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motors geared together

Started by unk March 11, 2017
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

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com


"unk" <me@privacy.net> wrote in message news:oa1pif$c3$1@gioia.aioe.org...
> 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?
On Sat, 11 Mar 2017 21:18:07 +0000, unk wrote:

> 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?
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. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
unk wrote:
> > It's not uncommon for RC models to have two electric (brushed) motors > geared together to one shaft. >
** Its not common at all, but was done occasionally with model boats.
> 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, >
** 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
On Sat, 11 Mar 2017 21:18:07 +0000 (UTC), unk <me@privacy.net> wrote:

>It's not uncommon for RC models to have two electric (brushed) motors >geared together to one shaft.
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 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?
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