Running AC pullmore motor on DC?

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July 19, 2012 10:09 PM

I know that the pullmore is supposed to be a universal motor but what are the advantages and disadvantages of using DC instead of AC?
Thanks

Dave
 

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July 19, 2012 11:24 PM

They run smoother on DC with better low speed performance. The disadvantage is you have to install a bridge rectifier and an optional capacitor if it will fit. . Not always easy to fit even a bridge in some PW steamers. Some of them you have to un ground the motor lead connected to the chassis. Diesels are easier. 

 

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July 19, 2012 11:37 PM

you can run a universal motor with DC current, it  will run stronger on lower voltage, better at slower speeds. Drawbacks are the horn or whistle will constantly be on and other accesories that employ a relay will be on constantly if using track power. The way around this is like Dale said and isolate the dc to the motors by doing the rectification on board and not from the source.

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Joe Geiser

 
 
 
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July 20, 2012 6:42 PM

Thanks for the info guys.
I guess that means I can use a ERR DC Commander in my dual motored F-3.
Will I still need to add the capacitor across the motor poles for good operation?
 

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July 20, 2012 7:47 PM

Not really a requirement. If direction is reversed by DC polarity you need a non polar cap if doing it. You can make one out of 2 polarized ones. Just put 2 like terminals together,then use the other 2 across the motor. K-Line actually did this on some of their engines using 2, 1000uf 35 volt capacitors. 

 

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July 20, 2012 8:16 PM

A universal motor WILL NOT reverse direction when you reverse the polarity of DC driving it.  You can add a fullwave diode bridge on either just the field or just the armature to make this happen.

 
 
 
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July 20, 2012 8:36 PM

rec089

 

 In the late 70's and early 80's I did this to all my engines. I was using DC track power. The main reason I did this so they would run cooler and I hate F-N-R-N-F.

 

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July 20, 2012 9:11 PM

Dale M,

I guess I don't really understand how a universal motor works compared to a DC can motor. What is the need for the rectifier?
Are you saying that the universal motor will only spin in a "forward" motion regardless of DC polarity?
If that is the case, what is it about the motor that makes it prefer a "forward" motion, relative to reverse, when DC polarity is not a determining factor?

I'm not questioning your logic or experience. I'm just trying to learn.
 

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July 20, 2012 9:26 PM

Flash

 

   If the motor is wired to run clock wise, changing the polarity will not change direction, if wired to run counter clockwise, likewise changing the polarity will not change direction.

 As Dale said you have to add a full wave bridge as I have done in the above drawing.

 

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July 20, 2012 9:36 PM

Ok Bill, So direction in this case is determined by the way the wire is wound around the field and armature? In your drawing, the armature winding is still on AC. The field winding is DC. Changing polarity of the field winding will change motor direction?
 
 
 
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July 20, 2012 9:39 PM

No wait, you said you were running DC track power. So both the field and armature were getting DC only. So what exactly is the rectifier doing to the circuit?
 
 
 
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July 20, 2012 9:41 PM

Flash:

 

A universal motor has the field electromagnet coil and the armature electromagnet coil in series.

 

The E unit, when cycled, takes one or the other of those coils (the field or the armature, depending on the type of motor) and reverses it with respect to the other coil.  That's what makes the motor run in reverse.

 

If you reverse the input DC to a stock motor, (assuming that you don't want to use the E unit) both the field and the armature magnets get reversed, but they stay the same polarity with respect to each other. So, the motor will still run in the original direction.

 

By putting a full-wave bridge rectifier between the supply and the field, when you reverse the track polarity, the armature polarity reverses, but the field polarity stays the same.  So, the motor will reverse.

 

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July 20, 2012 9:45 PM

Ok I think I've hot this now. In your drawing... The field winding polarity remains constant regardless of track polarity. Therefore, changing polarity to the armature creates the change in motion. So for the multiple posts. Took me awhile to trace out the rectifier circuit. Not I think I understand.
 
 
 
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July 20, 2012 9:49 PM

Arthur, I was thinking and trying to type while you posted your reply and eventually came to the same conclusion. Thank you for helping me understand the flow of current through the AC motor better.
 
 
 
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July 21, 2012 10:16 AM

YW

 

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July 21, 2012 11:45 AM

At the time I got my first "collector series" set back in 1988, I only had DC transformers (MRC Trainpower 6200). I too ran into the "horn sounding all the time" issue, but seeing as the locomotive (Lionel Pullmor-motored SD40) didn't have a bell, I just threw the polarity switch and the loco continued to run as before with its E-unit, but without the horn (which wasn't that much of a loss).

 

So, what was I doing "wrong"?

 

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July 22, 2012 6:04 PM

Originally Posted by RailRide:

At the time I got my first "collector series" set back in 1988, I only had DC transformers (MRC Trainpower 6200). I too ran into the "horn sounding all the time" issue, but seeing as the locomotive (Lionel Pullmor-motored SD40) didn't have a bell, I just threw the polarity switch and the loco continued to run as before with its E-unit, but without the horn (which wasn't that much of a loss).

 

So, what was I doing "wrong"?

You weren't doing anything wrong. The horn (which I don't like either) is triggered by a positive DC offset superimposed on the AC with the whistle button. If the "polarity" on the AC transformers with whistle control is backward, the horn doesn't work either, but the engine will still run.

 

The PullMor (universal) motors work by creating an electromagnet as opposed to using a permanent magnet They reverse reverse the phasing (polarity) of the electromagnet and the motor direction is reversed. That will happen under a DC or AC source. Lionel's are a little harder to modify internally because the locomotive frame is tied into the process and used as a "common" for the motor armatures as I recall. They do run smoother, quieter, and with better control under DC.

 

By the way, Frank Ellison's Delta Lines (revered by 2-railer and 3-railer alike) featured converted Lionel locomotives (running outside third rail) running on DC power.

 

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July 23, 2012 5:49 PM

It's interesting that this question of AC vs DC for running engines keeps coming up. I published and article on this subject in the July 2006 issue of TTOS's The Bulletin. Most current can motor engines with electronic E units work just as well on DC as they do on AC, so that's not much of an issue. Universal motors are a different story. In order to reverse the rotation of a universal motor, the polarity between the stator(field) coil and the armature needs to be reversed. That's what the E-units did in that F-N-R-N-F sequence. Lucky for us DC enthusiasts, when powering a universal motor with DC, a full wave rectifier module is a neat and simple solution with the AC input in series with the field coil and the DC output connected to each of the amature brushes. The bridge rectifier isn't doing any rectifying, it just happens to contain four diodes in a neat package that is easy to connect. You can obtain the same result with four properly connected individual diodes, which is what I did in the early days. There is no question that universal motors run better with smoother control on DC and make for easier lashups. The whistles, bells, etc. is wher you run into some issues. In general, the sound responds to one of the DC polarities and is quiet with the other.

Bob Walker

 
 
 
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August 13, 2012 2:58 PM

Flash, I did this with some Lionel Alco and used the 103 e-unit.  Add the bridge to hold current steady through the field.  You actually have some choices and can reverse the field, or the armature current, depending where you want to apply the FWBR.  You also need to be careful if this was a 2 position e-unit that has two field coils.  The engine runs fine, but what you loose is top end speed because of the significant voltage drops via the diodes of the electronic e-unit and from the full bridge rectifier.  I was doing this mod on 2 position mechanical e-unit engines.  Works fine.  G

 

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