slooooowing it down- diodes in AC engines??

I'm not an EE Mike, but I don't believe the diode trick will work on AC "Pullmor" motors.  I hope some of the bright stars on this forum read this and chime in...

I've been brainstorming different ways to tame speedy postwar steam locos for years.  The only credible suggestion I ever received was to rewind the field and armature with more turns of a higher gauge (thinner) wire, which would increase the voltage and torque.  Never tried it, outside my range of skill.

Locos originally made with a 2-position e-unit actually have two field windings of opposite polarity.  A few years ago I wanted to upgrade one of these to a three position E-unit with neutral.  Because of the 2 fields, you end up with some extra wires.  At first I tried connecting both field windings together end-to-end, which increased the resistance of the field relative to the armature.  I think the top speed of the loco was reduced, but it didn't improve the starting or running characteristics at slow speed.  So I just ended up using one field winding, and taped off the ends of the other one.  This is the accepted way of doing such an upgrade.

In the default configuration, the armature and field are wired in series with each other.  One of the experiments I wanted to try was to power the armature and field separately, to see whether a smoother start could be achieved.  This would have to be done on a bench with two phased transformers connected to the armature and field respectively, or perhaps one transformer feeding rectifier diodes and two rheostats.  This would be closer to the idea in the title of your post.  [Readers try this at your own risk!!]  I've also wondered whether modern electronic E-units such as ERR's AC commander, and the Legacy boards in Lionel's Pullmor-motored GP-9s still drive the armature and field in series under all circumstances.  (Yes Virginia, Lionel did sell a few AC motored locos with Legacy, and unfortunately they don't have Odyssey speed control )

In 2013, I tested a 2056 Hudson from one of those commemorative sets.  This loco was newly produced in China around that time.  It was a little balky at the lowest speeds, but seemed to maintain a consistent medium speed better than any postwar model.  Like the original, it had an AC parallel plate motor with spur-gear drive.  I think it was geared just a bit lower (maybe 11:1 vs 10.5:1 for the original), but I'm convinced that the electronic E-unit was doing something to manage the changing load as the loco navigated my floor layout that most definitely wasn't level!  Now I don't mean fine speed control like PS2 or Odyssey, but much more consistent than the runaway roller-coaster effect I observed in back-to-back tests with my postwar 2026, 2034, etc.  Unfortunately I never took that loco apart or tried retrofitting its e-unit into anything else, so the mystery of its improved control remains.  Some other folks have posted that the mechanical tolerances in the motor were poor, which might have accounted for the low speed balkiness.

It's also known that recent issue Lionel motor brushes have a much higher resistance than the originals.  It's so much higher that the new brushes are unsuitable as postwar replacements.  It could be that Lionel redesigned their last few AC motors for higher voltage operation, and together with the high-resistance brushes, these motors are less sensitive to changes in voltage and changes in load?

Mechanical improvements that would really make a difference are a 5-pole armature (or a "skewed" armature), and a lower (numerically higher) gear ratio.  But unless I win the lottery and buy my own stamping plant, we probably won't see these in our lifetimes. 

Not sure why pairs of back-to-back diodes wouldn't work for AC motors, but it sure seems they would.  However, low speed performance with Lionel Pulmore motors isn't easy to accomplish.  FWIW, I saw MUCH better low speed performance when I added a second motor to my Phantom locomotive, and the swap out of the LCRU to the ERR AC Commander helped as well.  With those two changes, the Phantom has surprisingly good low speed performance, much better than I expected, and way better than with the single motor and the LCRU.  I believe having two motors helps even out the effects of the 3-pole armature a bit.

If you have command control you can convert to the locomotives in question to ERR AC Commanders. I have converted 4 Lionel diesels with separate motors and trucks (2 GP9s (one postwar, one modern), a LTI era F3 (vertical motors) and a 2343 F3 (horizontal motors)). In 100 speed step mode all of them will run as slow as 25 scale mph, but not much less than that without stalling on 072 curves. 

In my case I have probably the best Pullmor motored drivetrains for getting slow speed performance (along with the twin vertical motored FM, GG1, and early NW2/Alco FA). Good luck getting a late Alco/NW2 or spur gear steam locomotive to run slow.

Hmm...  I just had the Phantom out on an O60 test loop, and I could get reliable operation at way slower than 25 scale MPH, perhaps 8-10 scale MPH.  Mine has dual vertical motors and the AC Commander.  I'm a bit surprised you can't get better than 25 scale MPH, time to do a lube?

The last time I had a carefully-leveled platform layout, I found that my MPC-era 8361 with a single Pullmor motor could navigate an O42 oval at ~10 mph.  That matched the performance of a much newer MTH Premier diesel with PS1.  So much for scale models and 20 years of progress. 

The O72 test loop on the floor of my loft wasn't level.  (The whole building was tilted, 1970s construction.)  It was hard to average 25 mph! Conventional locos, especially those with AC motors, would race down one side and slow to a crawl near the top of the loop where I had the power feed.  Legacy worked great.  But one PS2 Berkshire exhibited bucking on the downhill side- something about the timing of the feedback loop created an ugly cycle of hesitation and surging.  Self-locking worm gears are evil (and so are rubber tires, but I digress...)

Weakening the field will cause the armature to speed up, but I think you will lose torque.  If you decide to try these experiments please share your results!

I have found that the use of a powermaster and cab1 makes all old lionel run much better including my dad's 103 year old 706

gunrunnerjohn posted:

Hmm...  I just had the Phantom out on an O60 test loop, and I could get reliable operation at way slower than 25 scale MPH, perhaps 8-10 scale MPH.  Mine has dual vertical motors and the AC Commander.  I'm a bit surprised you can't get better than 25 scale MPH, time to do a lube?

Oh all the Pullmor motor locomotives I converted with AC Commanders will go that slowly, just not all the way around the layout with 3 dummy units and 12 freight cars in tow. Hit a 054 or 072 curve at 10 scale mph and they stall about half way through the curve with the above train in tow. 20-25 scale mph is about as slow as they will make a complete circuit without stalling. 

Try the above test with your Phantom. I'm curious if it'll respond the same. My stuff is regularly lubed so it's fine, but my layout is not 100% level (basement floor is sloped).

Mike Wyatt posted:

Would installing diodes inline to an AC engine (postwar Pullmor or open frame, not can motor) - would that have the same effect as it does on a DC motor- that is a slow-start??  probably a dumb question, but inquiring minds want to know!!

An unqualified YES I have used them for years. Used in pairs  wired head to tail with both ends tied together.  They will work on universal AC/DC or straight DC motors. In conventional  mode they force you to give more throttle to get the loco moving and with your Lionel transformers normally starting off at 6-7V each pair of diodes drop the voltage about ,75V so four pairs would lower your transformers minimum voltage by about 3V. This allows you to set the voltage more precisely at the point where the motor just barely turns over. It does not actually overcome the mechanical limitations of the locomotive and it's motor / gearbox just allows you to adjust your transformer more precisely.   Another way to improve low end performance in a universal motor is to run it on DC and use two diodes on your Lionel transformer so it is putting out half wave DC. Now if you do this you loose control of whistles and bells and I don't know how any version of RailSounds would behave.  It will however give you very fine control of low speed actually overcoming the normal mechanical limitations of the motor and gearbox. In HO scale this was referred to as pulse power. Some HO power packs had a switch to go from full wave DC to half wave pulse power. A couple even did this automatically gradually shifting to full wave DC as you turned up the throttle.  I posted drawings months ago on how to wire a universal motor to run on DC with reversal achieved by switching the polarity of the power going to the rails. I don't have time tonight to track that drawing down but you can by going to my members page and looking through my previous posts. When running on half wave power you can still add pairs of diodes to lower the minimum voltage your transformer can output.             j