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Unread 26-06-2003, 07:58
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Joe Johnson Joe Johnson is offline
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Dr Joe speaks...

Wow, a lot of good discussion.

Here are some of my thoughts.

Jnadke is 100% correct in saying that the a differential is mathmatically the same as planetary geartrain.

Those of you who say, "the torques add and the speeds average" are essentially correct (assuming you account for ratios -- and don't forget the SIGN of the ratio too, sometimes you are adding in a negative number... ...also known as subtracting!).

People need to keep in mind that motors have a speed-torque curve for every voltage. For a given voltage and torque, each motor will run at a given speed, period. Don't forget that the speed-torque line extends into negative torques and negative speeds. By this I mean that if you can run a motor faster than its free speed by applying a negative toque. Similarly, if you apply a high enough torque, a motor will run backwards!

This second idea (that a motor can be made to run backwards even if it is trying to run forwards due to the voltage being applied), is why Paul C. has said you can only use a worm gear to drive your ring gear. Worm gears with low lead angles have the property that they are like mechanical diodes for torque -- by this I mean that torque can drive from worm to worm gear but not the other way around (it is not an exact analogy -- to be honest, I just thought of it and I have not noodled through whether I like it or not). Essentially, a worm/worm gear combo prevent the weaker motor from being driven in reverse.

IN ANY CASE... ...I will state again that there is no real magic to 2 motor systems. They act like one motor with a different speed torque curve. This is not to say that there are not advantages -- there are -- but that they do not allow you to make bricks without straw.

Here are my benefits in order as I would rank them:

1) Higher current limit (due to multiple circuit breakers)
2) Higher power (more torque at the same speeds or more speed at the same torque)
3) Cooler motors (for a given amount of mechanical power, you have more surface area to dissipate the electrical power lost -- HEAT-- AND you have the potential to run the motors nearer to their peak efficiency point which means that you have less heat generated per unit of mechanical power)
4) The ability to shift the speed-torque operating point between the two motors (this can be done in software by giving different voltages to the different motors OR in hardware by mechanically have load sharing as in the case of a differential or planetary geartrain OR by both software and hardware) -- note that this does not make a "continuously variable gearbox" but it does give you some properties of a CVT (provided that you don't look too closely at it).

I really need to write a white paper on this subject.

Joe J.
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