Quote:
Originally posted by gwross
Can you elaborate on the kind(s) of problems you would run into when the motors ARE running at free speed?
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Basically, the faster motor will try to spin the slower motor, and the slower motor will act as a generator (slowing the faster motor down and wasting electricity). Although drivetrain friction is only a controlling factor, Matt is correct. Now for an explanation.
Remember that Torque is a force that is rotating. Like all forces, it tries to accelerate things (make them go faster). Lastly, remember that a force accelerates according to the equation Force = Mass * Acceleration. Also remember that, when dealing with motors, torque decreases as speed increases. Therefore, as robot velocity increases, the ability of the motors to increase this velocity goes down.
As Matt said, drivetrain friction will, for the most part, keep these differences under control. Drivetrain friction is a negative force, meaning it tries to decelerate things. For the most part, the faster motor will pick up this slack. While the slower motor has already reached its free speed (and cannot produce any positive force), the faster motor will still try to accelerate the system. The generator effect may or may not happen depending on the severity of your drivetrain friction. Even then, you won't notice any severe negative effects (staying under the 40A maximum) unless the free speeds are more than +/- 30%.
In the end, no matter what, you're not going to come up with a gear ratio that matches the free speeds perfectly. I'd error on the side of making the drill motor being the faster of the two for three reasons:
1. It is more powerful, and therefore it will pick up more slack.
2. Since it has timing, you'll incur less negative effect when operating both motors together in reverse. (Internal friction due to the timing will cause the drill motor to be slower, we want to minimize the difference)
3. The CIM motor has a higher internal wire resistance, so the generator effect will be less severe if you spin it faster than its free speed.
Overall, I'd aim for a gear ratio in the area of 3.4 or 3.5 to 1 when gearing the drill (raw) to the CIM. I wouldn't use the drill gearbox when gearing motors together because internal gearbox friction can be unpredictable. The less gears between them, the better.