"Universal" Motors: AC Vs. DC

I'm sure this has been discussed before, but since every site I've looked at seems to have a different answer, I thought I'd ask here.

Does the universal motor in an American Flyer engine actually run any different or better on DC?

I have a large and very nice Heathkit power pack, which runs my DC trains wonderfully. However, although it can run my AF engines very well, it does show a tendency to trip the "overload" circuit when powering one of these AC/DC locos. These same engines run fine with my MRC AC pack, with no signs of "overloading". Is this due to amperage, or what is the cause?

Any thoughts or experiences are most welcome. Thanks.

Mark in Oregon 

Original Post

Mark,

The open frame motors in the Gilbert locomotives. especially steamers with a smoke unit, require a lot more current than modern can motored diesels or even steamers.  If your diesel has two motors, you can almost double the power requirements.  Your Heathkit power pack may not have enough wattage to handle a locomotive, especially if it has a few passenger cars behind it.  Check the power output on the Heathkit compared to the MRC one.  I'll bet the wattage on the MRC is higher. 

 

 

Thanks Jerry

The info on the MRC (Tech ll Trainpower AC) shows "0-17VAC Variable /18VAC Fixed Total Output 40VA"

The Heathkit (RP-1065) shows 0 to 15 volts and "Overcurrent Trip point greater than 1.5 amperes"

Does that help?

In any case, I remember seeing a "Tracks Ahead" episode featuring the "Robinson Layout", and the narrator says that running on "filtered DC" makes the trains run quieter, pull more, and run cooler. It then goes to show Mr. Robinson explaining how installing a rectifier (available at Radio Shack!    will do the trick.

It's just something I've been curious about...

Mark in Oregon

Your HealthKit power supply of only 1.5 amps is no where enough.  Need a DC supply with at least ten amps capable.  Your MRC power pack is only some what better at only outputting 2.3 amps at full volts.

https://www.rapidtables.com/ca...o-va-calculator.html

Volt-amp and amp are different.  You can argue the technical aspects, but I think VA is often used just because of the higher numbers involved; the numbers seem more impressive.

Ten amp is big transformer.  I'd say it's "what you want" more so than what you need (minimalist need). But buy big and the future is ready for "anything"  

Ac will tend to run cooler than dc. Everything, even wire, gets a tiny break from ac's rising and falling of voltage each cycle (60hz.. 60 cycles per second) Dc is a constant push, not much rise and fall of voltage beyond some latent "bumps" from flipping the negative ac wave and smoothing all the voltage peaks with a capacitor. (batteries have no lumps... pure dc)

A Lionel ZW, KW or A.F. 18b, 32b  should be plenty for postwar A.F. (with bridge rectifier used for your dc trains..$5-ish, over rate it to near double for a cool, non-cooking, long lasting component ). 

"Still trying to not shoot my eye out"

 

"Nursing insomnia one railcar at a time"

My aroma therapy? Smoke Pellets.

 





Okay then Gary, I think you're no doubt correct.

I just tested the Heathkit (again) by running an AF #310; I tried this engine because it does not have a smoke unit, so I figured the current draw would be (somewhat) less. It did fine for about a minute, then it over loaded. 

 I re-hooked up the MRC; it ran the 310 fine. I'm always impressed with how quiet this engine is. 

 I tried (2) different locos with smoke; a 312 and then a 302AC. Both ran well, with plenty of smoke and room to spare on the throttle. I will say that this unit does have very nice low-end speed control.

Dunno it this proves anything, other than I guess the Heathkit needs to stick with my other (DC) stuff... 

Mark in Oregon

VA means volts times amps.    A power pack rated at 25 VA means 2.5 amps at 10 volts, or any combination of numbers that multiply to 25.    That means if the loco is drawing 5 amps (highly  unlikely), then the max voltage the power pack will put out is 5.    Conversly if the loco draws 1 amp, the power pack theortically could  support 25 volts but probably has circuitry to limit it to about 12.   

The amperage draw from any loco/item is what determines the required power pack/breaker rating.    The heathkit at 1.5 amps is a pretty lightweight power pack.   It probably will work pretty well with a lot of more modern locos with more efficient motors but not so well with older stuff, espically multiple units.

The Heathkit transformer I’m thinking of was designed with HO trains in mind (single-motor, open frame or can, DC) from the 1960s and ‘70s.

The max amperage draw of the typical HO locomotive of that era would have under 1 amp and, more importantly, would have responded more aptly to a lower voltage setting, operating realistically at less than 12 volts, thus allowing a higher amperage draw within the VA rating.

Your AF locomotive probably runs at close to 15 to 16 volts and also demands more amps at that voltage. My O gauge locomotives with dual can motors can draw between 2 and 3 amps at normal speeds and under load from my MTH Z-4000 transformer, which displays both the applied voltage and amperage.

You can’t simply use any DC power source to run a universal motor, and you always need to determine the amperage requirements in any electrical application.

Jim R. 

My thanks for all the suggestions and comments. The only reason I brought this up in the first place was that if indeed DC was "better", I thought I might look into that as an option.

I have a couple of post-war transformers, but they both start around 5-6 volts (I think), so the locos tend to "leap" into action.

As I mentioned, the relatively "little" MRC Tech ll runs my locos very well, and at all speeds. What I like best about it is that it actually starts at "0", so even these old '40s-'50s beasts are capable of nice, slow speed starts, with plenty of "high end" for decent smoke action as well.

Guess I'll stick with it for the time being.

Thanks again.

Mark in Oregon

There is a cure for the jumpy 5v start up; diodes on the output to the track.

+5volt .           --->l---->l----- +3.5v

+5volt(same) -----l<----l<----  +3.5v (same...connect ends)

A general purpose diode ("a one way check valve") will eat at least /about .75 v . So 2 or 3 inline eats about 1.5v-2.25v.DC. ....For AC, since it's power moves both directions, you need double the diodes, 4 or 6 diodes, faced to opposite each other and connected, + to - & - to + (arrows/lines above) to form an inline bi-dirrection string. The string can go on one output (if equipped with 2+) or just as often at the common to lower multiple tracks/acc.s..

  Bridge rectifiers are just 4 diodes in one package and can be an alternative choice. Maybe cheaper and more common with heavier amps. Overating on amps gives a cooler mod (the power loss =heat) over on voltage is good for spike resistance. Rate to the max transformer output. (Parraleling diodes for increasing amps doesn't work well IMO. They tend to fail that way. I'd run two of a same batch/same brand parrallel of I was barely over min tolerance though.)

Now you'd start at 3.5v-2.75v (etc) while the transformer is at 5v output. Your max voltage will be shy the same amount fyi. 

This trick , plus removal of diodes temporarily is one way of getting AC  with a an offset wave output (+ > -) or (- > +). Lionel uses that offset with a special DC only relay for blowing whistles.

    Especially where a linear motor is used (solinoid coil and plunger... Reverse unit?), DC can more likely cause the plunger to become magnetised and stick to a steel strike pad/stop. It happens with AC too, and DC usually don't use an e-unit, but an AC engine run on DC might be a different story as the fields are more constant.  

And back to cooling, AC coils are expecting the cooling break. They run hotter and pull harder on DC.

This cooling can apply to motors too fyi. That would a matter of case by case as to which way is "best".   (Just like old Williams Pittman's on half wave vs full wave AC: heat/power at X-speed via timing of coils is a balancing act.) (Half wave is like bumpy DC/ or one direction of diodes on AC...or a major offset thats skipping one wave 100%) .   The point is that you may get more heat on DC, you may get less, but with heat, DC can be more brutal to equipment in ways

"Still trying to not shoot my eye out"

 

"Nursing insomnia one railcar at a time"

My aroma therapy? Smoke Pellets.

 





RoyBoy posted:

Just get a postwar American Flyer or Lionel transformer and put a bridge rectifier between the out put and the track. Simple and cheap.

True - but without some filtering on the BR's output you then have a huge 120Hz ripple in the resulting DC.  It certainly will work - but with that amount of ripple I'm not sure that any "benefits" of running a universal motor on DC would still accrue.

IT consultant by day, 3rd generation Lionel guy (raising a 4YO 4th generation Lionel Lil' Man) by night in the suburbs of the greatest city in the world - Chicago. Home of the ever-changing Illinois Concretus Ry.

JTrains posted:
RoyBoy posted:

Just get a postwar American Flyer or Lionel transformer and put a bridge rectifier between the out put and the track. Simple and cheap.

True - but without some filtering on the BR's output you then have a huge 120Hz ripple in the resulting DC.  It certainly will work - but with that amount of ripple I'm not sure that any "benefits" of running a universal motor on DC would still accrue.

The "Tracks Ahead" fellow did indeed mention "Filtered" DC...whatever that means.

Mark in Oregon

A filter in this case is smoothing the ripple of voltage peaks in AC to DC convertion. On an osililscope the AC waves appear like:  nununu . After a rectification of the negative wave you have  nnnnnn  (or you can produce a  "negative" version uuuuuu )  but ideal dc looks like ------- . A capacitor can be compaired to a battery, storing power fast and releasing power fast. Added to a rippled ac line, it can help maintain the peak voltage as a smoother line on scope by suppling voltage during the valleys between the supply peaks, then it returns to charging up again, very quickly, during the next peak. The result is little voltage fluctuation or low ripple.

  Ripple and other instability can be confused with intentional modulation for signal/communication purposes by some modern components...they like "clean" "filtered" "smooth" power.

  Filtering ac is more about choking off erratic slopes and spikes in a line, and developing a consistency of voltage/hertz

"Still trying to not shoot my eye out"

 

"Nursing insomnia one railcar at a time"

My aroma therapy? Smoke Pellets.

 





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