Hi,
Quick question about motor curves. I understand that a mechanism should be designed to run the motor at peak power output. Looking at this motor curve for 775pro https://motors.vex.com/vexpro-motors/775pro, it seems that to achieve peak power output, the motor would need to be run at ~73 Amps. This is way above the maximum current from the PDP. Am I missing something?
You typically shouldn’t design mechanisms to operate at peak power for the reason you found: they draw way too much current. A better practice is to design around a specific time to travel or rotational speed and refine your design around that. Sure, you can design a climber that lifts the robot in a quarter of a second but that isn’t substantially faster than lifting in two seconds and the slower mechanism won’t cause breakers to trip constantly. You can also design your mechanism around drawing a more manageable amount of current. That amount depends on what kind of mechanism you’re dealing with, how often it’s running, and how much current the rest of your robot will be pulling while you’re running it.
A mechanism should not be designed to run at peak power output. It should be designed to run at Max Efficiency. For a 775pro, this is at (17422 RPM, 0.049 N-m). Of course, you can choose to run it at less than Max Efficiency. For instance, you can choose 40A sustained current which is (13301 RPM, 0.20 N-m). This gives you plenty of room in case you temporarily exceed 40A.
The motor can’t sustain peak power for very long. It starts to get hot and you drop from 350 W to 200 W after about 10 seconds. The motor completely fails at 90 seconds.
Operating at max efficiency isn’t ideal for most mechanisms because the power output will be somewhere around 70 watts. At that point you may as well use a Bag motor. The gear reduction you’d need for most mechanisms to run at a 775pro’s peak efficiency is also quite high. That means extra weight and space all at the cost of performance. I think generally the halfway point on the power curve between peak power and peak efficiency is where you should start, and make adjustments from there to better suit the mechanism.
We generally plan for about 80% of free speed…while ignoring friction, and other things that are hard to calculate when estimating the load on a motor.
One neat thing about using the Versaplanetary gear boxes is that if you plan well, you can then test the mechanism, and change the ratio a bit to see if it works better with the motor going a bit faster or slower than you planned.
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Thanks for all the replies, that makes a lot of sense. Quick question, (probably stupid) but the Andymark 775’s are very comparable to Vex 775pros right? https://www.andymark.com/products/andymark-775-redline-motor-v2
Don’t downplay yourself like that, it isn’t a stupid question. Seems weird that two very, very similar but marketed differently motors exist, right?
Here’s what Andy Baker had to say on the similarity to the 775Pro.
That’s really interesting actually. I was confused because the JVN calculator has both an AM 775 and a 775pro option with radically different specs. Thanks for clearing that up
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AM 775 is a completely different motor from the 775pro, Redline, or new Redline (used to be Redline-a).
As far as max power vs max efficiency, you generally want to be running somewhere between the two values. To calculate the exact value you want, I suggest using the mechanism calculator in my design spreadsheet.
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It was funny to keep scrolling and see someone call 775pros the future of drivetrains, resulting in an argument of their worth in that application vs CIMs.
Weird how time flies.
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This is a great rule of thumb. You almost always want to design your system so the motor lives in the top third of the motor’s RPM band. This our (REV’s) FTC motor guide. All of the basics are applicable to FRC brushed and brushless motors.
Agree that in general the design should be somewhere between max efficiency and max power. Systems which run briefly can move a bit towards max power, those which run continuously should shift towards max efficiency. For FRC purposes, the limits are to make sure you don’t do these things:
- Trip the main breaker.
- Brown out the robot by drawing so much current that the battery voltage falls below the RIO’s limits.
- Trip secondary breakers (usually accomplished with a current budget of, say, 50A on each 40A breaker)
- Overheat your motors to where they fall considerably down their performance over time (see motors.vex.com for info on this).
For non-FRC purposes, you also want to consider battery life in greater detail. If you want a 15Ahr battery to last an hour, you have a 10A budget overall to work with!
Just to be clear: this isn’t necessarily true either, depending on your goals.
I don’t typically design for max power or max efficiency. Max power is too much current draw, max efficiency isn’t really important to me because I’m not exactly trying to preserve battery capacity or anything like that. Typically I design for a total current draw of each motor (to ensure no individual breakers trip), for the entire mechanism (to consider the main breaker), and based on the design of the motor (i.e. some kinds of motors sustain higher current loads for longer than others without degradation). It’s somewhat experience based.
For a 775pro specifically, there are degradation concerns about running it drawing too much current for too long - so I would design for a lower current under load for continuous operation than I would for other motors.
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