Shooter burning motors...but why?

[matthewdenny]

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?

[speedshot]

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.

[Ether]

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?

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.

[IKE]

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.

[Ether]

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.




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

[Jon Stratis]

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.

[matthewdenny]

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.

[MrForbes]

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

[FrankJ]

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.

[matthewdenny]

I will definitely check the air vents also.

[Ether]

running at anything more the %70 of stall torque amps is cause for concern.

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

[matthewdenny]

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

[Sefunx]

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.

[EOC]

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

[MrForbes]

motor info

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

[Ether]

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

motor info

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

or here:

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

[FrankJ]

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.

[Jimmy Nichols]

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.

[matthewdenny]

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.

[protoserge]

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.

[artdutra04]

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.Plastic mounting screws are only half the battle to having a fully electrically-isolated motor. You also have to isolate the motor output shaft via either plastic gears or timing belts.

[protoserge]

Plastic mounting screws are only half the battle to having a fully electrically-isolated motor. You also have to isolate the motor output shaft via either plastic gears or timing belts.

This is true. Thanks for the addition :)

[drwisley]

Plastic mounting screws are only half the battle to having a fully electrically-isolated motor. You also have to isolate the motor output shaft via either plastic gears or timing belts.

Or an isolated coupling.

[ToddF]

We lost one of the RS-550 motors on our shooter, too. We have two driving a single shaft, and one of them started making funny buzzing noises. Upon close examination, we found that the plastic fan had become detached from the motor shaft, and was not providing full cooling. We don't know if this was the root cause of the overheating, or a symptom.

I should note that this was after several hours of intermittent running, calibrating the shooter speed to distance from basket, for our vision based shooter control. We replaced the motor, and placed a couple victor fans so they blow continuously over the exterior of the motor cases. Our motors stay cool now. Got to love forced convection.

[protoserge]

We lost one of the RS-550 motors on our shooter, too. We have two driving a single shaft, and one of them started making funny buzzing noises. Upon close examination, we found that the plastic fan had become detached from the motor shaft, and was not providing full cooling. We don't know if this was the root cause of the overheating, or a symptom.

I should note that this was after several hours of intermittent running, calibrating the shooter speed to distance from basket, for our vision based shooter control. We replaced the motor, and placed a couple victor fans so they blow continuously over the exterior of the motor cases. Our motors stay cool now. Got to love forced convection.

Yes, the root cause is overheating. I assume you are using a cimulator. If so, you should mill the face of the mounts down to allow air to flow into the motor intakes.

A good idea is here:
http://www.chiefdelphi.com/forums/showthread.php?t=101996

[J.Warsoff]

We had a similar problem with our BaneBots for the shooter. They would get hot very fast due to frequent running at high speeds. I came up with a solution using fans. I found a box full the small fans that they use on the Victors and only had to use 2 of them. I extended to the wires to they could reach down from the top of the robot where the shooter is to where the electrical board. Since the fans on their own count as electrical loads, I had to wire them to Spikes wired to the PDB. As for mounting the fans, all I did was use velcro strips (looped strip on side of fan, hooked strip under the motor) to keep the fans in a position where they blow up and against the side of the motors.

THe fans have worked wonders for the motors. They will either stay relatively cool during extended use, or will cool in under a minute after heating up. Weight was no issue at all, being that both fans and Spikes, along with the wires, weighed in at only 0.4 lbs in total.

This may be a good solution to overheating motors. I'll put some pictures up soon as a reference.

[Ether]

They would get hot very fast due to frequent running at high speeds.

High speeds is not the problem. High loads is.

[J.Warsoff]

High speeds is not the problem. High loads is.




I stand corrected. The load and the gear ratio combined can also cause problems, I believe.