We had a demo today. One of our drill motor gearboxes locked up, stalling the BDM. No big deal, we’ve done it before.
However, when we did it before, we had a 30 amp circuit breaker on.
With the 40 A circuit breaker, we fried the Victor 884. Based on comments on this list, I have been a bit hesitant about using the 40 A breakers. Now, we have evidence that they do not adequately protect the 884 when a motor stalls.
With the 40 A circuit breaker, we fried the Victor 884. Based on comments on this list, I have been a bit hesitant about using the 40 A breakers. Now, we have evidence that they do not adequately protect the 884 when a motor stalls.
At stall those drill motors should be drawing enough current to trip the breakers. I really do not remeber the number off the top of my head but it is a big enough that it should have tripped the breaker. I can’t get onto the First website so I really can not check the stall torque on the motor. Do the circuit breakers have a time delay in them??
*Originally posted by Adam Y. *
**Do the circuit breakers have a time delay in them?? **
The drill motors draw 120A at stall. The breakers can withstand 3x their rated current for ~1 second before they trip. 2x their rated current for ~5 seconds and 1.5x their rated current for 8-10 seconds.
*Originally posted by Jnadke *
**The drill motors draw 120A at stall. The breakers can withstand 3x their rated current for ~1 second before they trip. 2x their rated current for ~5 seconds and 1.5x their rated current for 8-10 seconds. **
From what I’ve heard, the 40A breakers aren’t very consistent with their limits. Apparently, team 254 got more than a few bad ones, some of which tripped at ~20A. Although, this is all hearsay, so please ingest a grain of salt, now.
*Originally posted by FotoPlasma *
** Apparently, team 254 got more than a few bad ones, some of which tripped at ~20A. **
Yeah, I remember that. They had a few rounds of furstrated confusion on why that was happening. They were using some kind of liquid gas in a can to cool down their motors and breakers. I guess they got so cold that they condensed, and water can do some pretty nasty things when mixed with electricity. Overall though, I think the 40 amp breakers were great. Used one on each of our 4 motors and we were fine. Never broke them, or caused the “magic smoke” from our motors. 
Could you describe the situation? How long had you been running? Was that motor being used frequently (as in, was the breaker/Victor already warm)? 'Bot acting abnormally? Any problems with retrofitting the 40 amp breakers? Had you been using cool-in-a-can? Could you describe the Victor?
-Andy A.
Could you describe the situation? How long had you been running? Was that motor being used frequently (as in, was the breaker/Victor already warm)? 'Bot acting abnormally? Any problems with retrofitting the 40 amp breakers? Had you been using cool-in-a-can? Could you describe the Victor? -Andy A.
We had just transported the robot across town. The robot was turned on and was being driven about 20 feet across the room. Some screws had backed out of our gearbox during transport. As the robot reached the end of its travel, the gearbox bound up, stalling the drive motor. Arcs and sparks ensued. No breaker trippage.
This Victor was our competition speed controller on our competition drill motor. We tested the no load current of the motor after all competition had ended and saw no significant degradation. All of our Victors have looked and acted normally during competition and since.
We have never used foreign substances on the breakers (they’re too embedded in the robot). We have also never heard or experienced the 40 A breakers trip. Last year, we routinely tripped the 30A breakers, so we hopefully know what to look and listen for.
We have not yet opened the speed controller for inspection. However, the odor of burned speed controller is still detectable. The input section of the Victor continues to function (the power light comes on and gives a solid signal when a valid PWM is present, blinks when no PWM is present). However, we have no output to the motor at all. There are no metal chips that we can see in or around the Victor.
Maybe the fan was not working at the time? Those things can heat up pretty fast, and with no fan and a current draw of above 100 amps for a second can easily fry the thing. The only factors are how long the fan wasn’t running while on, and how much current you were drawing before the gearbox locking up. Only problem is that if the fan was working as usual, there could be a deeper, nastier problem somewhere. The Victors shouldn’t fry that easily.
Since it had only driven 20 feet, I doubt that the victor it’s self was very hot. We ran the better part of a match with a damaged fan on a drive Victor, and it survived.
I would suspect either a faulty victor (perhaps a tough competition season has finally taken its toll and something gave), or that 40 amp breaker. My bet would be on the breaker, since from what I gatherd, this was the first time your bot had used them. If thats all that had changed between competition and this demo, then its the likely culprit.
When your gearbox bound up, was it a sudden thing or does friction slowly build up?
-Andy A.
I would suspect either a faulty victor (perhaps a tough competition season has finally taken its toll and something gave), or that 40 amp breaker. My bet would be on the breaker, since from what I gatherd, this was the first time your bot had used them. If thats all that had changed between competition and this demo, then its the likely culprit.
The Victor was the competition Victor. The breaker was the competition breaker. We had no over-current problems in competition. This was the first real current spike that we had with our robot.
When your gearbox bound up, was it a sudden thing or does friction slowly build up?
The gearbox bound up suddenly. We went from more or less normal friction to a hard bind in about 2-5 seconds.
My point behind starting this thread is that, this is exactly the situation that the breaker is supposed to protect against. And it didn’t.
We have done exactly this kind of thing with other robots and with the 30A breaker. The breaker trips first.
We have thrown out our 40A panel and breakers and put all of our drive motors on 30A breakers. If we start getting breaker trips or fry another Victor, then we’ll look into this further. However, I expect that we’ll see smooth sailing from now on.
Andy,
I think if you look, you are likely to find some other reason for the failure. The 129 amps at stall is not enough to kill a contoller by itself and the breakers are sufficient protection for the controller. High on my list is failed motor brush assembly, hardware across the circuitry feeding the motor, or metal flakes in the speed controller. All of these have the potential for exceedingly high currents at the speed controller.
High on my list is failed motor brush assembly,
We did have a failure in the brushes. I thought the breaker was supposed to protect against this failure frying the speed controller.
Andy,
A failed brush assembly provides a dead short across the output of the controller. This results in several hundred amps as opposed to the 129 amps at stall from the motor alone. Since the short is there constantly, even running a relatively slow speeds causes high currents to flow. High currents results in high temperatures at the transistors, enough to melt the silicon inside. This causes high internal pressures, blowing the case (of the transistor) open and causing the arcing and smoke you saw. I remember seeing a spec sheet that the controller was able to handle short duration loads of 600 amps but with a sustained short, the components just can’t handle the stress.
A Problem we didn’t know we had until the next season, occurred with the old drill motors, which were fastened to out home-made gearbox using a friction “collar” around the motor. When the demand was made for quick acceleration, we saw later that the motor turned (Thanks three times to Sir Ike). All I heard during the season was “the motor power lleads are falling of by themselves”. Once we saw the motors turning as muh as 90 degrees, we saw the reason the leads (using 0.187 spade female QD connectors) were “falling” off. It was decided to mount the Drills using the mounting screw-holes provided on the front end-bell of the motor.
A short to the case or gearbox via disconnected leads, as above, for example, could cause the same high currents Al describes. IFI used to brag about the 883 Victors handling several hundred amps, as a charging robot was put into full reverse, and with three FETs in parallel in each leeg of the “H”-bridge, each capable of carying a coninuous 50 A and more. it would take a verylarge current to blow them before the breakers went.
Some industrial SCR / Triac A.C. power circuits actually use semiconductor diodes as fuses, because they’ll blow faster than a fuse.