Dual motors with one decoder

[Jim Starbuck]

So I’ve been experimenting with the little Milwaukee electric yard goat and made an observation that I don’t quite understand what’s actually going on.
I originally powered the truck with 4x8mm coreless cell phone vibrator motors wired in parallel and both ran equally when powered by the decoder.

Original motors:
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I broke a solder connection from the powerkeeper at the decoder while installing the lights so I took it apart for repair. While it was apart anyway I changed the motors to 4x8mm coreless drone motors that I acquired after the original motors had been installed. The new motors are slightly smaller than the originals which had kind of rectangular exterior cases. The new motors mounted lower accomplished a better worm to gear engagement and also allowed the removal of a .030 shim on top of the truck side frames that was visually bugging me.

New motors:
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The new motors run nicely and I expected the same behavior as the others but here’s what I don’t understand. When powered up one motor runs while the other does not. If I physically stop the running motor, the other one starts. The thought was that since both motors had equal load and voltage that they would balance and run together. With no axles installed, I can slow one motor down by lightly pressing a toothpick to the spinning worm and make the other motor start and they both run. With the geared axles installed the front motor runs hard enough to spin the wheels while dragging the rear axle. I would have expected the load to shift voltage to the rear motor.
I desoldered the rear motor and replaced the geared axle tube with a plain one and the critter runs nicely on just one powered axle so I’m happy with it as is but I was just wondering what might be going on from an electrics point of view and why one pair of motors behaved differently than the others.
Again, this whole project was an experiment and to be honest, I was surprised it worked at all.

Thanks for any insight,
Jim

[woodone]

So there are two motors per truck?   You stated that the gear mesh was tightened. I would power up one truck at a time and measure the amp draw of each. What I think you will find is one truck taking more volts to get running than the other- To tight on the gear mesh?
Electricity will, take the path of least resistance. Once one motor is running and you increase the voltage it just turns faster, the second motor just will not start. Because it is easier to turn on the one that is already running.

[nickelplate759]

I'm guessing the motors are in parallel.  Would the behavior change if they were in series?

[wazzou]

So there are two motors per truck?   You stated that the gear mesh was tightened. I would power up one truck at a time and measure the amp draw of each. What I think you will find is one truck taking more volts to get running than the other- To tight on the gear mesh?
Electricity will, take the path of least resistance. Once one motor is running and you increase the voltage it just turns faster, the second motor just will not start. Because it is easier to turn on the one that is already running.

There is only one truck, one motor per axle of that truck.

[Jim Starbuck]

I'm guessing the motors are in parallel.  Would the behavior change if they were in series?

Yes, they are wired parallel but with the polarity reversed on one. With one pointing forward and the other back, they need to turn in opposite directions in relation to each other but the same direction in relation to the truck.
I’ll experiment more with series wiring when I get home later.

[woodone]

Bryan, I,am looking at the truck- one motor driving one axle- a second motor driving the other axle- thats two motors per truck the way I count them.

[peteski]

Those rectangular look more like standard 6-pole motors than coreless.  Coreless motors are by design cylindrical (the magnetic slug is inside the winding basket, and the cylindrical case acts as magnetic field concentrator. It would name no sense to make the case rectangular. Where did you pick them up? Any specifics you could share?

If they are in parallel then I would expect them bot to run at the same speed (unless the load is different on each).

If you have couple more of those motors, wire them up the same way as they are in your model, connect them to a decoder and run them without any load. Do they behave as expected (both run)?

[Jim Starbuck]

Peteski, I believe you are correct about the earlier motors not being coreless. Brushless yes but not coreless.
The new motors are 4x8 coreless 3v 70000 RPM super cheap off of eBay from China. A little experimenting revealed that they will both start when wired in parallel with no load but because of slight differences in each individual motor they start at slightly different voltages. I think that’s what was going on with the yard goat. These motors are not at all designed to go slow. When I cranked them up a bit they both ran steady.
I did wire them in series and also got them turning opposite directions but they did more of that one then the other under load.
I think I’ve got a pretty good handle now on why they’re behaving like they are.
Thanks

[peteski]

Jim,  brushless motors have stationary windings, and the rotor contains the magnet. They have 3 or more terminals and require external electronic driver to drive them.  As I mentioned earlier, the rectangular shape motors are likely 6-pole motors, with small magnets in each of the 4 corners of the case and regular commutator with brushes.  If you have spare motor, and it was inexpensive. sacrifice one to open it up to see what is inside.  That is how I learn all sorts of interesting things.  I'm perpetually curious. 

And yes, the small coreless "pager type" or mini-drone motors are designed to run  tens of thousands rpms, not for slow speeds.  They also have very low torque while 6-pole motors have surprisingly high torque for their size, and they can also handle running slower.

Here are photos of a disassembled 10mm x 15mm 6-pole motor. I suspect that your square-shape motor is similar.



Unlike 5-pole or 3-pole motors which have only single magnet (usually constructed from 2 separate magnets arranged in a way to have N and S poles facing the armature), this motor has 4 magnets (molded as a single unit) located in the corners of the motor's shell.



Also notice the unusual (90 deg.) angle of the brushes. 5- and 3-pole motors have brushes 180 degrees apart. 

EDIT:  I have looked at the photo of your model with the original motors and Those might not be 6-pole motors after all.  Those motor's case is not rectangular, like the motor in my photos.  Those might be regular 3- or 5- pole motors with iron rotor.  Or maybe they are tiny coreless motors after all. 

[mmagliaro]

I sampled a variety of little motors like this for the "Rocket" project, before I found one that would:
a) Run continuously under load without burning up
b) Start at something like 1/3 of its rated voltage or less, so it wasn't just an "on/off" switch.

I think those two properties are very important to making this work.  And there is a huge variation in the
performance and behavior of these little buggers, because mostly they were never meant to be continuous-duty
power plants for a model train.

If your 3v motor doesn't start until you get up to about 2.5v or something, honestly, I'd look for another motor.

[Sokramiketes]

Do you think the coreless motors are more picky to outside magnetic interference?  You have an interesting use case with them parked side by side.

[peteski]

Do you think the coreless motors are more picky to outside magnetic interference?  You have an interesting use case with them parked side by side.

I doubt it. The ferromagnetic case makes a good magnetic shield, and it also keeps most of the magnetic field from the inner magnet inside the motor.

[Jim Starbuck]

This is excellent information. Thank you gentlemen.
Peteski, I have a couple of those six pole motors and they run great. Here’s a bit more information in the original vibrator motors. The case dimensions are 4.1 x 4.32 x 9.15 mm with a .7mm shaft diameter once the vibrator weight is ground off.

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The round drone motor case is 4.08 x 9.1 mm and also has a .7 mm shaft diameter.

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Max,

I’m always looking for small motor sources. Do you have any recommendations from your searches?

Thanks,
Jim

[peteski]

Jim, I'm having second thoughts about your original square-shape motors.  They might be coreless after all.  Are those still available from your source?  If I could get few I would take one apart.  Or if you have spares and are willing, you could take it apart. That would positively identify their construction.

The cylindrical motors are without any doubts coreless.