One 18v7 - Two Motors?

I’ve looked the documentation over and searched the forum, but don’t see a direct answer to my application. The examples used for two motor applications are generally ones where the motors will vary in speed and run independently, so I can clearly see why two motor controllers are needed.

My intended application is somewhat different as it is in a model train locomotive (a Bachmann 1:20.3 scale 2-truck Shay, FWIW). It has two trucks, each powered by a motor built into it. The motors would typically turn at the same speed, with the mechanical drivetrain of the locomotive linking both motors so they turn at the same speed The total amp draw at stall is roughly 3 amps for both motors together, with power to be supplied by a 14.8 volt LiPo.

Control will be by RC, with no processing involved other than what the RC receiver and what’s on the 18v7 smc itself do.

I believe the stall amperage will be within the handling capabilities of a single 18v7 controller, which I have on hand. Then I read the documentation on wiring for two motors

Do I actually need a second 18v7? I understand that maybe this is needed simply to allow the feedback to work properly on the SMC, but perhaps that’s not an issue since both motors should turn at the same speed? If simple works, then maybe just one is really necessary since I have no need to independently control the motor speed or direction?

What’s my best choice here? I have no problem squeezing in another 18v7 if necessary, but if one works why bother?


Hello, Mike.

In applications where independent control of brushed DC motors is not necessary, it is fine to connect two motors in parallel to the same motor channel. This is a typical configuration for 4-wheel drive or 6-wheel drive robots, such as the Dagu Wild Thumper. The Simple Motor Controller 18v7 can handle a continuous current of around 7A, so it should be fine for your two motors.

You mentioned some kind of mechanical drivetrain that links the motors to make them turn at the same speed. One thing to keep in mind is that even brushed DC motors of the same model can have some variance in speed, around +/- 10% is common. This is not a problem for robots like the Wild Thumper where the motors are not mechanically linked, but it might be something to consider for your train.


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Hi Brandon,
Thanks! I think I’m set then.

It’s possible that there could be some slight variance, but nothing close to 10%. The Shay is a steam locomotive, but it was the 4x4 (or 8x8 to be accurate) of steamers. A two- or three-cylinder set of steam pistons hung on the right side. This drove a driveline from them to both trucks (or in some cases 3 trucks) that is very much like a real 4x4 truck. The model duplicates this, except that the power comes from the electric motors in the trucks and it turns the driveline for appearance’s sake. Thus any speed difference would be solely due to whatever very limited slack is in the u-joints in the driveline. My guess is something less than 1% difference and that only under load at certain points.

Very much looking for ward to this project, as it let’s the loco be independently powered and controled, so that it doesn’t rely on picking up power from the rails. Outside conditions play heck with wiring, so the result should be a very reliable and powerful locomotive. The Pololu SMC will make this conversion from track power easy and inexpensive to implement. I hope to follow up with a report in a month or so.