Another DC Electronic Load thread... - Page 1
Forum section: electronics / testgear

author: IDEngineer
author_rank: Super Contributor
timestamp: July 29, 2019, 05:19:03 am
content: I've read a bunch of the others but, like most applications for electronic loads, no one else's quite matches what I need to do.My son is an internationally ranked drone racing pilot. We need to do some stress tests on their hardware, including the LiPo batteries they use. When they shove the throttle to 100% to do a hard direction reversal (not a turn, a *reversal*), the peak currents can be over 100A. We want to study what happens to the supply rails, both VBat and the regulated rails in terms of transient feedthrough, under such extreme conditions. Obviously we cannot connect scopes etc. to a quad and then fly it around, so our thought is to use an electronic load instead of the 3-phase BLDC motors to develop what is nearly a short circuit across the battery.Most smaller loads top out around 60A, which is fine for this research. We will start at lower currents and ramp up as we gain experience.Our initial tests will involve a step function. Most loads support an A Level and B Level and the ability to switch between them, so we will configure (say) A to be zero load and B to be whatever peak current we want to test. Then we will trigger this externally, which means the load we buy must support an external trigger input - which not all of them do.Those that do support an external trigger don't always document it very well. For example, I was looking at the B&K 8500/8600 series. They have an external trigger input on the rear panel. However, the manual says the response time to the external trigger input is on the order of 10mS (?!?), which I interpret to mean that the load doesn't react to the trigger for up to 10mS after its leading edge. That's an eternity for a test like this.There are also discussions of loads with analog vs. digital closed loop control. I bet the 10mS trigger latency units are digital! But this raises the question of which one is better for this application. I'm strongly tempted to default to "analog is better" but that presumes the bandwidth of the feedback loop is better than the latency of the digitally controlled units... and nobody seems to really dig into specs like that.I'm willing to spend $1000-1500 on this load. I'm tempted to just design and build what I need but frankly don't have the time right now, and besides it seems logical that someone has already designed a unit like this. Anyone know about external trigger latencies, feedback loop bandwidth, etc.?Thanks!

author: PedroDaGr8
author_rank: Super Contributor
timestamp: July 29, 2019, 07:09:02 pm
content: You didn't mention the most critical value which is total wattage. 100A at 12V for example is ultra-high end load territory which means $$$$Additionally, you state:QuoteThis is less of a big deal than you are making it I believe. Response time doesn't always mean minimum time between changes, it could mean lag. So two commands given 1ms apart, are each delayed 10ms; which means they occur at 10ms and 11ms. For programmed load steps, this shouldn't be a big deal at all. The value you REALLY need to be worried about is slew rate. This tells you the bandwidth of the load and whether it can accurately simulate the load of an electric motor. For example, if the load only has 10A/s slew rate, then it will take 10s to reach 100a, which would immediately rule out that load.The only load I found that MIGHT fit is the Agilent/Keysight N3306A load card. This card requires the use of a suitable mainframe to function. This card goes from 0-60V, 0-120A, max 600W combined between the two and has a slew rate of up to 10MA/s with the right parameters set. The card would set you back around $800-900 used, while a compatible mainframe will run you a few hundred bucks and up. That being said, if there are VERY fast slew rates involved, then the wiring from the battery to the electronic load alone will have huge impacts on the reliability of your measurements.In the end, have you considered just strapping the drone down on a table and recording using a traditional oscilloscope? Even if not totally accurate, this would be far easier and if large current swings are involved far more accurate than a load. Note that a differential probe might be useful here to limit the amount of noise picked up from the motors.

author: nctnico
author_rank: Super Contributor
timestamp: July 29, 2019, 07:21:14 pm
content: You can take a look at the older Dynaload DC loads like the DLF 100-100-1500 I have. These loads are relatively cheap on Ebay and easy to fix. All the schematics are available. I fixed and refurbished mine in this thread:https://www.eevblog.com/forum/testgear/dynaload-dlf-100-100-1500-(100v-100a-1500w-dc-load)-teardown-repair/These loads have a modulation input and current measurement output but beware that on both connections the ground is connected to the - input terminal!The one I have can also work in a pulsed mode but I have never tested it. According to the spec the 10% to 90% risetime should be somewhere around 15us. The maximum frequency of the pulse mode is 20kHz.

author: IDEngineer
author_rank: Super Contributor
timestamp: July 30, 2019, 04:37:28 am
content: Quote from: PedroDaGr8 on July 29, 2019, 07:09:02 pmA 300W load should allow ~12A from 6S LiPo's (25.2VDC). And I can increase the current by running the LiPo's at nominal storage charge of 3.8VDC/cell or 22.8VDC for a 6S, which means a bit over 13A. Also, I don't know if the loads enforce instantaneous wattage or if they tolerate short duty cycles of higher values; I can capture the required data in a couple hundred milliseconds so if the load can allow the external trigger to switch from Level A, to Level B for (say) 200mS, and then back to Level A, that would be more than sufficient and perhaps a higher peak current could be tolerated.The alternative would be to skip the E-load completely and just use some sort of static load - maybe even a mechanical restriction on each motor - and trigger the test via the ESC rather than using a static setup. That would certainly be less expensive! Maybe I don't need to spend money on an E-load at all.QuoteI agree, IF the latency is constant. But if it's "reaction to the trigger could be 0-10mS", then it could severely impact a test like this. A test where there's just one "active edge" would be unaffected, but any test that is more of a pulse would have an unpredictable width.QuoteMost of these $1000-1500 loads appear to have a max slew rate of 2.5A/uS. For a ~13A test full current would be reached in under 5uS.QuoteYes, I've considered that, but I'm not sure (just as with the mechanically restrained motors idea above) it will adequately simulate conditions. The extreme condition occurs when the quad is flying at 100% throttle in a straight line, and then the pilot drops the throttle, rotates 180 degrees around the pitch or roll axis (now facing in the opposite direction), and then goes to 100% throttle again. This means the props are working AGAINST the previous velocity to bring the airspeed to zero, before accelerating in the opposite direction. This is a much more intense electromechanical condition than just "acceleration from zero" which is what you'd roughly be simulating with a strapped-down test.That said, based on where this thread is going I may just start out with the strapped-down test and see what we can learn from that at first. I haven't been able to dream up a way of simulating the in-flight reversal test with a strapped-down quad, which is why I was leaning toward an E-load approach.Thanks for the feedback so far, it's definitely helping me think through the options.

author: windsmurf
author_rank: Frequent Contributor
timestamp: July 30, 2019, 05:00:01 am
content: Maybe you can create a mechanical load on the motor to simulate a prop working against the velocity.It's too bad RC Heli hobby seems to be getting pushed aside by drone racing.I still think Heli's are a lot more fun...

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 30, 2019, 06:08:32 am
content: i was about to suggest building a custom rig similar to JPL except smaller scale to suit engine size, since i dont think motor load can be perfectly simulated with dummy load, and 1200W is heavy money to buy, and since OP mentioned studying battery transient effect, so this must have to do with dso probing. so a single massive electronic load will not suffice anyway. but since this thread is about e-load, then my suggestion maybe irrelevant, btw there is a youtube video i cant find anymore showing diy e-load using massive heatsink, googling youtube may give some light, ymmv...

author: PedroDaGr8
author_rank: Super Contributor
timestamp: July 30, 2019, 06:35:14 am
content: Quote from: IDEngineer on July 30, 2019, 04:37:28 amThat's much different than 100A that you mentioned before. The HP/Agilent/Keysight load combo I mentioned before could easily handle this and far more, but this doesn't make them representative of the load the motor will induce.Quote from: IDEngineer on July 30, 2019, 04:37:28 amI think at this stage, you are on the right path. Figure out the ballpark you need to play in before you layout the serious $$$.Quote from: IDEngineer on July 30, 2019, 04:37:28 amAgain, I think you are on the right track. While you can't simulate everything perfectly, you can figure out a rough approximate.Sometimes it makes sense to pay a lot up front, rather than pay a little many times. In this case, you don't know exactly know what your needs are. It makes sense to pay a bit up front to figure out your needs, then pay more later, if you need to, in the areas that make sense to spend the money.If the approximate isn't good enough, you can balance THAT against the costs rather than the much broader unknown you had before.Also, I forgot to say it before, but good luck to your son in his competitions.Sent from my LG-LS998 using Tapatalk

author: JohnG
author_rank: Frequent Contributor
timestamp: July 30, 2019, 01:59:25 pm
content: This might be worth a look:https://www.analog.com/media/en/technical-documentation/application-notes/an133f.pdfJohn