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

Saturday, February 7, 2009

How to bench test a Scale-Craft Universal motor

As noted in the post on Scale-Craft motors, I came to OO from HO, not from Lionel. All of my locomotives until roughly five years ago were permag, including my one Lionel 4-6-4 which a prior owner converted to a permag DC motor and my two S-C 4-6-0 drives.

When I finally purchased a locomotive with an S-C Universal motor on it I will admit it, I found those four wires out the back of the motor intimidating! I located the wiring diagrams (see the files area of the OO Yahoo group) [Update: also here] and while I think I might have actually got the motor that seemed to be in the best shape (this one in the photo) wired up correctly, I could not get it to do more than hum. It was a bit frustrating. I moved on to other projects.

But, I knew I had seen these models run with the original motors, and S-C shipped lots of these motors so it had to be me not the motor. Fortunately, Dick Gresham sensed from the other post the info I really needed and provided it--how to wire up a Universal motor to bench test it. I think others out there need this basic rundown as well:
Scale-Craft 12V AC-DC motor - I have a caution tag that I think came with this motor. It says "This universal motor operates on 12 volts A. C. or 9 volts D. C. Oil motor bearing holes with fine motor oil before using." I use the following procedure to bench test the motor. First I check that the coil wire is continuous by using an Ohm meter. You could also use a continuity tester or a test light, or just omit this step. Then I twist the end of one coil lead to the end of one brush lead. Next twist the end of the other coil lead to the other brush lead. Now you should have two wires to hook up to a DC or AC power supply. I apply only a few a few volts to the motor initially, and turn the armature manually if the motor doesn't start immediately. I've run several of these motors in this manner…. Some of the motors seem to be sluggish. I surmise that the grease in the transmission has dried up, and needs to be replaced.
So the key is that you need to wire one side of the coil to one brush and the other side of the coil to the other brush. THEN you have two wires to connect to your power pack. Hooking up the wires in this manner I applied power from the little HO train set transformer on my workbench and the motor would only hum. I took it over to the layout, and touched the leads to the track leads, and it would run! I don’t have a meter to see how many amps it is drawing, but it needed more for sure than the little transformer put out.

On DC the motor runs the same direction no matter what the polarity is. The way to reverse the motor is to reverse the coil wires, to hook them electrically to the opposite brush. Then the motor runs the opposite direction. Which now makes the manual reverser that S-C shipped with models make lots of sense, as to reverse the model you have to manually switch the relationship of the coil wires to the brush wires.

The other thing that now makes sense is that the model runs on AC exactly the same way as in DC, with the manual reverse. I hooked the motor up to the transformer I use with the very little O-27 I have (mostly Marx) and it ran like a devil! I am sure it is kicking out more amps than what I use for DC models, maybe too many really. But the bottom line is if you normally run Lionel OO on AC you can safely plop a Scale-Craft locomotive with the manual reverse and the Universal motor right down on the rails just like a two rail Lionel 4-6-4 and it should run fine. It will run on AC or DC.

But note: if the locomotive is set up so that the Universal motor is wired with a rectifier this model is only for operation on D.C. Again, as S-C noted in their 1941 catalog, “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.”

The whole AC/DC problem came up because while S-C introduced their line as DC in 1937, Lionel made their models in AC in 1938. The S-C DC motor cannot be operated on AC. Their solution was the Universal motor, which can operate equally well on AC or DC. [UPDATE: But, it should be noted, this was not a new solution, as I see in the 1936 Nason catalog their motor was also AC/DC and was set up to be used with a hand reverse as well--S-C was the one who tried to go against the grain with the DC motor].

Dick did also have some very interesting notes on the early Scale-Craft DC motor that I would like to share as well.
Scale-Craft 24V DC motor - I only have one extra 24V DC motor. Initially it would not run. There is a Bakelite brush holder attached to the motor with four brass screws. I removed the brush holder and discovered one brush spring was badly bent and useless. I think the spring came into contact with the commutator when the motor was running. There is a steel ball in the brush holder to reduce friction on the armature shaft. I wasn't expecting a steel ball. It fell out, and I had to retrieve it. I found a spring in my supply of extra springs that would fit in the brush hole in the brush holder. This spring was considerably different than the original spring. I reinstalled the brush holder with my replacement spring. The Bakelite is brittle. I overtightened one screw and broke the corner off. I glued the corner back on with super glue. The motor ran very smoothly. I was able to start the motor repeatedly on 1 or 2 V, DC. When I reversed the DC polarity, the motor ran smoothly in the opposite direction.
I will keep working on the two locomotives I have with the Universal motor. The motors are still are a bit sluggish on my 20VA DC packs so I may need to upgrade there, but they will operate on the layout with the original motor.

UPDATE: See this article for a few more notes on this topic. Short version: I purchased a new, larger DC power pack and while it works great, something about the output waveform won't allow the motor to run. Bench testing the Universal motor as described above is best done on AC.

UPDATE 2: And see this article for notes on running the Universal motor with a modern rectifier. For DC operation this is certainly the way to go, it is a smooth and powerful motor. It is for me anyway also the easiest to bench test motors when connected to a tender that has the correct rectifier setup.


mongo said...

I wired up my 12V universal motor as you've suggested and tried to run it on AC and it does indeed run. However, at 12V the motor does not run much faster than at 6 volts and there is a lot of heat generated. My question is once I connect each brush/pole pair together can I safely run those two leads to the track via the tender's wheels and run long term on AC power? I'm using a ZW lionel transformer for power and checking the output with a multimeter. I don't have a large enough DC transformer. Incidentally, if I choose to run on DC, how much current do these motors need to run at 9V?

mongo said...
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John Ericson said...

I hate to say firmly you can or can't run it safely on the ZW, I have never tried it in any long or short term trial. It seems to me people did run them that way back in the day, but by the post-war era almost everyone in OO was running DC. As to the current draw, I am not sure either and don't have a way to measure it myself. I have not set up a S-C universal motor beyond bench testing, I use DC motors on the layout.

mongo said...

What type of DC motor setup do you recommend I use with the original geared transmission for the 12V universal motor? I have a 4-6-0 scale-craft locomotive that I am working with. The transmission runs nicely, but the motor generates so much heat it troubles me because at times I can see smoke coming from the coil and I'm not really sure why so much heat is being generated by the original scale craft motor.

John Ericson said...

This gets at another problem as it is not easy at all to set up another motor to run with the original S-C transmission. The only one I have seen done involved I believe a prior owner hacking the motor off just leaving a stub of the shaft, then using an universal to connect a DC motor to that stub.

I have puzzled over this question at length as I would really like to for example re-power my S-C 0-6-0 with an upgrade DC motor. There has got to be a way to do it, but I have not arrived at an obvious solution. Other than figuring out how to get the original motor to function well...

I will have to return to this topic after New Years.

S-C was oriented to the scale market and DC and clearly ran these on DC with a rectifier as the optimal setup (that is what they sold for power supplies--1940 catalog for example sells 24 volt DC power supply for OO).

When I bench test these with my big main MRC power pack on the layout they don't run well for reasons I am unclear about. On the other end of the spectrum, a train set transformer does poorly also as it does not put out enough amps.

My thinking is the key to this puzzle is getting a rectifier wired in with the locomotive and then running on DC. The one S-C universal motor model I ever saw running well was equipped with a modern rectifier from Radio Shack. As I write, a 4-6-0 just arrived with a modern rectifier in the tender. The motor is too stiff to turn over but I will, again, follow up more after New Years.