Rewinding a Compound-Wound Generator - Page 1
Forum section: electronics / general

author: Xnke
author_rank: Regular Contributor
timestamp: June 22, 2023, 10:36:28 pm
content: Working on a restoration project here. No, it is NOT worth the time or money as a "Generator" but it is worth it to learn how it's done, seeing as I'm a technical instructor now and it's a minor part of the curriculum.1920's design, six-pole, exciter-cranked generator. The exciter DC windings on the rotor are also the DC starter windings. It's inefficient, sure but it's how the thing was built. The reason for the rewinding is that the last guy decided to ignore the "32V starting" rating on the data plate, and tried to crank it with a common 12V car battery. Burned the exciter/starter coils right up, never did succeed in getting the engine cranked...Currently, the exciter windings are two turns of 13AWG per coil, wave wound, 73 coils. (73 commutator bars and 37 slots.) The generator is compound wound, so it's electromechanically regulated with a single relay, and in normal operation is very well regulated over it's allowable loading.I would like to convert the 32V starting system to a 24 or 36 volt system. The manufacturer originally had 12V, 24V, 32V and 48V starting variants, and I know the 12V units had a completely different rotor assembly, with a different number of slots. 12V units had single turn bar-type windings, instead of coils.The AC rotor coils are concentrically wound on the rotor stack first, then the DC exciter/starter coils are wave-wound over the top of them, in the same slots. There are 26 turns of 11AWG per each of the six AC coils.The Stator assembly consists of 6 poles/coils with 300 turns of 14AWG and 15 turns of 10AWG wire per pole. All coils are wound in series, and the diagram seems to show the stator coils in series with the DC brushes. Not 100% sure, the wiring diagram is a bit difficult to follow yet. I'll work on getting it cleaned up enough to post here.Given that the rotor assembly has 2 complete turns per each coil at 32V, I can't just ratio my way to a 24V starting coil on the armature. There's probably going to have to be some stator coil adjustment as well...can't very well have 1.75 turns of wire without some creative commutator work...I've ordered a few books and done some reading up on the subject, and I've rewound motors before-this is the first time doing one of these double-duty DC Exciter/Starter systems though.How should I approach the voltage conversion here?

author: Stray Electron
author_rank: Super Contributor
timestamp: June 23, 2023, 12:58:33 am
content: I would love to see some pictures! Was it made for the military or for civilian use? IIRC the old Wind Chargers and similar 1920s and 1930s farm-use, tower mounted, wind driven electric generators were designed to output 36 VDC so I'm guessing that your system was designed to be compatible with those.I can't help you with the rest.

author: Xnke
author_rank: Regular Contributor
timestamp: June 23, 2023, 10:00:53 pm
content: This is a Kohler Electric light plant, originally these were "farm plants" and did 110VDC and 110VAC, or 24, 32, or 48V DC for battery charging. Mine is a model 1E21H, which is a Navy Bureau of Ships emergency backup generator. Design output was 115V/13A single phase and 32V starter, with a 32V charging system with selectable "high" or "Low" charging.Attached is the original manual and some photos.Here's some shots of the stator coils, after removing them from the cast iron frame. The pole pieces are soft iron lamination and bolt into the frame, and they're easily removable as a group. The stator coils have both a heavy 10AWG series winding of 15 turns per coil, for 90 total turns, and a shunt winding of 300 turns of 16AWG per coil, for a total of 1800 turns for the shunt winding.And one stator coil after removing the original tarred cloth overwrap:This is the rotor assembly, including the AC winding and DC Exciter/starter windings:And the first excavated winding that wasn't heavily charred:The engine has been the primary project so far, as even with absolutely zero parts being available, I'm a machinist and machine tool instructor so we just make those parts. The rewind hasn't required making any parts, just removal of wire and replacement as it's *very* burnt double-cotton-covered magnet wire.

author: TimFox
author_rank: Super Contributor
timestamp: June 23, 2023, 10:44:14 pm
content: 32 V DC was a common farm voltage (from windmill generators) before rural electrification in the US.I once had a book on radio repairs that had a whole chapter on radios designed for 32 V DC.

author: Xnke
author_rank: Regular Contributor
timestamp: June 24, 2023, 03:01:50 am
content: Yes, it's a 8V battery stack, vs the more common 6 or 12v battery systems we're used to today. 4, 8V batteries in series. 8V batteries arespectacularly expensive, especially compared with a pair of used car batteries from the local scrapyard. Back then, farm batteries were usually railroad batteries composed of individual cells, so whatever cell arrangement you needed you just made up.These sets were designed during that "rural electrification" period, that's why they were offered in practically any voltage/starting battery combination that the railroads used. They were just in continuous production from 1920 until 1948, and mine was built 6-14-1944, for the US Navy.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 24, 2023, 03:54:32 am
content: Quote from: Xnke on June 24, 2023, 03:01:50 amNowadays, that would be 2S lithium.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 24, 2023, 08:49:26 am
content: Aren't Planté cells easily available in the US? That would be my choice for making an 8V or 32V battery. We still occasionally use them here in certain applications (emergency lighting central battery units, for example).

author: Xnke
author_rank: Regular Contributor
timestamp: June 24, 2023, 06:34:26 pm
content: Sure, but you need to make sure that the battery is a starting/lighting type battery, capable of extreme short duration discharge followed by long periods of trickle charging.There's a reason lead-acid batteries are still the dominant technology in automotive starting/lighting batteries, although I will admit lithium and LiFePo are making inroads.I just got a copy of Braymer's "Armature Winding and Motor Repair", published in 1920. It's a first edition, too, so that's really neat. It has an entire chapter on this style of motor/generator armature and it was published the same year that this model of generator was first sold, so I'm hoping while I'm on vacation this week I'll absorb some good information.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 24, 2023, 06:56:48 pm
content: Planté cells are a type of lead-acid cell, in fact the first. They are very reliable, even when abused.https://en.wikipedia.org/wiki/Gaston_Plant%C3%A9#:~:text=In%201859%2C%20Plant%C3%A9%20invented%20the,to%20the%20Academy%20of%20Sciences.https://ukbatterysales.co.uk/brand/enersys/reserve-power-emea/340-2/powersafe-plante-range/?doing_wp_cron=1687632964.4471731185913085937500

author: Xnke
author_rank: Regular Contributor
timestamp: June 24, 2023, 09:43:16 pm
content: Yes, but they're generally very expensive to buy individual cells vs buying a preassembled 6 cell battery, which is very cheap by comparison.For example, a pair of new 12V lead-acid batteries capable of starting the generator would run about 225$. Used could be as cheap as 50$.A single 2V lead acid cell of appropriate capacity would run about...225$. No one uses single 2V cells for stuff like this...so while they are available they are very expensive.A single 8V battery (Of which you'd need 4 in series) will run about 125$, or about 500$

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 24, 2023, 10:41:39 pm
content: Have you considered rewinding it to 16V and using a 4S R/C lipo pack?

author: AVGresponding
author_rank: Super Contributor
timestamp: June 25, 2023, 07:39:37 am
content: How about traction batteries (for forklifts etc)? These are often made up of individual cells, I'd bet they're cheap enough from a scrapyard.

author: TimNJ
author_rank: Super Contributor
timestamp: June 25, 2023, 07:10:03 pm
content: Is an electronic solution possible? Not sure how this engine works, but i assume it requires a relatively high “cranking current”? Any idea how much?Rewind to 16V? 24V battery with some chonky TO-3 transistor to drop it to 16-18V or so? e.g. MJ11032G or something.

author: Xnke
author_rank: Regular Contributor
timestamp: June 25, 2023, 10:10:46 pm
content: The problem is that not only are we changing the DC cranking voltage, we are also changing the DC exciter voltage that's used to excite the generator's field. The interaction is the biggest issue with getting the rewinding data correct.That's also why I'm trying to stick with the 24V or 36V system, both of those voltages were used with this generator system in other models and both voltage choices used this same rotor hardware, so I know that those two voltages can be done and there is factory data on setting the regulator up for them. I'm working on charting out what the differences are in the regulator/controller between the various voltage models. Some of these units were so rare that it's hard to find the factory manuals for them, like the 48VDC units...very uncommon!