Shooter burning motors...but why?

So… we have a shooter powered by a 550 motor. (We have destroyed both a FP and AM motor.) The motor is geared down about 7:1 using parts from a Modulox lunchbox, and then powers 2 8" dia wheels on a hex shaft. We have destroyed 2 motors after only a couple shots each, and cant figure out why. Any ideas?

What kind of bearings are you using and are they aligned with the shafts properly. Bearing to shaft alignment is critical for reducing the load that the motor sees when operating.

It’s easy to figure out why, if you gather some data:

What is the free speed (i.e. steady-state speed with no balls being launched) of your shooter wheel? If you don’t know how, or don’t have the equipment to measure the speed of a spinning wheel, then what is the current being drawn by the motor?

At that free speed, what %PWM are you commanding?

You’ve already given the 7:1 geardown ratio, and that’s a good start. Just make sure that’s the right number and includes everything all the way from the motor rpm to the wheel rpm.

Are the motors hot when they fail?

If you have the ability to measure rpm, this can be a really good diagnostic tool. Run your shooter at “free speed”, full power. Use the gear ratio to calculate the actual rpm of the motor, and compare it to the motor data sheet. If you are running at 50% the motors free speed, then you are also running near full power for the motor which will cause it to get hot fairly quickly. If you are running below 50% free speed, you are in the death zone for those motors. You will need to figure out how to free up your shooter mechanisms to reduce friction. You will want to get the free speed of the assembly at or near the calculated free speed.

With 7:1, free speed of that assembly relative to motor free speed should be approximately 2100 rpm. If you are around 1100 rpm, then you are running at or near peak power. Below 1100 rpm is certain death. At around 1500 and above, you stand a pretty good chance of the motor living.

If you do not have a way of measuring rpm, remove just the motor and try turning the assembly. It should spin relatively freely. If not, check for bound bearings, shafts, or other areas of frictional losses.

Also, do you have a way to measure current? Current will tell you a lot about how much power is going in. If you an measure those values and post, someone here may be able to help diagnose further. Pictures would be helpful as well.

Also make sure the AIR VENTS on the 550 ARE NOT BLOCKED.

With a 7:1 reduction, it should be very easy to spin the wheels by hand - if not, then there is some mechanical binding that needs to be fixed.

I think everything is assembled properly. It spun nice and smooth (before the motor bit the dust). We have been hooking the motor directly to the battery for testing so I would guess thats 100% power. Unfortunately I don’t have anything to measure RPM with. I will try to measure current in an hour or 2 when I’m in the room with the robot.

The AIR VENTS are the four slots on the mounting face of the motor, around the bearing where the shaft comes out of the motor.

You should probably be using the control system so you can limit the electric power to the motor…that would be the first thing to do before you burn up another motor

Measure the current to the motor. Knowing the current & voltage going to the motor & the motor curve you can get a good guess of speed. Free running at anything more the %70 of stall torque amps is cause for concern.

Maybe you have too much compression on the ball? When the ball hits the shooter does it stall the motor & it is inertia loading that is killing the motor?

The FP are pretty durable. They take a lot of abuse. Holding them full power at stall for any length of time would be abusive.

If you run thru the PD board, the CB will give you a little protection from burning up the motor.

I will definitely check the air vents also.

Running at anything more than 50% of stall torque amps is a cause for concern. Or possibly even less.

Could anyone point me in the direction to find the motor specs documentation? A link would be great.

We found, (through a burned hand), that all motors will get hot, so its a good idea to have a Spike or use a speed controller to, overall, turn off motors when they don’t need to be used.

We burned up a 550 shooter motor and found the cause to be a bad bearing.

motor info

http://www.usfirst.org/sites/default/files/MotorInfo4.1.pdf

or here:

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

Running at anything more than 50% of stall torque amps is a cause for concern. Or possibly even less.

You are right. I was actually thinking of 30% stall torque amps which would be about 70% free speed.

We burned up a couple because we blocked the air vents. Good ole CD search turned that one up quick. We are mounting our motors in the sideplate of our shooter, a couple holes in the sideplate inline with the vents and problem solved.

At least the banebots aren’t as much as the tetrix motors from last year.

Turns out we were covering he vents. Works great now. Thanks everyone.

If you have time to pick up some hobby motor heat sinks from towerhobbies.com, there are black ones that fit the 550 series diameter perfectly. I think we had to cut them to fit. http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXNAB1&P=ML

Pretty much all failures with BaneBot motors are attributable to excessive heat buildup. I am working on an FRC MTBF plots for the BaneBots using our motors, understanding there are various uses and setups. If I have time, I’d like to put a call for data from the community.

Another thing to watch out for is case shorts. Once the motors heat up too much, the enamel used for sealing the shaft melts off and then the current is allowed to flow from each terminal to the case of the motor (and then your robot). Make sure you consider mounting with nylon screws and washers or other non-conductive hardware. Please note that a case short does not indicate the motor is gone. We’ve run case short motors for a reasonably long duration before they finally failed.