Testing VFD diodes without DC bus terminals

Jerry mahan

Member
J
Join Date
Feb 2020
Location
Florence
Posts
8
Hello,
I can test/check the diodes in a VFD if the VFD has DC bus terminals available. All of the VFDs I see sold today don't provide access to the DC bus terminals. Does anyone know of a method of testing diodes in these VFDs?
 
Normally they are packaged , It is not a good idea to try to check diodes thru DC+ and DC - terminals, the DC capacitor may be connected to those terminals and maybe the DC Choke is connected between diodes and DC capacitor. Some manufactures also include a bleeding resistor to avoid over current while charging DC capacitor for the very first time.
 
You should still be able to check the diodes between the L1,2,3 terminals and the terminals on the bus capacitors... If there's a choke, the resistance is low enough when in diode check mode to barely notice it.

It's funny, but most General Purpose VFD's I run into have the +/- bus terminals, so that you can add a DB module or resistor... Maybe they leave them out of HVAC specific models, since you rarely see DB used on fans and pumps.
 
Gentlemen, Thank You for responding to my question.
A very popular industrial VFD, the Flex 4 made by Allen Bradley, doesn't provide +/- bus terminals. None of the common industrial VFD's seem to have them.
My question is: How do you check the diodes if no +/- bus terminals are present.
 
It has digital inputs and analog inputs for control, 3 phases in, 3 phases out. There are no DC bus +/- or Br +/-.
I have used AB160 vfds that had DC bus +/- and BR +/-, but none of the drives I've seen today have them unless there very expensive.
 
Go to AB's support sight @ https://rockwellautomation.custhelp.com/
Towards the top of the page you will see a search box. Do a search for "504644" (without the quotes.) The KB article I linked above should be the only one that comes up.

I'm not sure, but you may need to login or create a tech connect account to view the article. You can create an account without having to pay for full access. The free account will just allow you to see the KB articles that are listed as "Access Level: Everyone"
 
Curious, what is the idea of testing the diodes ?
Is there a problem with the diodes going bad ?
Are you possibly thinking about reforming the capacitors, if the drive has been inactive for a long time ?
 
I am an industrial motor control instructor and I'm trying to teach my students how to troubleshoot problems with VFD's. The "Quick Check" for VFD's requires checking the converter and inverter diodes in a VFD by connecting to the DC bus + and - terminals with power off of course. The VFD's I used to show my students how to perform the Quick Check had DC bus + and - terminals, but none of the VFD's I've seen for some time seem to have them. I thought maybe someone had some technique for the Quick Check on these newer VFD's without having to take the thing apart.
 
IMO, if a vfd does not have bus or DBR terminals, it is NOT a general purpose drive, it might be an HVAC drive, but not suitable for heavy duty use. There are too many applications in the general purpose world which require dynamic braking via simply adding a DB resistor, or in some cases a DB module (DB transistor and resistor).

My advice is pick a better drive to teach with. Apparently the way to test the input diodes on the PF4 is to see if the fuses are blown to the input.
 
"My advice is pick a better drive to teach with."
I need to prepare my students for any type of drive they encounter.

"Apparently the way to test the input diodes on the PF4 is to see if the fuses are blown to the input."
An open diode in the converter stage won't blow a fuse.
 
A lot of the smaller VSD's use a single block that has the input diodes and the output transistors combined into one unit.

So if the diodes fail you would need to change the entire power module.


Alan.
 

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