We have a motor-alternator that we haven't been able to determine the model or type. All we know is that it uses 32v. DC input and outputs 115v. AC (~2Kw output). Here are photos of the Motor-alternator and the control panel for it.





Can anyone help identifying the model? Does anyone have a manual for this? We know that it runs, but it seems to need a little work in the controls as it wants to spin way too fast. Any help would be much appreciated.

I assume you have figured out that the motor-alternator is a Safety Car Heating and Lighting Co. product. These appear to be numbered in the MG series, for example MG-214 is a 32vdc to 115vac, 3000W unit. I am going from a list of copyrighted items maintained by the Library of Congress, which includes a parts list for the above circa 1950. The N&W Historical Society has several wiring diagrams available for Safety motor-alternators on the N&W's fleet of passenger cars, which might be of some help if one of them matches your unit's voltage/wattage. You can search their on-line archive and see if anything looks promising.

No help identifying any source of electrical schematic, but I can identify some of those parts.

The white, multi-finned item is a selenium rectifier. Changes AC to DC. They are obsolete and chances are it would be defective.

I'd say the long, flat squiggly thing at the right is a heat dissipating resistance element.

The two items with round coils and arms that move up and down to the coils are cut in/cut out relays.

The white tubular item between them is a resistor.

Selenium rectifiers are obsolete technology. I've worked on a few items that had them in them in recent years... mainly vintage battery chargers. They were all bad.

You'd think that replacing them with diodes of the proper size would fix that issue, but it doesn't. The electric systems in those battery chargers, for instance, wouldn't function correctly with diodes. Seleniums are not efficient as diodes, so the chargers put out considerably more voltage due to the efficient diodes, which overcharges the battery. It's all due to the fact that the old transformer is designed to put out way more voltage than its modern counterparts due to the inefficient selenium rectifiers.... so when you put diodes in the circuit... they let more voltage thru...

So, is the selenium rectifier an example of the mythical "unobtainium"? Is there a source for new ones that isn't prohibitively expensive? I'm pretty sure this is beyond the capability of volunteers to reproduce. This motor-alternator has to provide power for strain gauges, the car's recording table, and the all important crew refrigerator so I would be hesitant to replace the rectifier without knowing that the replacement won't adversely affect the system.

Related question; could this be why the motor-alternator seems to want to spin way too fast; if this rectifier isn't functioning?

Well, disconnect the leads and test the diodes in the rectifier. It's most likely a stacked bridge rectifier. It's pretty easy to test a diode.

If it is the rectifier, then you need to find the data sheet for that rectifier, note its traits, and create an equivalent out of other components. For instance, if your rectifier has a forward voltage drop of 1.6 volts, and you find comparable modern diodes with a forward voltage drop of 0.8 volts... just put two of them in series. As you can see, not rocket science.

Safety Electric is still around. They are part of Wabtec now. Last I had contact with them they had a service center in the Meadow Lands area of New Jersey. Might find someone there willing to help. Both the old TRAIN people and the private car association have put out lots of information on pre Amtrak passenger cars. There is a lot of this equipment still in use, it is just a mater of finding the right person to talk to.

As an electrician who has worked on more than a few old AC generators, I would not assume that the selenium plate rectifier is defective. Yes in battery chargers they had a high failure rate due to them be subjected to over current from trying to charge batteries that were internally shorted. In old AC generator applications they typically supply DC current to an auxiliary DC field which may be similar to the application in the original post. In this application they are not typically subjected to over current and have a very long lifespan. Case in point about 15 years ago I restored a 3.5KVA Kohler engine driven generator as a hobby project. It and subsequent similar models for many years used a selenium plate rectifier to provide this aux. field. The unit I restored was built in 1948 and had it's original control intact. A few years later I had a customer that needed a slow speed long life generator for back up on an off grid power system so I sold it. Last I knew it had over 1200 hours since it's overhaul with the original rectifier. Kohler used to provide a diode type bridge rectifier as replacement and may still which as I recall included an additional resistor to make it match the output voltage level due to the increased efficiency as stated by another poster. On old battery chargers when replacing a tungar tube rectifiers with a bridge rectifier made of diodes one has the same issue of needing to add an additional resistor. FWIW I used to send selenium plate rectifiers from battery chargers to a company called Philbin Mfg. in Portland, Ore. to be repaired.