipment; the red cabinetry shows a beautiful 1930s design style

* In "condense mode" [3], the turbine's penstock valve ("wicket gate") is closed off, the turbine volume filled with compressed air to displace the remaining water, then the generator operated as a massive synchronous motor. In this mode, the excitation controls can provide marketable power factor correction, aka "reactive power" for the grid.

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[1] http://bayimages.net/view-photos/electric-power-generators-h...

[2] http://www.hydroworld.com/articles/hr/print/volume-29/issue-...

[3] http://www.bpa.gov/Finance/RateCases/BP-14RateAdjustmentProc...

Permanent magnet excitation is a possible way to generate the required field flux for a generator, but it's not a reliable way.

If the permanent magnetic field isn't strong enough you can't even generate enough voltage to power-up the excitation circuit at all, and that's the circuit which would feed the current back in.

So whether this is feasible or not depends on the generator design.

I've got memories of some of the Snowy Mountains scheme's hydroelectric generators being designed to start though self-excitation.

Wikipedia also has a bit about it [1]. It seems that there is usually a backup facility to run a DC current though the field winding before starting, thus magnetising the magnetic paths, for when the residual field has decayed too much. That backs up your comments about reliability.

[1] http://en.wikipedia.org/wiki/Self-excitation

Residual flux is usually enough to start, but if you lose all flux for some reason (when the core is completely demagnetize) zapping it with DC can help restore it.

Theres a lot of word choice issues in this discussion. A generator, like on a old fashioned bike light, does not require a field winding it uses a magnet. Magnets can't be controlled as you'd guess so they're not to popular above a couple watts. Even cars don't use them, at least not for decades now.

Generators have more parts and are pretty finicky and delicate compared to alternators which are mechanically pretty simple beasts in comparison.

Alternators are electrically controllable and have a field winding.

Something like a diesel locomotive has a couple stages with a small 32 volt generator providing enough field current to run a small alternator which provides the field for the big alternator which connects to the drive motors. In the old days they used DC drive motors but for decades now they basically have multiple giant rolling VFDs and ac motors. VFDs are more reliable than mechanical commutators, which sounded crazy in the old days but its true.

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