Odd behavior with constant current loadbank - Page 1
Forum section: electronics / general

author: Syllith
author_rank: Regular Contributor
timestamp: January 04, 2023, 08:31:39 pm
content: I work for a company that tests rectifiers and we were using some older analog load banks that used kanthal and were essentially glorified toasters. Well the company that made them no longer produce these and all the ones we found online were digital and from my understanding, they use MOSFETs to control the current. We ordered one to see if it would be a viable replacement. We hooked it up and noticed there's an fairly large amount of voltage drop, even at smaller loads.For example, with the analog load bank, we can have a 100A rectifier and put an 80A load on it and it'll maintain around 57VDC or so. With the digital one, the ammeter will report the exact same current draw, but the voltage dips to around 47VDC or lower. Even if we draw say 40A, the voltage will still dip to 50 or so. We don't get this behavior with the analog ones. My coworkers and I are racking our brains as to why this is happening. Any ideas as to what is going on here?

author: Whales
author_rank: Super Contributor
timestamp: January 04, 2023, 09:39:33 pm
content: How are you measuring the voltages and currents?Where are you measuring the voltages and currents (in both setups old vs new)?What equipment are you using to measure the voltages and currents?An extra 10V drop at 80A is a mere 0.125 ohms of difference. Different measurement practices (eg measuring voltages at source vs load), different measurement equipment (different ammeters) and different cables (different thickness + cleanliness of connectors) could explain this.

author: Syllith
author_rank: Regular Contributor
timestamp: January 05, 2023, 11:18:07 am
content: To measure current, we're using a clamp meter on the 4 gauge wire running to the load bank. We are using the same 4 gauge wire for both the analog and digital load bank. Quality of connectors are good and cables are not old. To measure voltage, we have the load banks themselves that tell us, a DMM measured at the terminals of the load bank, and the controller of the rectifiers themselves. All are consistent readings, so it seems as if everything is accurate. These rectifiers will automatically adjust their voltage based on how much drop there is. Not sure if this is worth noting, but if we slot 2 rectifiers into a cabinet, this voltage drop goes away and everything is normal. It almost seems to me as if the way the digital load bank manages the current is causing the rectifiers to be unable to determine that it needs to increase its voltage.Perhaps since the digital load bank is switching (kind of like a SMPS except with current), when the MOSFETs are on, the load is very heavy, but only for a brief moment, and that's prevent the rectifiers from recovering? I don't know, I'm just trying to throw theories out there. I believe our measurements are accurate, just because we have so many different pieces of equipment measuring it, so it makes me think the way the load is applied that it's causing the rectifiers to act strangely. It doesn't do it on every rectifier either. Some of them just work fine (notably the 24v ones). Certain brands don't seem susceptible to it either. It's a 50-50 shot every time we try it one a different rectifier that we'll get this huge amount of voltage drop that's inconsistent with our analog load bank.The model of digital load bank is LB-48-200-CC if that helps in any way.https://www.tequipment.net/Eagle-Eye/LB-48-200-CC/Capacity-and-Load-Banks/

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 05, 2023, 05:23:12 pm
content: Quote from: Syllith on January 05, 2023, 11:18:07 amHave you looked at the voltage with a scope? Also put the voltage probe at the output of your rectifier and check the voltage.

author: Whales
author_rank: Super Contributor
timestamp: January 05, 2023, 11:02:28 pm
content: QuoteAre these synchronous controlled rectifiers with variable voltage output? Or am I thinking of something completely different?Do they perform any waveform smoothing or just chop up the AC into something RMS equivalent to the DC that you want?QuoteQuoteSounds likely. See what the voltage and current waveforms look like on an oscilloscope. Many control systems can't handle anything but low frequency V & I fluctuations properly. Multimeters may also misread complex waveforms.

author: Syllith
author_rank: Regular Contributor
timestamp: January 09, 2023, 01:59:07 pm
content: I have hooked everything up to a scope and applied an 80% load. I tried this with a 24v and 48v rectifier.These are the results of the 24v (normal):https://skydrive.digi-safe.co/files/02EE6393-0AB5-41C7-B25E-EA86EACE3319.jpegHere is the 48v (abnormal):https://skydrive.digi-safe.co/files/D97DEE7A-662D-44B4-93C2-2E1E438250D4.jpegAs you can see from the 24v image, there's a big current hit at the beginning, then it recovers until the voltage stabilizes. On the 48v, you can see it's just a saw wave, unable to recover fully. Perhaps this is because the 24v provides double the current, allowing it to recover faster (Better transient recover time?). The saw wave almost looks like caps draining and filling rapidly.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 09, 2023, 02:19:01 pm
content: looks to me like at least one if not two unstable control loops, resonance or switching noise.By rectifier you imply some simple analogue power supply with no regulation however I bet it is not, perhaps you could provide more detail or at least an internal picture for educated guesses.

author: Syllith
author_rank: Regular Contributor
timestamp: January 09, 2023, 04:10:47 pm
content: Sorry, rectifier is more of a generic term we use here as most of the manufacturers label them as that on our spreadsheets. It is a normal SMPS with filtering and all. We actually had to use 2 different PSUs for this test since they don't make a 24v version of the ones we're having problems with, but this voltage drop can be seen on basically any 48v PSU.48v PSU:https://www.vertiv.com/4ae6fa/globalassets/products/critical-power/dc-power-systems/r48-2000e3-rectifier-data-sheet_00.pdf24v PSU:https://www.alpha.ca/solutions/products/power-modules/rectifiers/item/24vdc-cordex-3-1kw-rectifier

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 09, 2023, 06:54:19 pm
content: Well there you have it, a power supply and a load both with active feedback circuits, one trying to maintain constant voltage & the other constant current, what could possibly go wrong!!Neither of the product specifications mention transient response so you are left requiring an empirical solution, given the frequencies you have observed with your scope my first line of attack would be connecting leads (inductance), try keeping them as short as possible (certainly <1M) and a twisted pair.If that doesn't work try adding some C (low ESR & ESL) directly across either or both the psu or loads terminals, again by frequency I reckon your looking at film but you might need 10-100uF but compared with what you been spending so far peanutsGood luck

author: Syllith
author_rank: Regular Contributor
timestamp: January 09, 2023, 07:14:17 pm
content: I think I understand what you're saying. The 2 active feedback circuits conflicting makes sense and has been my suspicion since the beginning. If I'm reading this right, you're saying to add a few 1-100uf film caps on the output to add additional filtering to see if this will correct the voltage drop problem?

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 10, 2023, 07:35:14 pm
content: Quote from: Syllith on January 09, 2023, 07:14:17 pmI couldn't say exactly but with the ripple on your scope anything could be happening causing unexpected errors. Try the short cables first, it really does look like this loadbox is unstable under certain unspecified conditions and you didn't say how it was connected to the UUT. The capacitors are another attempt at stabilizing the load box by isolating it dynamically from the UUT's own feedback loop.If all else fails bin the loadbox and go back to heaters, hell it's winter here anyway I would love an excuse to run a loadbank or two in the workshop