Exciter

What is an exciter? Context: The machinery was part of the installation when the first man was electrocuted in the electric chair in 1890. "The apparatus consisted of a stationary engine, an alternating-current dynamo and exciter"

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I had heard of Field Ecxiters of Motorized Circuits years ago but have never really seen one. I Believe they are designed to facilitate monitoring or additional circuits to be added at relatively long distances. =AE

In the context of a dynamo, it provides the initial magnetic field to get the thing going. Without it, there's no magnetic field so you can spin the dynamo all you like, and it doesn't generate any voltage. In an old car dynamo, this would be provided by the battery, or sometimes some residual permanent magnet in the dynamo. Once it's got started, the dynamo generates enough spare electricity to maintain the magnetic field itself, and the exciter is no longer required.

It is a small DC generator which provides the field current for the (AC) alternator. Typically it was mounted on the shaft of the main alternator but in 1890, it could well be a seperate machine. On smaller machines it was self excited while on large machines its field was provided by a smaller self excited DC generator. In time it was replaced, first by amplidynes, then magnetic amplifiers, and, finally to solid state rectifiers and "brushless exciters where the AC was converted to DC for a fixed field winding, feeding a rotating AC 3 phase winding which was connected through rectifiers to supply DC to the field (rotating)-making the Alternator effectively "self exciting" The latter approach eliminated high current moving contacts.

| It is a small DC generator which provides the field current for the (AC) | alternator. Typically it was mounted on the shaft of the main alternator but | in 1890, it could well be a seperate machine. On smaller machines it was | self excited while on large machines its field was provided by a smaller | self excited DC generator. In time it was replaced, first by amplidynes, | then magnetic amplifiers, and, finally to solid state rectifiers and | "brushless exciters where the AC was converted to DC for a fixed field | winding, feeding a rotating AC 3 phase winding which was connected through | rectifiers to supply DC to the field (rotating)-making the Alternator | effectively "self exciting" The latter approach eliminated high current | moving contacts.

I've been wondering why it is that the field needs to be powered by DC. Obviously with DC, the field is effectively fixed position on the shaft, and thus what you have is a rotating field that rotates exactly with the shaft rotation. But what if the exciter was powered by AC instead, and the field winding connections organized so that the field rotates around the shaft in the same direction as the shaft turns, resulting in a field that really rotates at twice the shaft rotation speed. This could then be driven by a prime mover running at half speed. This would eliminate the rectifiers.

There are limited applications where that, or a variation has been done. Often used for frequency conversion equipment. One variation is to apply a separate 3-phase AC to the three rings of a wound-rotor motor, while the shaft is driven by another motor with speed regulation. The output taken from the stator can handle a fair amount of power over a range of frequency.

Considering all the inventiveness of the early pioneers, if it *could* be done, it *has* been done. (somewhere....;-)

daestrom

------ You still have to get the supply to the rotor and this involves moving contacts- not as bad as a commutator but a very serious issue when the field current on many machines is in the 1000's of amperes. Compared to this, rectifiers on the rotor are cheap and rugged. To do it without moving contacts the existing scheme would have to be partially used. The brushless scheme uses a stationary field which is driven from SCR's for control and this field doesn't involve a very high current. This excites a rotating 3 phase winding which supplies the main field winding through a set of rectifiers. There are no moving contacts. In addition, the rotor 3 phase winding does not necessarily need the same number of poles as the main winding as it does not have to be at the same frequency-this gives some design flexibility which your proposal doesn't have. Your proposal would work and as Daestrom has indicated, similar things have been done but development went the other way- mainly from experience and to some extent from mind set.