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Originally Posted by Matt Adams
Now with the above illustration.. don't think of motors as people rowing a boat, where if one is rowing harder the boat will turn. Think of the motors as a bicycle with two riders. If one works harder, that's fine. But if someone's legs just can't physically move fast enough, it's bad news![/font]
Matt[/color]
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I generally agree with what Matt has to say, but I want to point out that the free speed of a motor is not like the speed of sound in which there is some major discontinuity in the behaviour of the motor. A motor does not have an opinion about what speed it spins at. In particular, it has not qualms about going faster than its free speed.
What I think folks need to keep in mind is that the speed/torque line does not end at the Y-intercept, nor the X-intercept. If you couple 2 motors together, they are going to run with some (fixed) relationship between their speeds. SO... at each given speed, the torques are going to ADD. They add even if you are opperating at a speed higher than the free speed -- BUT, the motor running faster than its free speed is providing NEGATIVE torque at that point.
As Matt said, this is not a 100% tragedy. The motor does not suddenly blow up like a it was in a cheap action movie. Just as Matt said, it hurts the effeciency (because, just like the speed/torque curve, the effeciency parabola does not end either -- you have NEGATIVE effeciencies for that one motor in this range -- which only makes sense, you are putting still putting positive electrical energy in and you are also adding work via the shaft i.e. getting negative energy out). If you opperate in this range very long, you will smoke your motor but that is more of a gradual thing not an explosive process.
Another thing to think about is that up to this point, we have only been comparing motors as if there is only ONE speed/torque curve. This is not true. There is a family of speed/torque curves based on the voltage applied to the motor. It is possible to give more or less voltage to one motor or the other to reduce some of these problems - it is not a perfect solution, but it can be a way to help the motors share the load a bit more effectively.
Joe J.