The online magazine on the history and operation of vintage scale model trains in American OO gauge

Wednesday, March 13, 2013

Wiring the Scale-Craft Universal Motor to a Modern Rectifier

One thing I have found that I have had to do as an American OO enthusiast is from time to time recreate lost knowledge. In particular I have found the Scale-Craft Universal motor to be intimidating (more on S-C motors in general here). I had only seen a model running with one of these on DC power one time, years ago on the layout of Bill Chapin. He had wired the model to use a new rectifier he had purchased at Radio Shack. I had not figured out how to do it, and was only able to bench test the this motor well from an AC transformer (more on bench testing here). But I run DC on the layout.

Fast forward to a recent purchase. Oreland No. 8 is a Scale-Craft 4-6-0 with a Nason 2-8-0 tender. It was clearly set up by the builder to run well on DC and has a few nice extra details (the ash pan detail just visible in the photo for example). Bench testing with DC power showed it should run, but the wiring used in the tender was brittle and shorting out.

Yesterday I rewired the tender, as seen in the photo. The result is I was very impressed how smoothly the engine runs! No wonder Scale-Craft stuck with this motor for so many years. it is a smooth and powerful motor.

This is a drawing of the wiring as seen on Oreland 8.  (Click on it for a better view). Note the original Scale-Craft pins used to connect the wiring between locomotive and tender, connecting to the four wires from the motor brushes and winding.

There are a few other notes to glean from Oreland 8. First note that all power pick-up is from the tender. I have seen this arrangement before and it works really much better than I thought it would. This model has Nason trucks on the tender (the two-rail type with the square, insulated bolster) but the builder had equipped them with more reliable Scale-Craft wheelsets. The bronze sideframes allow not only for better electrical contact than would S-C sideframes but also allow for soldering the pick up wires directly to the sideframes. This requires in turn very flexible wires or the locomotive won't track well. In my case, initial testing says the red wires seen above are a little heavy, so my next step before reassembly is to replace those wires.

It should not be too hard to find a similar rectifier to duplicate the installation, and I will update this article with further information as I work out other models.

UPDATE: This rectifier looks similar and would seem to be in a rating range to work.

UPDATE II: And it works fine. It still took some trial and puzzling on my end. It pays off to briefly bench test the motor on AC power (make sure the motor works first!), but the critical thing seems to be that in the diagram R is "+" and B is "-" on the new rectifier. Plus and minus both are connected to the coil on the motor and one of the ~ wires goes to a brush. The other brush wire and the remaining wire from the rectifier are what connects to the rails. Connected in this manner these motors will give good service with a DC power supply of sufficient capacity.

Also note that most likely this engine was built by Newton Guerin, an active member of the North Jersey group. A photo of his layout mentioning Oreland junction may be seen here.

UPDATE III: See another description of a model now running with a modern rectifier, a Scale-Craft 0-6-0. And note this photo! These four frames with trucks and rectifiers mounted were all in the same recent eBay lot. Someone was working on a big installation of these rectifiers in vintage Scale-Craft engines -- the original S-C banana plugs are wired in and also note the trucks. All four frames have Nason trucks on them, mostly the "square bolster" type, which are the way to go for pickup, the bronze does better than the metal used by S-C.

UPDATE IV: I used the one above with the green wires in a rebuilding project as it is. This type of installation works really well too. Compared to the drawing, R is +, B is minus, the leg closest to the + is Green and the remaining leg is yellow.

1 comment:

Anonymous said...

Hi, really enjoy this blog even though I don't model OO - the retro modelling aspect is fascinating. I notice in some previous posts about these motors they are described as 'series-wound', but you describe wiring them with the field winding and the armature in parallel. I suspect that is the source of heating issues and heavy current draw. In the circuit you have drawn for the 'oreland' engine, the field winding is in series with the armature, and the direction of current through the field winding is set to be always the same by the bridge rectifier (regardless of track polarity). Any bridge rectifier with a rating above say 2 amperes should work fine there (including the one you picked). If you need help figuring out which wire goes where speak up...