To help lower the barrier of entry for understanding the relationship between different motors and the speeds and forces they impart I made this reference sheet of common FRC motors and gearboxes. Before printing out a giant version for hanging on the wall though, I wanted to see if you guys could spot any errors that I may have missed or equations that I might have botched.
On the first sheet is a list of most every legal motor, physical specifications, performance specifications, and comments as well as a short list of relevant equations needed for motor calculations.
On the second sheet is the majority of common gearboxes, physical specifications, and comments as well as an estimate for loaded fps for a transmission.
Each motor has a series of colored dots next to it. The different colored dots correspond with the different gearboxes on the “Gearbox” sheet. If a given motor has a given gearbox’s colored dot next it, it mean that those two are compatible.
On the “Gearbox” sheet there is a single colored dot by each gearbox name. If the motor name on the “Motor” sheet has a dot of the same color next to it then it means that the given gearbox is compatible with the given motor. It is so you can immediately see what the gearing options for a given motor are. I can update my first post to mention that.
I thought about doing the mapping in reverse, that is, having each motor have one dot and map each motor to all the gearboxes it is compatible with. I decided against that as I think the average use case will be to first pick the right motor for the job and then pair it with an appropriate gearbox.
I was actually trying to find more information on that. I have heard that the reason that you shouldn’t stall those motors is that there is an internal fan that spins only when the motor shaft spins. Wouldn’t this then mean that the AndyMark 500 series and 775’s would suffer from the same overheating? Or is the fan design specific to the BaneBots motors and the AM ones are different?
How durable each motor is depends on the design and quality of motor construction. That said, I believe the AM 500 / 775 series motors are also cooled by an internal fan slaved to the motor shaft. Stalling these motors should be avoided if possible. If you must stall, it is best to stall a motor at the lowest possible voltage you can get away with.
There is something I have been wondering about this information, if someone could enlighten me.
The Stall Current and Max Power values don’t seem to line up. For instance the RS-550 motor pulls 85A at stall. Given a 12V supply, the power consumption should be: 85A x 12V = 1020W. But the value listed for Max power is significantly lower at 254W.
I know that at stall a motor acts like a (small?) resistor and that without the back EMF resistance the current draw jumps up. Do the Max Powers listed only apply to non-stall situations?
Ah okay, wow that is really different than what I was thinking. I’m not entirely sure what you would use that wattage for.
Is max mechanical power useful for calculating force applied to an object or something similar to that? I have an equation on the sheet for calculating linear force based on the stall torque of the motor, do you know of another one I could add using max mechanical power? Or an example of what knowing the max mechanical power can tell you?
The max output power of the motor is often useful for selecting a motor that will be capable of moving your mechanism at the desired rate at the expected loading conditions… or showing you that the motor you planned to use will not be capable, no matter how you gear it.
This paper has all the commonly-needed motor equations, and an easy-to-use calculator:
Power is work over time. You can calculate how much mechanical power it takes to do a certain work in a certain amount of time. The power rating of a motor will let you make a first approximation of whether or not motors can meet your objectives regardless of gear ratio.
Let’s say you’re trying to lift a 50kg mass robot 1 meter off the ground in 5 seconds. You can quickly determine the work this takes - the negative of change in potential energy. In this case, the energy required to do the work is (50 kg)(9.81m/s^2)(1 m) = 491 J. Doing this much work in 5 seconds means your power requirement is 98.1 watts, at the minimum. This doesn’t account for various efficiency losses.
What can you do with this number? Compare it to the motor spec sheet. For example, with a perfect gear ratio, can a window motor possibly lift 50 kilograms 1 meter in 5 seconds? No, as its max power is far less than 90 watts. Could a mini-CIM? Absolutely, its max power is more than double the requirement. From here, you would move on to figuring out a good gear reduction for this motor to best accomplish your task while meeting your other constraints.
Practically, max power is a quick way of determining which motors are capable of which jobs. Bigger number means more powerful.
Ah thanks for pointing that out, I will add it to the sheet.
Wow, that’s fantastic, thanks. I think I will leave that information separate from this spreadsheet, but it is great to have a a reference for later. Until now we have just be going on the hand-waving “vary voltages will result in varying speed and torque” but seeing the equations now, they are incredibly simple.
Alright, I might look into getting some general work equations on the sheet and reference the max power in those.
And I need to brush up on my motor skills apparently, I haven’t even been factoring time into any calculations we have done. Without a mechanical engineer on the team us Comp E’s will have to get serious about these issues
