Pullmor vs can motor - what's the difference?

Although Jerry Calabrese talks of "legendary Pullmor" motors, I hesitate to use that term, which was American Flyer's name for what are now called "traction tires".  What it seems to refer to are 3-pole universal motors.  These are series-wound motors, with laminated magnetic circuits so that they can be run on AC as well as DC.  They are similar to the type of motors traditionally used on prototype electric and Diesel-electric locomotives.  They are flexible in that they adapt their speed to the load being pulled.  It is not necessary that multiple motors, whether in one locomotive or in different locomotives of a multiple-unit lashup, be at all matched in their characteristics.

The "can" motors are DC motors with permanent-magnet fields.  They have become practical for AC powered toy trains only in modern times because of the availability of very strong magnet materials and of compact, efficient semiconductor rectifiers that can be carried in the locomotives.  These motors lack the flexibility of series motors, in that they want to run only at a speed proportional to the applied voltage.  This means that if two motors pulling the same train would run at different track speeds if used separately, when used together, the faster one will draw all the current and the slower one none.  The slower motor would in fact turn into a generator if the usual worm drive allowed it.  When two motors are used in the same locomotive, the manufacturer should have tried to match them.  When you run two locomotives, both with can motors, you should match them yourself by running them uncoupled and unloaded to see that their speeds are nearly the same.

A can-motor locomotive and a series-motor locomotive will run together, with the series locomotive adapting to the can locomotive.

Birds

What is the difference between a Pullmor motor and a Can motor?

"PullMor" or AC open frame universal series wound AC/DC 3 pole motors:

Birds

Why do many people consider one to be better than the other?

It's not a matter of "better"  Just different.  Can motors are much cheaper to make, some(Pittmans, better Mabuchi's) are extremely durable, but most are non-serviceable and are throw-aways.  Can motors:

Rob

Can Pullmor motors be equipped with speed control (i.e. odyssey II) and can they have the ability to move slowly like the newest Legacy equipped engines?

Regards,

John

There is nothing in principle to prevent including a universal motor in such a control loop.  Whether it is practical to do so with what is available on the market, I don't know.

My personal preference is for the train to respond to loads, grades, and curves at least qualitatively in the same way as the prototypes, giving me as the "engineer" the job of maintaining and adjusting speed to suit the circumstances.  I can see however that others may prefer to set and maintain a speed automatically.

You can get a PW open frame loco to crawl around a layout by using a power supply like a TPC.  It won't be as smooth as a can motor but it will crawl around the track.

Thanks for the feedback.  What are the properties of a can motor that allow it to move so much more smoothly?

Regards,

John

 I don't want to divert the thread with John having asked such a good question about the properties of a can motor that allow it to move more smoothly...

but I do want to thank everyone for their in-depth responses, and thank Rob for the photos.  It is much appreciated.

Chris
 

Here's an interesting link:

http://www.trainweb.org/girr/tips/tips5/motor_tips.html

The short answer is:  lower gearing combined with higher motor speed, and more poles.

Just to elaborate a bit on what's been said and to try to put it into some historical context ...

What we now call the "can" motor had its origins in WWII military research and the need for smaller, lighter, more efficient motors, especially for airplanes (warplanes). After the war, under the rubric "precision" motors, they were adopted (and presumably further developed) for such demanding civilian applications such as tape recorders and movie cameras.

In the mid-1950s, model railroaders (primarily in HO) began to buy these motors and fit them into their locomotives to give all-round better performance than the open frame permag motors then in use. (These are like Lionel's universal motors, but with a permanent magnet field rather than the wound field of an AC-DC motor.)

The can motor is the result of much closer tolerances in general, curving the permanent magnet tightly around the armature (becaue of the better tolerances, the field can be much closer to the armature), and the armature often had five or seven poles, rather than the three-pole design of many less expensive permag motors. As mentioned in a post above, the whole motor was then enclosed in a cylindrical metal case (the "can"). By the 1970s, lots of modelers were installing can motors into their locomotives, and manufacturers (slowly) began using them as original equipment. Today can motors (or a couple of close relatives -- "flat can" designs) are more or less standard equipment in HO and N locos.  In the toy train/3-rail AC community they gained a bad reputation, because Lionel in the 70s began using them in their cheapest trains, which could then run only on DC current. All "better" trains continued to use open frame (unfortunately now called "pullmor" motors). In the mid-to-late 80s, the electronic e-units were developed, which rectified the AC into the DC needed by a can motor and also provided the familar three-postion reversing mechanism. But the "cheap" stigma contiued until recent years, when it became completely clear that can motors perform better than universal motors -- they draw less current, run more smoothly, and generally have a better response to input -- and manufacturers began using them in premium trains.

Martin

when it became completely clear that can motors perform better than universal motors -- they draw less current, run more smoothly, and generally have a better response to input -- and manufacturers began using them in premium trains.

Lionel did try to update the open frame motor with what would have been an Odyssey motor had it been economically feasible.  The motor did reach pre-production but was never manufactured.  Lionel never explained why these were pulled but the Odyssey name was retained for Lionel's version of closed loop back speed control.  Units were demo'd and did work.  It's assumed that this project was canceled when the production costs were deemed too high or the r/d costs were going to be too high to amortise, or both.

There are some advantages to a universal motor (like the automatic ballancing when using multiple units and higher lower end  torque).   I've noticed that many of the higher end cordless pwer tools use universal motors internally even though they run off of batteries while the cheaper units use can motors.

A comment on Martin's excellent history:  The electronic e-unit was not a prerequisite for using DC motors in AC trains.  It is entirely practical to combine a traditional electromechanical e-unit with a rectifier for that purpose.  However, as far as I know, this was never done.

Thanks for the great responses!

Regards,

John

lionelsoni

... It is entirely practical to combine a traditional electromechanical e-unit with a rectifier for that purpose.  However, as far as I know, this was never done...

I have at least 6 Williams diesels with  traditional electromechanical e-units(NOT equipped this way from the factory) and full wave rectifiers w/ filter caps(big ones - extra weight - no traction tires on these).  The problem was that if the motors were wired in parallel(as from the factory), the locos moved off in the prior direction before the voltage rises enough to pull the e-unit pawl up to roll the drum into neutral.  Wiring the motors in series pretty much eliminates this issue.

Rob

Rob, are your e-unit coils powered by AC or DC?  With big filter capacitors, the DC voltage will be about 40 percent higher than the AC RMS voltage, which would produce the effect you describe.  I put both the e-unit coil and the motor on the DC side.  I find that 5 or 10 millifarads is enough of a filter and small enough to get into any locomotive that I have converted so far.

I also have wired some motors in series, but to get better low-speed control rather than to cure the twitch-before-neutral problem, which I have often seen in unmodified locomotives.  An unloaded postwar locomotive often wants to start at just a couple of volts. 

lionelsoni

Rob, are your e-unit coils powered by AC or DC?

 AC.

lionelsoni

With big filter capacitors, the DC voltage will be about 40 percent higher than the AC RMS voltage, which would produce the effect you describe.  I put both the e-unit coil and the motor on the DC side.  I find that 5 or 10 millifarads is enough of a filter and small enough to get into any locomotive that I have converted so far.

I also have wired some motors in series, but to get better low-speed control rather than to cure the twitch-before-neutral problem, which I have often seen in unmodified locomotives.  An unloaded postwar locomotive often wants to start at just a couple of volts. 

It was just a side effect for me.  I wanted the low speed performance.

Rob

...been meaning to get back to this.

Bob, do I correctly conclude that 'modern' era engines (beginning with year ????) cannot be double, or triple headed because of the characteristics of their 'can' motors? (Obviously I don't have any...yet).

runtime

runtime

...do I correctly conclude that 'modern' era engines (beginning with year ????) cannot be double, or triple headed because of the characteristics of their 'can' motors? (Obviously I don't have any...yet).

runtime

That depends.  Spur gear drive(can motor-in-truck) designs like  K-line MP-15 / Alco / S-2 are more forgiving in the differences between motors.

Worm gear drives - Williams, eg.,  with shallow gear pitch designs are more likely to want to run at their own speed and will fight with other motors.

Rob

They "can", but only if they would all run at about the same speed on their own.  Otherwise, the slower ones will be a "drag" on the faster ones, causing mechanical wear and possible electrical overloading.  One way around this is to use a command system to make each unit separately want to run at the same speed.