Common Motor/Gearbox Reference Sheet

[Chris is me]

Originally Posted by Monochron

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.

[Monochron]

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?

[Chris is me]

Originally Posted by Monochron

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?

Max Power is referring to max mechanical power, half the free speed in radians per second times half the stall torque in newton meters.

[Monochron]

Originally Posted by Chris is me

Max Power is referring to max mechanical power, half the free speed in radians per second times half the stall torque in newton meters.

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?

[Ether]

Originally Posted by Monochron

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?

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:

http://www.chiefdelphi.com/media/papers/2432

[Chris is me]

Originally Posted by Monochron

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?

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.

[Monochron]

Originally Posted by nathannfm

You might want to include the BaneBots CIM-U-LATOR Gearbo[/url]

Ah thanks for pointing that out, I will add it to the sheet.

Originally Posted by Ether

This paper has all the commonly-needed motor equations, and an easy-to-use calculator:

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.

[Monochron]

Originally Posted by Chris is me

Practically, max power is a quick way of determining which motors are capable of which jobs. Bigger number means more powerful.

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

[nathannfm]

You might want to include the BaneBots CIM-U-LATOR Gearbox, especially because it is one of the ones eligible for the banebots voucher.

http://banebots.com/pc/FIRST/S24K-C1-5
http://banebots.com/pc/FIRST/S24K-C1-7