Turning motors on and off in the KitGearbox?

[Andrew Blair]

I was considering how to make the kitgearboxes run more efficiently, and I realized that running two CIMs in the boxes all the time draws a lot of juice. Would it be possible, and more importantly, efficient, to run only one CIM in the gearbox, and then, using a current sensor, kick in the other CIM when the current draw reached a certain point? The motors would be on coast, so I'm assuming they should b easy to bakdrive. Has anyone done this?

[greencactus3]

Originally Posted by Andrew Blair

I was considering how to make the kitgearboxes run more efficiently, and I realized that running two CIMs in the boxes all the time draws a lot of juice. Would it be possible, and more importantly, efficient, to run only one CIM in the gearbox, and then, using a current sensor, kick in the other CIM when the current draw reached a certain point? The motors would be on coast, so I'm assuming they should b easy to bakdrive. Has anyone done this?

even if they are on coast the inertia of the motor armature itslef is a lot. 2 minutes isnt a long time. running dual motors shouldnt drain the battery enough to worry if you gear it right. if you lose juice too fast gear it down more. and learn how to drive.

[Andrew Blair]

Well, by cutting out a motor 50% of the time, at say 80% efficiency, you save about 40% of the juice you'd need to run both motors all the time. Which is not neccessary usually. 40% savings is pretty good in my book, and relatively easy to implement, using just a couple of sensors and a bit of code. Running 4 CIMs and 1 window motor last year, all of them being used pretty much the whole match, we could get 2, maybe three matches before the battery needed to be replaced. So, that kind of energy savings could be very useful.

[roach1051]

The motor that is not powered will be a parasitic drag on the motor that is driving. The powered motor will be spinning the other, the dead motor will become a generator. It takes additonal power to turn the unpowered motor. How much actual battery power you save is questionable. If you could mechanically disconnect the dead motor from the working motor, the battery savings could be real. But then, if you just keep a couple of spare batteries on chargers between matches, you should have plenty of power. We ran 4 CIMS with the kit gearboxes and the door motor for our lift last year, and had plenty of battery power at two regionals and nationals. The only time that we had to hussle to keep batteries charged was during eliminations matches where you run back to back matches. If you take care of your previous years' batteries, you should have enough by now that you don't have to worry about having enough charged between matches.

[Al Skierkiewicz]

Andrew,
From an electrical standpoint there is not much to gain from turning one motor off. With both motors supplying power, the electrical load is effectively shared between the motors. There are some electrical losses but it becomes a wash as to whether the electrical losses overcome the mechanical losses of a motor in the drivetrain but not powered. As to electrical load from the motor that is off, those are negligible since there is no electrical load on the motor in "coast". Since the motors are about 90% efficient transferring electrical to mechanical power I think you are talking relatively small savings overall. By sharing the load, you don't have one motor at a much higher temperature than the other and with individual currents being lower, the electrical voltage drop in the wiring is smaller. It is like feeding a single motor with two wires in parallel. However, it might be useful if you wanted a "turbo" overdrive for high speed running at the push of a button, but with the normal playing field size the advantage of double speed is small.