It's not uncommon for RC models to have two electric (brushed) motors geared together to one shaft.
The motors are almost always controlled by (a pair of) some type of open- loop (no feedback) PWM driver. Can someone explain to me if, or if not why, the inevitable differences between motors and electronics do not result in the motors attempting to turn at different rates and wasting energy, perhaps to the point of component failure?
If they are PMDC motors, and they are identical (same turns, magnetic field, etc.), then they will have the same EMF at the same speed, and the same current flowing through equal DCRs. Much as LEDs wired in parallel, by adding resistors.
Not ideal, but more effective than, say, two PWM drivers and two inductors, in a switching converter: the inductors have much less DCR than the motors do, so the circuit is much more sensitive to PWM mismatch (which is inevitable due to timing errors and component variances).
Tim
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at a guess, the controller doesn't short when off.
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It's pretty damned uncommon NOW -- the current standard is brushless motors up to a couple of horsepower output, being driven by an electronic speed control. Only the cheapest of toy airplanes use brushed motors these days, and AFAIK none of them use motors geared to a common shaft.
The best way to run motors together like this is to drive them in series; second best is to use nominally identical motors driven in parallel. If you drive motors in parallel, then any differences in their characteristics will be accounted for by the armature resistance, which should provide enough compliance to make up for differences in the motors' back-EMF to speed constant.
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** Its not common at all, but was done occasionally with model boats.
** The motors will turn at the same rate, obviously, with each adding a fraction of the output torque.
With no load, the slower one will take a little torque from the other to bring it up to the same speed - however, the load torque will be much greater than this and swamp the effect.
.... Phil
The only model I have ever seen in this configuration, was an electric helicopter from Graupner, way back in the mid eighties. It had two 540 size motors.
The motors are not constant speed. They will spin slower as load increases. As long as the motors are fairly well matched, they will both contribute fairly evenly to drive the load.
-- RoRo
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