This illustration is from the 1940 Scale-Craft catalog, and essentially the same illustration appeared in the 1939 catalog as well.
As a first note, over the years I have not spent a lot of time thinking about S-C motors. As of now [2009--see UPDATE 3], both of the S-C locomotives that I have that I run are equipped with aftermarket, D.C. permanent magnet motors connected to the original transmission. The others that I don’t run have the universal motor, and I have probably a half dozen extra of these motors that I suspect someone saved when they converted a bunch of locomotives to aftermarket D.C. motors.
I had not noted until fairly recently that the original S-C products produced 1937-38 were shipped with the relatively rare Scale-Craft DC motor. In the 1938 catalog they state (p. 23),
All the Scale-Craft “OO” locomotives have a 7 pole armature permanent magnet motor, which assures abundant power when needed. A feature of the permanent magnet motor is that automatic reversing is accomplished by merely changing the direction of the current. The motor operates on 12 to 24 volts, direct current only.That was pretty incompatible with Lionel and AC.
So, the 1939 catalog introduced their new motor. In the listing for the 4-6-0 they state “The new Scale-Craft 12-volt A.C. motor is standard equipment; however, the permanent magnet motor may be substituted at an additional charge of $2.00.” This points out a couple things. One, A.C. was cheaper! But also now they had a product that was potentially at least compatible with Lionel and with their older products, depending on how you set things up on your system.
This photo is of both motors side by side. They are pretty easy to recognize. Note that the transmission design was modified somewhat for use with the Universal motor.
Wiring this motor required according to instructions for the 4-6-0 dated 10-6-39 a OOX-9059 reverse switch for A.C. motor operation. Essentially this switch seems to be a D.P.D.T. center-off switch. In practice it is in the tender and requires a hole to be cut in the tender to allow it to be thrown manually. These instructions may be found in the Yahoo OO group site in the files area.
By 1941 Scale-Craft also marketed a bridge rectifier. It looks like a stack of small discs with five nibs for wires coming out. The catalog description (page 91) states
The use of the bridge rectifier for polarity reserving with direct current is the latest development in remote control practice. It is trouble proof, simple, and cheap to install. These little rectifying units are placed between the motor field and line; they preserve a constant direction of field current, regardless of line reversals. The result is, that a line reversal reverses the motor—the same as with a permanent magnet outfit. It must be remembered, however, that the bridge rectifier cannot be used with alternating current. It is made for direct current only, and will be burned out in a few moments if used with an alternating current.A wiring diagram for the “reversing rectifier” is presented on this page of the catalog. [UPDATE: the diagram may be found in this post]. They also note on this page that they have discontinued manufacturing the permanent magnet motor and the universal motor is now available either as 12 volt or 24 volt.
Jumping ahead to the end of their OO production, in the final S-C catalog (the Round Lake catalog) they list three locomotives. The 4-8-4 has a 7 pole Universal motor that operates on 16 volts A.C. and 12 volts D.C., and the 4-6-0 and 0-6-0 models have a “Pittman per-mag motor.” So, actually, there must be a few late S-C models with factory shipped Pittman D.C. motors.
In conclusion, I came to OO from HO and direct current so for me personally the most mystifying thing just looking at the S-C Universal motor is the four wires coming out! Two extras! I don't think I am alone on this. I will be posting more on how to get these motors operational again, this is a puzzle worth solving.
UPDATE: See the follow-up post for more, How to bench test a Scale-Craft Universal motor.
UPDATE 2: The original DC motor runs great! See this article for more.
UPDATE 3: See also this article for more on running the Universal motor on DC with a rectifier. It also runs great! Scale-Craft clearly put a high priority on shipping out locomotives with a great motor.