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Originally Posted by dawilliams
I have used Victors with Y-cable to synch the PWM input to each Victor and never had a problem. This was with much higher current than Victor 884. IFI lists max currents for Victor 885 for 1 and 2 seconds. Since none is listed for Victor 884, where are some of you getting this over-rating info for the Victor 884? There are other threads that discuss burning Victors in matches (not just reversing polarity) which does indicate some teams are drawing too much current for some time. And defensive bots do push for quite some time.
I like the analysis done by Joel J (team 229) but I suspect the problem may lie in the desire/need for speed higher than 6.2 fps (that's less than 4 mph ie medium fast walk). Then a similar analysis would show the wheels don't slipand higher current would be expected. I also think the line:
"When you reduce the free speed of the CIM motor (5310 RPM) 22:1, you get a final RPM of 239.245, which translates to a ground speed of 6.2634 fps"
should use the motor speed at the load equivalent to 40A current, not the free speed.
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www.andymark.biz
Look at that company's (created by FIRST veterans from team 45) website for info on shifting gearboxes that give you the option of having a speedy gear and a pushing gear. You can buy one of their shifters, or you can design your own (this is not a simple thing to do, if you've never done it before). You shift into the pushing gear (low gear) to play defense, or to hold your position, and then move into the higher gear to fly around the field. The drivers would have to remember to shift to low to do any pushing, as they would probably pull too much current in high gear.
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should use the motor speed at the load equivalent to 40A current, not the free speed.
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Well.. under an amount of load that would cause the robot to pull 40A, it would certainly be moving near the rated load speed for the proportional amount of torque (multiplied by the efficiency of the gearbox); however, under normal driving conditions (that is, when not pushing), the motors are probably pulling much less than 40A.. they are pulling something closer to the free speed current draw divided by the gearbox efficiency. So, in my example above, with an AndyMark transmission, the robot would normally drive around at 4.1 fps in low gear, but when it started to push, it would slow down to a slower speed. I bet you can visualize this happening if you've ever watched a robot get into a pushing match.