Suicidal Victor 884's...

[Lloyd Burns]

The reason for the 40 A breakers is to delay the onset of "Puff". When those engines start (with a Drive-it-like-you-stole-it driver), or when some Idiot tries to push the end wall and your wheels have absolute traction on the carpet, the motors aren't turning very fast, the back emf they generate is zero, and they draw the "Stalled" or "Locked rotor" current in the specifications. For how long depends on the fuse. One sheet said that when one of these motors (I forget which) was stalled, it acted like a .0999 ohm resistor, or, using Georg Simon's wisdom, it draws blessed 120 A ! ! !

Once they are moving the 'bot somewhere, and their back emf approaches the applied voltage, the current drawn becomes a lot less (like "20 A"). This is the "under normal driving conditions" state, where the fuses don't blow anyway.

And, when the load is primarily braking (sudden shift to speed = 0 on a 'bot going all ahead full), the current in the Victors is in the loop between the Victor and its motor. As stated above, this circulating current can be very high, and it doesn't flow through the breakers.

These are not TOYs, but they get the respect Rodney Dangerfield gets ! Pfui !

[Al Skierkiewicz]

I would believe that the move to 40 amp breakers is in response to many teams complaining that the 30 amp breakers were tripping too often to allow reliable operation during a match. In robots during the past two years that were tripping 30 amp breakers, most often it was due to mechanical designs that demanded that motor be operated too far out on the power curve and robot designs that had extremely high frictions when the robot turned. In these case, even the 40 amp circuit breaker would not have changed things very much. The current draw on those robots still exceeded 80 amps per motor with peaks above that and many robots were tripping the 60 amp breaker.
All teams should be aware of the high currents that these motors can draw. Even with the 120 amp main and 40 amp motor circuit breaker allowed this year, some teams will still be tripping breakers. Each of these breakers can handle 200% of rated current continuous for only 15-20 seconds before trip. The main breaker will trip off at 200 amps after 10 seconds minimum to 30 seconds maximum. On the resettable breakers, once trip temperature has been achieved, lower currents will trip the breaker.
Having a design that calls for a gear ratio that gives extremely high speed on a flat surface will most assuredly run the drill motors to 90-100 amps on the ramp. A drive system that uses two drill motors with a low gear reduction, when in a pushing match will trip the main breaker well before the end of a match.

[Sachiel7]

My guess for the reason that they included 40 amp breakers this year is that in the 2001 Season, teams constantly had issues w/ motors tripping while going up and over the teetertotter. Last year, they removed all non-horizontal obstacles, but since they are adding in another one this year, they decided to give you a little more power

[Steven Carmain]

Was this a 883 or a 884? The 884 can take the 40A, but the 883 can only take 30A

[hi_im_sean]

my team had a similar problem with our main breaker....
our conectors were not properly installed and it arced causing it to spark and smoke after about 30 seconds of running and drawing major current without our breaker popping. you might want to check all your connections.

[Ralph Lambert]

We just had an 884 fail. I was just trying to reset the cal to the defaults. I was holding down the cal button, we powered up the electronics and in a couple of seconds it was tossing sparks. Now it controls in one direction but not the other. I have not taken the fan off yet to survey the damage. Can these things be repaired or is that it?

[Al Skierkiewicz]

It is best to check in with Innovation First on repairs on these devices. So far the arcing and sparking complaints could be metal shavings in the controllers or could be a bad run of FETs. If the later is the case the guys and Innovation First will know and will tell you what to do. Do not attempt repairs without checking with Innovation First. If you try, I would virtually guarantee no warranty coverage.

[Andrew]

Since a bunch of people are having similar problems, I'm not sure that our problem from last year relates to this year's 884 deaths.

we had two victors (one on each drive motor) connected. We had a short through case between the M+ on one victor and the M+ on the second victor. This created a current loop which blasted one of the victors (the weakest).

I always demand that we do a continuity test between M+, M- and case before turning on any power. We slipped last year.

Anyway, that's something else to try.
Andrew
Team 356

[Al Skierkiewicz]

Andrew,
When you say "short through the case" what do you mean? Did you have motor wiring accidentally shorted to the robot frame? If that is the case and you had the motors running in opposite directions I can see how that resulted in the failure. Did the Victor that failed have shorter power wiring feeding it than the one that survived?

[Fridge]

Our team just had a Victor 884 fail. We heard a pop and then saw and smelled smoke. I'm fairly sure it wasn't a short because I couldn't hear any clicking from the 30 Amp breakers and the wires were well insulated and not close to each other. We were using a van door motor hooked up to a 30 Amp breaker. I'm going to have to take a closer look under the fan, but I don't know what could have caused this.

What should we do?

[Andrew]

We just tested all of our Victor 884's (7 in all, 4 from the kit, 3 ordered from IFI later) on our simulator to make sure they are OK before putting them on the robot. We used a van door motor and fused it with a 20 A breaker.

All of our victors performed fine under these conditions.

Since so many people are experiencing difficulties, I would suggest doing a functional test before installing and checking all wiring on the robot for shorts to case before turning them on.

We will probably also do a 40 A breaker + drill motor functional test prior to full install, just in case.

Andrew
Team 356

[Andrew]

Originally posted by Al Skierkiewicz
Andrew,
When you say "short through the case" what do you mean? Did you have motor wiring accidentally shorted to the robot frame? If that is the case and you had the motors running in opposite directions I can see how that resulted in the failure. Did the Victor that failed have shorter power wiring feeding it than the one that survived?

This was a flukey short, but here's what it was.

One drill motor had a short between its motor terminal and our aluminum heat sink shroud which surrounded it. This shroud was mechanically (and electrically of course) connected to our aluminum frame.

On the second drill motor, we had a short between the ring connector and the mounting screw for the speed controller. The mounting screw was mechanically and electrically connected to to the same case, although on the opposite side of the robot. (Note: we now use plastic 6-32s to mount our speed controllers to the frame).

I'm not sure whether we had M+ on one motor connected to M- on the other (in which case forward would have been bad) or whether we had M+ connected to M+ (in which case opposite would have been bad).

I'm sure the length of wiring was different, but not by much. I just assume that the weakest component started to flame out first.

Andrew, Team 356

[Al Skierkiewicz]

Andrew et al,
This is one of the very reasons I think inspectors should check (with an ohmmeter) for power wiring to chassis shorts when performing inspections on the robot. All teams should perform this test before "power on" when in construction phase and repeat if any wiring passes through chassis parts. (tubing, enclosed spaces, etc.) This first bit us in 1996 when wiring that was passing from one side of the robot to the other through chassis tubing was shorted after a drill and screw operation was made to the tubing. (This played havoc with the controllers at the time due to just increased noise in the ground lead.) That problem is what brought me to the team. My son, a first year student, dragged me out of the stands to help out and I have been here ever since.

[Ron Woodward]

We also had one blow up on our first robot test, However it was not connected to anything. We had mounted all the electronics and wired input power to them. Only our drive motors were hooked up. The speed controller had no motor connected to it at all. Power and pwm signal where there no motor and it blew all by itself. All the others were fine. We removed the circuit breakers for all the others just in case for the rest of our testing.

From this it looks like the controller suffered a shoot through failure - Where both the high side and low side FETs are on at the same time causing a dead short in the device itself. The circuit breakers will not protect for this type of problem, they take too long to respond to protect the FETs. This problem is usually caused by not allowing enough dead time between switching on and switching off a design issue in the speed controller.

[Al Skierkiewicz]

Ron,
Was there a software adjustment of the deadband by any chance? (the issue with 883 vs. 884) Did you have any particular indication on the Victor. (no light, all orange, etc.) Did you try to reset to factory calibration? All of this is assuming you didn't have a flame out, of course.
I am just looking to get a track record on the failures for future reference. If anyone returns 884s to Innovation First and gets any info on the failure, please post.
Thanks,