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Postby Plantarum » Mon Jul 16, 2018 10:13 pm

Another machine that we bought recently. I bought it off a very bad picture as an induction motor.
I noticed however that the stator arrangement was adjustable and that does not make sense in any induction machine.
Upon arrival it turned out to be a synchronous generator. At first I thought it was 2 phase because of the way the leads come out of the housing, but it is 3 phase.
The slip rings are situated on both sides of the salient pole rotor. There is no exciter. The machine is supposed to be separately excited.
I gather the adjustable stator is for synchronization purposes. However, synchronizing a 1,5 kVA machine to any grid does not make much sense to me.
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Re: Oddball

Postby ppppenguin » Mon Jul 30, 2018 2:54 am

Plantarum wrote:However, synchronizing a 1,5 kVA machine to any grid does not make much sense to me.

When I was at university we had small 3 phase generators (<5kVA) in the lab that we could synchronise to the grid as an exercise. They were driven by DC motors and we had 3 lamp synchronisers. All good fun.
Jeffrey Borinsky www.borinsky.co.uk
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Re: Oddball

Postby Lucien Nunes » Thu Aug 02, 2018 1:25 pm

Clearly the distnictive feature is the rotatable stator that allows the phase of the output to be adjusted relative to the mechanical drive. If the drive were a simple prime mover such as an engine, this doesn't seem useful. During synchronising, the phase is likely to drift faster than the stator drive could keep up, and it can't move far enough to correct any arbitrary phase difference. So it seems more likely that the angular phase of the drive has some significance while in normal operation. It could be driven by one part of a machine, driving another part or parts via synchronous motor, with the relative timing adjustable by rocking the stator, avoiding the need for a sliding helical sleeve or differential gearing.

A similar concept was used on machines that required many rollers or drums rotated with variable speed or phase differential, such as papermaking machines. Synchronous unit drives locked to a master generator would have freely rotatable 'stators' fed by sliprings, that could be slowly driven by smaller variable-speed motors to create a small, precisely adjustable differential at the rotor relative to the master. The master could be a motor-generator set driven by a variable-speed motor, that defined the overall speed of the line.
Black was always meant to be a phase. The neutral phase.
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Re: Oddball

Postby Million Horsepower » Mon Feb 18, 2019 5:52 pm


A classic use of an arrangement like this, is to drive a mechanical rotating rectifier.
Arms in the shape of a Maltese cross make a commutator/rectifier coupled to the synchronous motor. Used for very high voltage rectification in something like an electrostatic precipitator.
I used one at Little Barford A station during training in 1980 ish. It was a circa 1930's station

You ran it up to speed switched the EHT on and then adjusted the phase angle to stop the arc running around the commutator segments.

When the commutator was damp the arcs ran all the way around it and it lit up like a Catherine-wheel.

Not saying this was that application but that it is typical of that time.
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