Beckman-Coulter Microfuge 22R Repair question (Motor Driver) - Page 1
Forum section: electronics / repair

author: Rob Morley
author_rank: Newbie
timestamp: March 11, 2024, 04:56:36 pm
content: Looking for some advice on how to diagnose a 3phase motor driver issue. Also hoping someone has dealt with this issue before.I am currently repairing a Beckman-Coulter Microfuge 22R Centrifuge, circa 2013 (https://acmerevival.com/wp-content/uploads/2021/08/BC_Microfuge_22r_Manual.pdf). At the moment the problem being reported by the machine is there is "no tach signal". However, I have tested the tack signal, and it oscillates as the drum spins. So not the sensor. Next, I moved on to the motor driver board since it won't see a tach signal if the motor is not spinning. This is where I am stuck at the moment. Attach is a picture of the motor driver board and a reversed engineered schematic of the single phase of the motor driver.Where I am having problems is with the HCPL-316J (https://docs.broadcom.com/docs/AV02-0717EN). I'm testing the gate drivers by manual setting the Vin± inputs. According to the datasheet the Vout pin should be high or low depending on the logic levels provided on Vin±. However, when I set the Vout signal to high it oscillates at 110Hz with a duty cycle of 60%. The only reason it should do that is if there is a fault detected. I am not seeing any movement on the fault pin, and this happens on all 6 of the gate drivers. So, I am not sure what is going on at this point. If anyone could shine some light on to this issue it would be much appreciated.ThanksRob Morley.

author: fmashockie
author_rank: Frequent Contributor
timestamp: March 12, 2024, 01:32:46 pm
content: Hey Rob. I am a Lab Instrumentation Engineer for a biotech company. We have two of these centrifuges. Though I've never had the issue you are describing with them. I've never seen the 'no tach signal' error before. And of course, the manual for this instrument is no help - it just states to power cycle the unit...This is so typical in my line of work - lab equipment manufacturers not providing enough materials/resources for the end user to repair it. But thats a different topic entirely.When I have these kinds of problems I like to check the easiest stuff first. Have you checked the motor? Have you checked the windings? Have you inspected the hall sensors (not the tachometer)? I would check those things first before suspecting the motor drive circuitry.Also, are you sure you are testing these HCPL-316J drivers correctly? The fault pin should go low if there is a fault. Are you seeing that? The chances of all six of these going bad is slim to none - so when you start seeing things like this it is time to question your method of testing. The datasheet provides test circuits for the HCPL-316J. Might be worth taking one of them out of circuit and testing one to get a better idea how they work. But I would be starting with looking at the motor first.Curious to see how you make out!-Frank

author: Rob Morley
author_rank: Newbie
timestamp: March 12, 2024, 04:37:57 pm
content: Hi Frank,QuoteYeah, we do a lot of scientific instrument repair in my group and the manuals are horrible, and the schematics are non-existent. So, everything is a big puzzle to figure out what's really causing the problem. Sometimes it's quick fix other times like this one it takes more effort.QuoteThanks for reminding me to check my test circuit. I had forgot that there was SN74HC366 buffer IC between the connector and the isolator. Once I notice that I fixed the problem, and it is working as describe in the datasheet.QuoteI've checked all the easy stuff first. Safety relays and switches, power (AC and DC), motor windings and everything up to that point is testing ok. As far as I can tell the tach signal is coming from the hall sensor. As I manual spin the motor, which is disconnector from the board, while power is on, I can see what I assume is the tach signal pulse high low on every rotation. I think the only thing left test is the drive circuit on that board before I move on to the controller board.Thanks for your help Frank. If you have any more words of wisdom let me know they have been helpful. If I find what is wrong with the centrifuge I'll post again.CheersRob Morley.

author: Rob Morley
author_rank: Newbie
timestamp: March 12, 2024, 08:34:25 pm
content: Ok, may have spoken too soon. I did fix the problem with the test circuit; however, it didn't fix the testing problem 100%. Only the top IGBT of the pair is turning off and on as commanded. The bottom IGBT pulse at 100Hz still. I've attached the oscilloscope screen shot of the gate signal for the bottom IGBT. I'm using a differential probe capture the signal on channel 1. The positive probe is connected to the gate and the negative probe is connected to the emitter. On channel 4 is the Fault signal.Any insight on this behavior would be great. If I'm doing something weird on the measurement side, please let me know.ThanksRob Morley

author: fmashockie
author_rank: Frequent Contributor
timestamp: March 13, 2024, 03:54:30 pm
content: Nice work Rob! Sorry I didn't mean to assume you didn't check those easier things first. I know I get tunnel vision sometimes when troubleshooting issues and sometimes it is best to take a step back - thats all I was offering!I can open one of our 22R microfuges and take some measurements for you if that would help?-Frank

author: Rob Morley
author_rank: Newbie
timestamp: March 13, 2024, 04:51:14 pm
content: Hi Frank,No offence taken, I get tunnel vision too, and taking a step back was what I needed to do. When you mentioned test circuit in made me go "wait a minute" I did miss something. Having someone say "did you check this, this and this" helps get you out of that tunnel vision in a repair. It did get me one step closure to fixing problem, but I am still having the pulses on the bottom IGBTs. I am still working on it, but I don't know if a solution will present itself yet.QuoteIf you have the time that would be amazing. I don't have a second working one to compare too so I don't know if I am chasing a problem or design feature.Thanks again for the advice/help.Rob Morley.