[Jnadke]

Originally posted by DougHogg
I have been trying to learn about multiple-motor drive trains (and single motor drive trains actually), so the above has been very helpful. Thank you to everyone who has been posting in this thread.

Thank you. I will try to answer your questions, as I wasn't exactly clear in my reasoning.

If the drill motor is faster and more powerful, then the CIM motor will be dragging it back a bit, but I don't understand the reference to "slack".

Also, does the drill motor lose some power when it is geared down to the CIM's speed?

By slack I meant drivetrain friction. Since it's a negative force, the faster motor will focus more work on overcoming this. Generally, you want to "error" on the side of the more powerful motor being the faster of the two, so the torque at equivalent speeds will be more closely matched. The exception is if one motor has timing, because of reason #2. For example, if the drill motor has a 10% difference in speed from forward to reverse, if you make it 5% faster in forward operation, it will be 5% slower in reverse operation.

Yes, the drill motor does lose some power. Spur gears are anywhere from 95-98% efficient.

Could you divine "timing" in this context. (Does it relate to why the drill motor goes faster in forward than in reverse?)

Yup, you nailed it. Timing is when a motor is "optimized" in a manner of speaking. Remember that as a motor spins, it's coils are constantly pulling/repelling the internal magnets, which causes a force (torque). In a perfect world, we would switch direction of current in the motor windings at the point where the magnetic field is the strongest. However, because of the inductive (resistance to a change in current) and resistive properties of our coil of wire, the magnetic field takes time to build up. Therefore, we switch the direction of the current before we reach the point where the magnetic field is the strongest, so that, by the time the coil is at this point, it is starting to up its own magnetic field, and we can maximize its benefits (Magnetic field strength decreases as distance increases). Obviously, when you operate the motor in reverse, the magnetic field will be severely unoptimized, so you lose much of the power.

Motor timing is measured in degrees (how many degrees before the point of maximum magnetic field strength the current direction is changed). Timing is one way to maximize the efficiency of a motor. The drill motors we use have timing because it takes more torque to screw something in than it does to take it out.

Makes sense.

Yup. According to Lenz's Law and Back EMF theory, as you pass a magnet through a coil of wire, it creates a voltage potential which sets up a current that repels the movement of the magnet. Higher resistance means lower current, hence less back EMF.

Okay, so here is the homework assignment I gave myself. Given the above information, to synchronize a Fisher-Price motor instead of the drill motor (in this year's game, we rotate our telescoping arm with a drill motor), you would want the CIM's to be a little faster, since they are more powerful. On the other hand, the Fisher-Price would have a higher resistance. Therefore I am surmising that it would be good to just get them as close as possible. How about a ratio of 2.75 to 1 for Fisher-Price to CIM? Does that sound like a good target?

In general you'll always want to have the motors properly matched. It varies case-to-case depending on what gears you can find, etc. In this example, I'd error on the side of the more powerful (CIM) motor.