There is a curious item on page 60 of the November CTT that suggests shutting off the e-unit to avoid “depriv[ing] power to the motor”. I can understand shutting off an e-unit to prevent accidental cycling or to silence the buzzing, but this idea doesn’t seem to me to hold water.
The author says that “[t]he E-unit creates a controlled short circuit…” It is of course not a short circuit, controlled or otherwise, but simply an additional load. Whether the motor is affected by it at all depends on the size of the voltage drop due to the current that the e-unit draws, through the effective series resistance of the source–the transformer, the track, and the wiring.
I tested some e-units. A typical one has a resistance of 13 ohms. However, it has an inductive reactance at 60 hertz of 30 ohms. These together are an impedance of about 33 ohms; so the current drawn at a likely operating voltage of 16 volts is only about 500 milliamperes. But this current is mostly reactive, because of the considerable inductance of the e-unit coil. That part of the current, being 90 degrees out of phase with the voltage, produces a voltage “drop” that has virtually no effect on the voltage at the locomotive. The in-phase component of current, which does produce a real voltage drop, is about 200 milliamperes. Even if the track resistance is as high as 1 ohm (which is very high), this results in a voltage drop of only 1/5 volt, not exactly “rob[bing] the motor of power.”
Like the Chief, I didn’t follow everything, but I always read your posts regarding electrical questions (like the rheostats). Its exactly why I’m grateful for a forum like this. Thanks for taking the time to share your knowledge. I read the CTT article and as a layman, I didn’t understand why anyone would design something that required it to short circuit in order for it to operate correctly.
The only thing I can think of is that perhaps an E-unit coil could develop shorts in it’s windings over the years leading to lower resistive resistance which could contribute to significantly more “Load” while perhaps still performing satisfactorily. I have several coil operated accessories and even 6019 uncoupling tracks that are 50+ years old and when I operate them the lights dim all around my neighborhood (J/k) but they still work. I suspect this is the case with them.
It’s the stepping switch that Lionel used as a reversing unit. Many prewar locomotives came in versions with and without them. Lionel put an “E” after the model number of the version with a reversing unit, which seems to have led to the convention of calling it an “e-unit”.
Shorted turns in a coil operated with DC simply reduce the number of effective turns while reducing the resistance and therefore increasing the current. The coil remains just as effective that way, but consumes more power. But shorted turns in a coil operated with AC act like a shorted transformer secondary winding and drastically reduce the effectiveness of the coil, while greatly increasing the current drawn. An e-unit with shorted turns would not be of much use. I rather suspect that someone at CTT just imagined that the things draw a lot of current and somehow slow down the train, without checking that hypothesis for plausibility, and exaggerated the tiny contribution of the e-unit to voltage drop at the locomotive.
I too was mystified by the idea that an E-Unit was some kind of short circuit. And, I couldn’t see it appreciably altering the effective voltage to the motor. Glad to know that I wasn’t the only one. (It’s been quite a few years since my college electonics classes, but I didn’t think I had forgotten that much.)
On a related note: On the same page of tips, did anyone else take exception to the “avoid the singles” tip. Properly maintained and lubricated postwar single engine locos are quite respectable ‘pullers’. And in many cases are better runners than their twin motored siblings.