Banebots RS-775 Case Short

[Tom Bottiglieri]

We have a Banebots 775 motor with its power leads shorted to the case. You can imagine why this would be an issue. We are not sure whether this is the one from the kit or the additional one we ordered.

I've seen one other team who posted about a problem with this... (http://blog.iamjvn.com/2011/02/jvn-b...75-motors.html)

Is anyone else having this issue? If you have NOT run your 775's yet, could you check them and see if they have this problem? It would be nice to know if they are DOA or if they were damaged while running in the robot.

[Jeff Waegelin]

John mentioned it in his blog post, but we saw this on some brand new out-of-the box motors, too. It wasn't limited to just the ones we had already run.

[Karthik]

1114 has seen the same thing with some of our unused and used 775s.

[Mr. Lim]

On a semi-related note,

We've also run into an issue where the "red dot" that presumably indicates where the positive terminal of the motor is is not consistent between the RS-775s.

We have a single gearbox that mates to two RS-775s, and had them both wired according to the red dot, and when we powered them up, they ran in opposite directions...

[artdutra04]

25% of the unused, still-in-the-box 775 motors we tested were case shorted.

[AdamHeard]

Zero of our 15ish were defective, but not have been installed and ran on a robot.

[JDL]

Although we aren't using any on our bot I'm gonna check ours just for kicks.


Side note: it really is good practice to check all motors for such issues before you use them, prevents nasty little surprises sometimes. I never apply power to a new motor (in FIRST or otherwise) without testing it for ground faults and other problems.

[MattC9]

Hmmmm.... Just our luck I went and checked our 3, 2 of which were conductive. What exactly makes them "bad"? I have not heard why exactly this a problem.

[Trent B]

Got in 3 today crammed in a flat-rate box without much padding. None have case shorting issues.

[MrForbes]

Originally Posted by MattC9

Hmmmm.... Just our luck I went and checked our 3, 2 of which were conductive. What exactly makes them "bad"? I have not heard why exactly this a problem.

Imagine two of your motors have a short to the case, and they're mounted to metal parts of your robot that connect...interesting things might happen.

I think it's also against the rules to have any power applied to the metal parts of the robot.

<R36> All wiring and electrical devices, including all control system components, shall be electrically isolated from the ROBOT frame. The ROBOT frame must not be used to carry electrical current (e.g. this is necessary due to polarity reversals that occur under certain operating conditions such as during motor direction reversals).

[trilogy2826]

Arrgh... 3 of our 10 were shorted to case. Thanks for saving us some trouble. Has anyone tried to go through the Banebots RMA process yet?

[rsisk]

Could someone please post the steps to check the 775s for a short?

Is there a fix? Or do the motors need to be returned.

Thanks

[Mike8519]

3/7 of ours were shorted

[JVN]

Originally Posted by rsisk

Could someone please post the steps to check the 775s for a short?

Is there a fix? Or do the motors need to be returned.

Thanks

Use a multimeter to check the resistance between one of your motor leads and the motor casing. If it is conductive, you've got a problem.

We pulled one apart and tried to fix it, but no luck.

We haven't done the RMA process and probably won't bother (the motors have all been modified for use). The ordering (waiting) process was painful enough.

We had one of these motors (via AM Planetary) driving the same gear as a CIM. One of my freshman was wondering why the "gear was sparking." I told him it was normal to see sparks coming out the back of an open motor like the 775. He corrected me -- "not out the back, on the front... on the gear." Ugh -- that was when we first noticed the issue.

-John

[Dale(294engr]]

Originally Posted by MattC9

Hmmmm.... Just our luck I went and checked our 3, 2 of which were conductive. What exactly makes them "bad"? I have not heard why exactly this a problem.

This is a mfr defect carryover from last year. Appears Banebots may be dumping known defective 775's on FIRST as I and others contacted Banebots
but they resisted acknowledging there was a problem.
I personally performed failure analysis, witnessed shorted turns to rotor core.

RS-775 rotors use epoxy coated core laminations as insulation, ineffectively.

(most motors use brown fiber insulation. check out CIM rotor. Epoxy, properly quality controlled, MAY allow a more compact motor & less labor to mfr)

Enamel windings short at iron lamination corners via break thru at tight bend.

Two windings shorted behaves as a shorted transformer winding
causing no load motor current to quickly increase, depriving motor of torque

if short to motor case = robot AL frame, it is Illegal and failed at inspection

(theoretical remedy: electrically isolate each motor to pass inspection, but problem will likely escalate as more turns shorted draws higher current & produces less torque; high localized heating leads to catastrophic smoke)

Shorts I measured were typically ~0.1 ohm to case (either terminal to case)
which requires a careful low ohm set-up

It's a smoking gun again this year.. This is a great well made motor otherwise.
PreTesting out of new box is essential if decision to use these.
At a regional last year 70% of new boxed spare parts 775's were shorted!

causing huge delays for teams counting on them (Team 207 et al)