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

Sunday, August 2, 2009

More on bench testing S-C universal motors

A prior post is on “How to bench test a Scale-Craft universal motor” and it remains one of the more popular pages in the site. People do seem to want to run these classic models.

This past week I got back to working on a pair of locomotive projects that both have S-C universal motors and tried to bench test them with my new power pack. What I found was that they only run weakly at lower voltages. At higher voltages they seem to lock up due to the way magnetic field worked. But on the AC transformer I use with O-27 they run fine.

The new pack is a MRC Control Master 20, which I love on the layout--a great pack. As a bonus, the large scale setting works well with early Scale-Craft DC motors, as it will put out more than 12 volts and those are 24 volt motors.

According to MRC it produces “smooth, flat line DC.” At 100 VA it is putting out plenty of amps to run Scale-Craft universal motors. But as noted they won't work with this pack. My guess is that it is something to do with the waveform of the DC and the fields of the motor. In one of the product listings I see this wording: "filtered and regulated DC output."

The bottom line is that S-C AC/DC universal motors seem to be touchy on modern DC, at least wired up for bench testing, but work fine on AC. In the post yesterday I noted that back in the day AC power was not uncommon in OO. I need to track down a rectifier; it may solve the issue. But for now I will stick totally with permag motors on the layout.

UPDATE: Use of a modern rectifier is the key to running the SC 12V AC/DC motor on modern DC. See this article for more, it took me years to learn how to wire a rectifier, and with a rectifier these motors are smooth and powerful.

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