How does the Pullmor work?

[thor]

Can some knowledgeable person explain to me how the Lionel Pullmor motor will run on either AC or DC?

I had always thought that the two kinds of supply required a different construction and having tried connecting AC to a DC motor and gotten the expected result - buzzing and heating up - I wonder how the Pullmor can run on either.

I found this explanation

http://www.phys.unsw.edu.au/hsc/hsc/electric_motors2.html

However it still doesnt really help me understand it! And WHY is it described as 'horrible' when it is (a) compact and (b) provides good power at variable speeds?

[Anonymous]

A DC can motor uses permanent magnets for the armature fields to work against. Mag field is only going one way and the direction of current affects the direction the motor will turn in.

An open frame "universal" motor uses a "field coil" to provide the magnetic field for the armature to work against. Since the electrical current it alternating, the magnetic field in the field coil is also alternating poles with every directional flip. By using a "split" comutator where the brushes make contact with the armature, you also reverse the feed to the armature to "match" the flipped field to the field coil. This way the magnetic fields are always working in opposition. You need an "E" unit to reverse the feeds to both the field coil and the armature to get the motor to turn in the opposite direction when using AC current. If you feed DC current to a universal motor it will still turn properly even though the current isn't reversing. The comutator provides a way to make sure the static field is always in opposition to the rotor's field.

The split commutator gets more complicated as the number of poles increases and you can get permanent magnets that are much smaller and more powerfull than a field coil. The open frame motor does have its own advantges and you will see them even in medium duty battery powered hand tools.

[lionelsoni]

The torque that a motor produces is proportional to the strength of the magnetic field produced by the stator and to the strength of the magnetic field produced by the rotor. For a permanent-magnet DC motor, such as the can motors now popular in toy locomotives, the stator is the magnet and does not change. So the torque depends only on the rotor, where the magnetic field is proportional to the current I through the rotor. Since an AC voltage is repeatedly reversing, it tries to drive the rotor one way, then the other. The result is buzzing, not rotation.

The universal motor, on the other hand, has a coil to produce the stator's magnetic field. This "field" coil is in series with the rotor, so the same current flows through both. When it is alternating, both fields reverse at the same time. The torque is proportional to I*I, not just to I as in the can motor. The square of I, that is, I*I, is always positive, regardless of whether I itself is positive or negative. So the torque is always in the same direction. It doesn't matter much to a universal motor whether the polarity reverses 120 times a second, or every hour; the torque never reverses.

To reverse the universal motor, you have to reverse the current in one of the two places, rotor or stator. That is what an e-unit does.

The reason for the laminations in a universal motor is to keep the iron of the stator poles from acting like a shorted transformer, since iron is a conductor. The separate layers interrupt any electric currents that might otherwise flow in the iron. A similar DC motor doesn't need laminations, since the steady current and constant magnetic field will not induce currents in the iron.

Why they said it's "horrible" I can't imagine.

[johnandjulie13]

Is it better to run a "universal" motor one way or another? While people have been running their Lionel trains for 70+ years on AC, would their be less stress on the motor by sending only DC? Intuitively, it would seem the motor would have less wear and tear if the magnetic field did not have to flip every 1/60 of a second.

Regards,

John

[Anonymous]

Hey Thor-

Whoever called the pullmor motor horrible should be dunked in molasses and left on an ant hill for a few days.[xx(] Just kidding. Maybe.[:)]

Pullmor-type motors made in the 1920s will be running in museums long after all the can motors in world made in China yesterday are in landfills.

I know it's a pain if you have any sound or electronics at all, but try running a universal motor on DC - better yet - battery power. You'll be surprised by smoothness. It can pretty much crawl. E-Units are quiet. I run my standard gauge and pre-war engines on DC. It's kind of a throwback to the earliest days of toy trains when a glass battery may have been the only electricity available.

Reading the link provided was interesting - I think the (horrible) writer was editorializing based on the example (2-pole sewing machine motor) at hand.

Someday, when I get some time (retirement?) I'm gonna' whip up a 5 pole armature for a pullmor and blow everyones mind...

Old 2037

[FJ and G]

A related question to "how Pullmor works" I have is, what are the typical number of "poles" in toy train motors?

I saw some R/C boat motors that advertised 12 poles.

Is there an advantage with having more poles? If so, I may later on swap out my motors.

Also, what are flywheels? Do they help the train coast?

Sorry to pepper this post with questions, as perhaps each deserves its own post?

[Anonymous]

David, read the link(s) provided by thor - there's an explanation of why more poles are better. All I know is that more poles gives more starting torque and smoother start. I'm pretty sure this is why dual-motored toy trains work better - two pullmors in tandem work like a six-pole motor...

A flywheel is an energy storage device. When the electricity is cutoff from the motor, the energy stored in the flywheel lets the motor spin down more slowly as that energy is dissapated.

[lionelsoni]

The more armature poles, the smoother the motor. The three in a Lionel motor mean that the commutator rewires the armature 6 times per revolution.

I am skeptical that flywheels do much for toy trains. The greater gear reduction with faster motors makes them more practical; and they may be of some help; but I think their effect is mostly psychological.

[FJ and G]

Old,

So a flywheel is like a capacitor then.

Bob,

So 12 poles ought to be pretty good.

---------------

One other thing about model train motors is that in the smaller scales, they are connected to the gearing with an articulated type "arm" that appears to decrease wobble. I'm not sure how this is done with toy trains.

[Anonymous]

The mass of the flywheels in toy trains is pretty insignificant when compared to the mass of the locomotive itself. This is not true of the flywheels used in HO locomotives where the flywheel could be equal to the weight of the rest of the model. HO loco's can coast almost half (or more) a locomotives length if you were to cut track power suddenly. On an O scale engine it might coast about an inch. On a toy train the flywheel helps smooth out performance while adding some mass to the entire package.

[martinden]

For those who doubt the effect or usefulness of flywheels in toy trains ...

Get your hands on an old Williams FM or other loco that has can motors without flywheels. You'll very quickly see their worth -- the first time the power is cut off suddenly, and the engine just stops -- no coasting at all -- and three or four of the cars behind it jump the track. (Always at the back of the layout, a loooong reach from the edge.)

With the flywheels, you get several inches of coasting to "cushion the blow." Believe me, they're worthwhile! To the extent that I've spent roughly sixty dollars (each) to replace the motors in a couple of older engines in order to get the flywheels.

[Anonymous]

QUOTE: Originally posted by FJ and G

Old,

So a flywheel is like a capacitor then.

Yup. Pretty much. You might even be able to replicate the effect of the flywheel with a big enough cap across the motor. Sort of, maybe.


Old

[FJ and G]

Old,

Interesting. I used to think a flywheel was some sort of electronic, spring-loaded mechanical device that released energy when train stopped. Would be interesting to experiment with caps.

My "Dummies" book says that recently, caps in the farad-range have been made, about the size of a 35mm film canister (1 farad is many many many times more capable of storing a charge than your garden variety cap--imagine you'd get a bit of a shock touching one even with power turned off).

The book further states that caps can be used (depending on the type) to:

1. create timers (I believe the flywheel application would fall into this category)

2. smooth out voltage (as in AC to DC in a power supply so voltage stays nice and level and smooth)

3. Blocking DC current (such as for use in a microphone--sounds like the opposite of a full-bridge rectifier, which blocks AC)?????

4. Adjusting frequency (to use as a filter to reject AC signals above or below some desired frequency)

[lionelsoni]

Those "super capacitors" are generally limited to fairly low voltages, which means both that they don't store as much energy as you might imagine and that they will give you no more of a shock than a flashlight dry cell. The energy stored in a capacitor is equal to one-half the capacitance times the square of the voltage. If the capacitance is in farads, the energy is in joules.

From a circuit-theory point of view, a capacitor is an integrator or differentiator, depending on whether you consider its input to be current and output to be voltage, or vice-versa, respectively.

[Anonymous]

Don't quite know what a "Super Capacitor" is, but the caps I use in my tube amp projects (2-200uf @200-500v) seem to pack a lot more power than one would think! Not only that, but the evil things will "charge themselves" back up even after a apparent full discharge! I kid you not.

Be very careful with capacitors... or any other energy storage device.

Old (and hoping to get older) 2037

[Anonymous]

Super capacitors are generally a half Farad or more but generally are only rated for 3 volts or less. They were developed partly for replacing batteries in computers as backup energy in power failures. Super capacitors have almost indefinate life and cycles. The BCR ( www.jandwelectronics.com ) developed by a 3rd party replaces nickel cad batteries in MTH PS1 and PS2 locomotives. It is a circuit containing several super capacitors wired in series to meet the require voltage.

Some locos with Lionel Pullmores can be converted to DC by installing a bridge rectifier and capacitor in the engine. There was an article in CTT some time ago on doing this. I converted a 2055 this way and it ran much smoother especially at low speeds. A bit tough fitting all in the shell though. Pullmore engines are really not AC only locos. AC power was used to allow for the whistle relay,plus way back rectifiers were expensive and not that reliable.

Capacitors can also be added to DC can motors, space allowing. For example 2, 1000uf or more caps can be wired back to back (+ to + or - to - ) in series creating a 500uf or more non polarized capacitor and installed across the motor leads. This will give the engine additional coasting power. If there is no reverse board (in a forward only engine),a single cap can be used.

Dale Hz

[lionelsoni]

Magnets have not always been as strong as they are today. When I was a kid, loudspeakers were still made with electromagnets; and I had a couple of model powerboats whose DC motors used plain iron magnets. When Joshua Cowen started, the only practical way to build the motors for his trains was with a wound field. They were commonly powered with DC from the start, using batteries, and later from Edison's DC generators.

So universal motors were not used because of the difficulty of obtaining DC, but because they were the best way at the time of making even a DC motor.

[4kitties]

Thor, I'm wondering if the author of that website accidentally left out the rest of the line that began with "horrible," i.e. "Horrible EMI. " I say this because after I read your post and the web page you linked to, I did a web search for universal motor characteristics, and on this website:

http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02nQXGrrlPbFqM

it mentions that one of the characteristics of universal motors is "Terrible EMI" (electromagnetic interference), which I think no one would dispute. Right next to those two words was a phrase about sparks and ozone.

The website you linked to was interesting reading, but I disagree with a couple of key points the author makes on his several motor pages:

1) You will never find an induction motor in a vacuum cleaner, at least not the kind we use in our homes every day. Induction motors can't run at the high speeds required to produce decent suction through a relatively small hose and attachments, concurrent with adequate airflow. However, induction motors work fine in dust collectors for wood shops, etc., where high airflow with low suction (not unlike a squirrel-cage furnace blower) is needed.

2) Universal motors have very high torque at startup. That's one of the characteristics that makes them ideal for portable power tools. They produce the most torque right at or close to the stall point, which is how the manufacturers come up with 5-plus horsepower ratings for shop vacuums.

I don't know where the author got his information about the number of poles affecting starting torque. I've never seen a universal motor with more than 2 field poles, and I'm not sure such a thing exists.

I'll take any kind of motor in my model trains, but as far as I'm concerned, series-wound motors rule! I also collect vintage fans and I am especially fond of the DC and AC/DC fans from the early part of the 20th century.

Joel

[lionelsoni]

Joel, did you know that Joshua Cowen made fans before he made trains? He admitted later in life that his fans were useless.

[Poppa_Zit]

FYI -- The name "Pullmor" was coined by AC Gilbert in the rearly 1950s to describe locomotives that included two or four driving wheels with grooves holding rubber "tires" for better traction. About the same time, Lionel countered the American Flyer traction tires with its "Magnetraction" system.

Apparently, after Lionel Corp. bought Gilbert's assets in 1968, it liked the name so much, it applied it to a style of motor.