Garden Railway Forum

I am spending a bit of time playing around with a "big big train" indoor layout for my grandson. I am thinking of converting one of the bigger (Hymek) locos to a twin drive system, by having two powered bogies (no mechanical connection between them). I intend to control these with a basic radio control system employing an ESC, with the thought that at some time in the future the grandson will want to run it on our garden railway and will need a bit more power to pul four coaches up the hills!.
As a retired electronics engineer, I envisage some "load sharing" problems if both motors are fed from the same ESC. My first thought is to put a low value resistor in series with each motor. Has anyone any experience of such a system?
Any thoughts would be welcome,
Peter

oldchadders wrote: Fri Apr 21, 2017 10:23 pm As a retired electronics engineer, I envisage some "load sharing" problems if both motors are fed from the same ESC.

Model motors are permag motors, so don't go into thermal runaway when run in parallel like semiconductors and don't need balancing resistors. They have an upper speed limit set by the magnet field strength, so also won't destroy themselves if run unloaded, like a series wound motor will.

Running two of them off an ESC is no different to double heading two locos on a track powered layout. The faster one takes more of the load and slows down and the slower takes less load and speeds up, so things balance out. Motors are never identical, but the only way things would go pear shaped would be if the motors were grossly mismatched in terms of operating voltage and gearing, causing one to stall, which won't be the case in this instance.

With my battery electrics, I fit a suitably rated PTC resistor in one motor lead to act as a fuse if the motor stalls for some reason and would fit one to each motor in a double motored loco.

Tri-ang used cheap 3V toy motors in the 'Big Big' range, so finding a suitable ESC for that voltage range might be more of an issue.

If the 'Big Big' Hymeks are anything like their OO counterparts of the time, a double motored one will pull a heavier train than two single motored ones double heading. A single motored loco will also pull more with the powered bogie at the trailing end than the leading end, due to the way the weight transfers back when pulling a train.

Regards,
Graeme

You got my point in your second paragraph - the two motors are unlikely to supply equal power or have identical gearing, so some sort of balancing would seem to be desirable. PM or wound field makes no difference to the potential unbalance (although I am more used to working with large horsepower motors on industrial plant. I had thought of using the back-emf as a measure of speed and controlling to equal speeds with a measurement of current as a "load signal" but this seems something of an overkill.
The motors on the Hymek normally operate from a 6v battery supply, so I reckon that should not be a problem. I also have some cheap ESCs which seem happy to work from a 3.7v Lithium Ion cell, so that should be ok for a 3v motor (but would need a separate supply for the receiver (or the operating range of the TX control could be limited, such that with a nominal 6v supply the ESC output was limited to 3v.
I see no problems on the control side, which cannot be overcome quite easily. I was bothered about the drive imbalance, which it seems difficult to simulate and measure on an operating loco.

Peter

oldchadders wrote: Sat Apr 22, 2017 12:29 pm You got my point in your second paragraph - the two motors are unlikely to supply equal power or have identical gearing, so some sort of balancing would seem to be desirable.

I've been a Tri-ang collector for many years, but that doesn't include the Big Big range, so I dug out the Tri-ang service sheet folder to take a look at the one for the Blue Flyers and see how they were put together.

You are right, they used a 6V motor. I was under the mistaken impression the Big Big range only used 2 dry cells, not 4. Sorry.

I assumed you were just going to take the drive bogie out of a donor model to fit to the working loco. The motors in that case will be near enough in performance unless one is faulty, and the gear ratio will be the same.

When I've double motored HO locos, I checked the current draw and speed of both motor bogies to make sure they were close. If the results are within 10%, they'll work satisfactorily together in a loco. If one draws much more current than the other, or runs at a slower speed, it is defective in some way. These 'Big Big' models use toy motors, so the lifetime is limited and they aren't designed for maintenance such as replacing the brushes. The designers would be very surprised at how many are still running after 40 years.

It's easier with track power, but it shouldn't be difficult to see what is going on in a battery powered two motor chassis on the track, just rig up a couple of small ammeters on the motors and check that there's no major mismatch in current draw when pulling a train and that neither bogie is slipping. Given the design of the Big Big body and bogie, it might be easier to temporarily screw two motor bogies to a piece of ply and sit a battery box, the ammeters and an on/off switch on top, until you are happy you have two motors that work together. If it passes that test, try it with an ESC, then finally proceed to pack it all into the loco.

I built a simple plywood 'chassis' for testing purposes when I was first starting out in garden scale, with the intent of using it for experimenting with simple manual speed control circuits for small battery powered railcars. It had a terminal strip that made it easier to fit meters, speed control boards and to test different batteries, without cranking up the soldering iron each time.

Don't overthink it, this is a model loco, not a steel rolling mill. You are building it for a small child to use, so it has to be simple, robust and most importantly, finished before he loses interest.

Regards,
Graeme

I have been down this route before. A twin motor bogie hymek runs fine on manual and worked equally well on an old fashioned speed control unit from an rc car.
The battery set up is slightly odd in that you have 2 parallel pairs of two cells in series so they will run fine with two or four batteries. (Think I got that right)

Permanent magnet motors (and parallel wound motors.) when fed at constant voltage, are in the ideal condition a constant speed device, they will draw whatever current needed to try to maintain that speed. Two similar motors ganged together, electrically and mechanically, will load share automatically, by drawing appropriate currents.

If one set of wheels slips, say, then that motor will speed up and actually draw less current.

Just make sure the motors are the same and you will have no problem.

If need be two ESCs can be driven from the same servo channel using a Y-lead.

(edited for science.)