1771's Jaguars

[Al Skierkiewicz]

Guys,
I would need to see the actual tested ratings for these connectors. None appeared in the link above. Although the 35 amp Power Poles would seem a bad choice remember the Power Poles are rated for continuous duty at that current. Have you noticed that the battery is only a 50 amp Power Pole? The two minute (say up to ten minute) rating for these connectors far exceeds the failure point of 35 amps continuous. In case you haven't tried, Power poles are easy to solder but if you use the West Mountain crimper, with #10 wire your connector resistance is already very low. If you solder, it is almost the same resistance as a wire. A bad crimp on any connector (or a solder job that only covers part of the wire) will get hot under load. Inspectors need to have numbers. You may have been able to get away with them in the past but if they are under rated, then you will need to change.
As stated earlier, I recommend using the least number of connections you can. We use Power Poles on motors (and in special applications) only to allow their rapid removal. Everything else is point to point.

For reference...
CIM Stall Current is 129 amps
Fisher Price Stall Current is 70 amps
Battery (charged) is capable of over 600 amps.

[martin417]

Originally Posted by Al Skierkiewicz

Guys,
I would need to see the actual tested ratings for these connectors. None appeared in the link above. Although the 35 amp Power Poles would seem a bad choice remember the Power Poles are rated for continuous duty at that current. Have you noticed that the battery is only a 50 amp Power Pole? The two minute (say up to ten minute) rating for these connectors far exceeds the failure point of 35 amps continuous. In case you haven't tried, Power poles are easy to solder but if you use the West Mountain crimper, with #10 wire your connector resistance is already very low. If you solder, it is almost the same resistance as a wire. A bad crimp on any connector (or a solder job that only covers part of the wire) will get hot under load. Inspectors need to have numbers. You may have been able to get away with them in the past but if they are under rated, then you will need to change.
As stated earlier, I recommend using the least number of connections you can. We use Power Poles on motors (and in special applications) only to allow their rapid removal. Everything else is point to point.

For reference...
CIM Stall Current is 129 amps
Fisher Price Stall Current is 70 amps
Battery (charged) is capable of over 600 amps.

Al,

For reference, I find that quite often, published specs are misleading, and in some cases outright wrong. I did a great deal of research with the CIM motor over the last year (not FIRST related). While the data around the higher efficiency end of the curve is reasonably accurate, as you get farther from that point, it goes off into the weeds. Using accurate instrumentation, I measured the current at several different operating speeds and loads. I was using a 95 AH AGM 12V battery with an internal resistance much lower than the FIRST batteries. I don't remember exactly how high the current got at its highest, but it was very near 30 Amps, no more than 35. That was when the motor was nearly stalled. At stall, the draw dropped to less than 30 amps. Not sure why the data sheet says what it does, but the CIM doesn't draw 129 amps.

Based on that, and the fact that Fisher Price toy cars (the main use of the FP motors) are wired with ~14 gauge wire, I don't believe the 70 amp rating on them. I know my kids were able to stall the motors out on the jeep they drove around, and no wires smoked.

[sanddrag]

Originally Posted by martin417

I don't remember exactly how high the current got at its highest, but it was very near 30 Amps, no more than 35. That was when the motor was nearly stalled. At stall, the draw dropped to less than 30 amps. Not sure why the data sheet says what it does, but the CIM doesn't draw 129 amps.

I find this very hard to believe. We have known and trusted and designed to these specs for 9 years now. For the CIM to stall at less than 40 Amps, would mean its peak power is significantly less than published, yet our robots seem to perform to expectations every time.

To be honest, I've never run one through any sort of current measuring device, but I certainly have tripped 40 Amp breakers with them. I know some teams have done current monitoring (and if I recall, some sort of FIRST-sponsored program at IRI 2008?). Do any teams have any current draw data they'd like to share?

[martin417]

Originally Posted by sanddrag

I find this very hard to believe. We have known and trusted and designed to these specs for 9 years now. For the CIM to stall at less than 40 Amps, would mean its peak power is significantly less than published, yet our robots seem to perform to expectations every time.

To be honest, I've never run one through any sort of current measuring device, but I certainly have tripped 40 Amp breakers with them. I know some teams have done current monitoring (and if I recall, some sort of FIRST-sponsored program at IRI 2008?). Do any teams have any current draw data they'd like to share?

As I said, in the range where a motor is used, the specs are fairly accurate. You don't USE a motor at stall. It doesn't do much. I am offering the results of a scientific experiment, you can choose to do with that data what you like. I can say, with absolute certainty, that a CIM motor will never draw 129 amps, or even 50 amps.

[sanddrag]

Originally Posted by martin417

As I said, in the range where a motor is used, the specs are fairly accurate. You don't USE a motor at stall. It doesn't do much. I am offering the results of a scientific experiment, you can choose to do with that data what you like. I can say, with absolute certainty, that a CIM motor will never draw 129 amps, or even 50 amps.

I'm not discrediting your test, as I have done none of my own. I'm more just intrigued by your findings. If your results are correct, it could really change the way many teams design things in the future. So you are saying that your data shows the motor is highly non-linear, deviating from the theoretical model of a permanent-magnet DC motor? Interesting. Do you have some experimental curves produced from the test?

Has anyone else ever put one on a testing rig?

[martin417]

As an exercise, measure the resistance across the leads of a CIM. I measured between .4 and .6 ohms. The voltage of a fully charged battery can be from 12-13 volts.

Applying Ohm's law, we know that (in simple circuit) I=V/R, so at worst case (highest current), we would have 13V/.4ohms, giving you 32.5 Amps. At best case, 12V/.6ohms gives you 20 amps.

As I said, I saw max currents in the low 30 amp range. If you would likem perform your own experiment.

Remember, take everything you hear (or read) with a grain of salt, and to everything you say, add a grain of sugar.

[Al Skierkiewicz]

Martin,
We did current testing on real robots in the past, both ours and many others. I can tell you that the CIM can and does indeed draw well over 100 amps in stall. I wonder if you had a bad motor. I can't find the data but I seem to remember that the motor resistance is off by at least an order of magnitude from what I remember. Seems it is in the order of 10 -20 mohms.

[Dad1279]

Martin, we've measured over 32 amps with fan blades last year, no where near stall.

All of our current/rpm measurements with fans were fairly close to the published curves.

Never measured actual current at stall, but we have 'popped breakers' in the past when we had transmission issues, and the CIMs stalled.

I'm confident they draw well in excess of 40 amps at stall.

[martin417]

I stand corrected. I just went to the lab and performed a simple test. I clamped a CIM in a vice, and locked the rotor with a pair of Vice Grips. I clamped on a current probe, and then connected the leads of the CIM directly to an MK 100 AH AGM battery. I did see current in excess of 100 amps.

Not sure why the results we got last time were different, but I suspect it had to do with the battery we were using. I am betting the internal resistance of that battery was high. At any rate, I would also suggest running this test using a FIRST battery and compare. I am not sure it is capable of supplying that kind of current, but, one test is worth 1,000 theories.

I hate having to eat my words.

[Al Skierkiewicz]

At least now we all know we aren't crazy.

[Andrew Y.]

i know in high powered brushless set ups with up to 120 amp continuous and over 200 amp bursts, the preferred connectors are Traxxas, deans, or bullet connectors.

Im sure the deans will be just fine.

but i always wondered what kind of difference we would see if we use higher quality connectors? anyone do any tests>?