How likely is a 775 pro from Vex to pop a 40A breaker? Usually we don’t use them because we’ve heard they pop breakers easily when they are stalled.
It would be used on an intake in place of a BAG motor that didn’t have enough power. Or maybe there’s another motor that fits on a versaplanetary gearbox that’s better than a BAG motor and isn’t a 775 pro?
It might be easier to make a recommendation if we could see what it is you are trying to power with the motor, and if you let us know what gear reduction you are using, and roughly what the load is on it.
If you set things up right, the motor won’t ever be in a situation where it will stall…
Just curious, what is your reduction on your BAG motor? It’s kind of curious to me that it would have trouble with the ball.
It stalls out when the ball is under the mecanum wheels on our intake, because it takes a decent amount of torque to move it to the center when it’s being pushed against the bumper by the mecanums.
We use a bag motor geared down 25:1 on our intake with mecanums to funnel it to the center of the robot with no problems. It is possible to still use a bag motor for this.
This depends on a variety of factors but the short answer is if the motor is stalled under 100% power for an extended period, it will likely trip the breaker and will eventually puff up in smoke.
Whether or not this stall condition is likely to happen depends on a few things - gear ratio, loading, etc - and what stalls a BAG motor at a certain reduction may or may not stall a 775. A 775 geared to a lower reduction that matches the speed of the BAG motor will have more torque due to the higher power of the motor, so it’s possible you won’t have the stalling problem you do now.
But if you think your intake will still stall even with a different motor / ratio, the 775pro is not a motor designed to be stalled at full power for extended periods. But how does your intake work at all if it is consistently stalling?
…somewhere…there must exist a picture of this robot?
Stall current for a 775PRO from Vex is 134 Amps (according to Vex)
Trip time for a 40 amp snap action breaker @ 3x load is 0.5 to 1.1 seconds. (according to AndyMark)
Whether you stall the 775PRO or not depends on your gear ratio, not the motor itself. Stalling a 775PRO is very bad for the motor as it has an integrated cooling fan on the shaft so as you stall the motor (highest heat output) you also stop the cooling fan. This will eventually let out the magic smoke.
One thing that should enter into this discussion is the design of the motor. The 775, like the Fisher Price motor, requires cooling air to flow through the motor. The integral fan requires a minimum RPM and the vents in the end of the motor need to be open for air to flow. Use carefully with those items in mind.
It looks like your options are to either intake slower or add a second motor. The 775 pro would be the most likely replacement. There’s a lot of theoretical gear ratio and motor curve stuff involved, but assuming you need the stall torque of a BAG to intake a ball and you’re geared for the same speed, a 775pro would be pulling 56 amps, vs the BAG stalling at 53A. Admittedly, the pro would still be turning at about 60% free speed at that point. But if you need even more torque than that to intake for a second or two, then yes, you’re going to start pushing the limits of breakers and speed controllers. It’s possible that the higher stall torque will let you power a ball through the mecanums faster and keep you from stalling and blowing things, but I wouldn’t want to bet the farm on it.
Gearing slower or adding a second BAG would get you more torque per amp per motor, and thus reduce your chance of blowing things. Two 775pros would probably be even better.
This is great advice.
That said, we love the 775pro for it’s power-to-weight ratio. We use 8 775pro’s and 4 CIM’s on our 2016 robot, haven’t tripped a breaker yet!
-Mike
For our intake we are using a MiniCIM geared at 4:1 on a VersaPlanatary. Like others have said, the 775pro is an air cooled motor, so it needs ample room for heat dissipation and air intake.
We used 8 775Pros on our drive train, 1 on the intake, and 2 on our scaling mechanism. The only time we popped a breaker was when we had only 1 on the scaler.
Keep in mind the even the CIMs have a pretty short duty cycle even when running at rated speed. Run any of the FRC motors at full voltage and stall and you will rapidly overheat and smoke them. The CIMs and their smaller relatives do better because of their greater thermal mass & reliance on conduction rather than convection for heat transfer. The motors with stall currents > 40amp will eventually cause the breaker to chatter or the speed controller to pull back the voltage to the motor (Assuming the speed controller has that function built in).
So if you are using any brushed dc motor for positioning you should keep this in mind. If it takes more than 20%-40% percent of the full voltage stall current to hold the position, you are going to rapidly overheat the motor. For motors than use fan cooling consider ducting one of the KOP fans to help with cooling
Say on sir. Had this motor been out earlier in the fall and had weight been an issue, I would have pushed very hard for a (8) 775Pros over (4) CIMs but without time to really suss this out, I was just too scared to advocate.
What are your ratio(s) (motor to wheel please), tires (size and grippiness), chassis configuration (# of wheels, any lowered if so how much, distances between wheels side to side and fore/aft)…
In general, open wide your kimono Mr. PiKaChU, open wide…
Dr. Joe J.
Their reveal thread should begin to answer your questions.
http://www.chiefdelphi.com/forums/showthread.php?t=144481&highlight=3360