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AKD DC Bus Link Creating n521 (Regen Over Power) Warning?

Using the AKD's DC bus link to share bus voltage across a series of drives is a good way to limit regen needs and unnecessary energy costs.  There are articles and app notes on KDN that show the connection's needed to use the DC bus link, and they commonly use the below connection diagram.


Note that the above diagram connects the DC busses of all 3 drives.  It does this so it can make use of a single external regen resistor with resistance RBext.  This setup also makes use of the AKD-x012's internal resistor, so your total available regen power is increased.  But using this connection scheme can cause a n521 (Regen Over Power) warning in the AKD-x012 drive.  It's a warning, not a fault, so there's no real issue beyond the nuisance of a n521 warning message.  But why does this happen?

If the AKD-x012 drive has a higher regen load and regens power back to the bus more than the AKD-x024 drive, then you will have a momentary situation where the AKD-x012 drive has a higher bus voltage than the AKD-x024.  It will be a small difference in voltage for a very short time, while the power distributes across each drive.  But the AKD regen circuits respond VERY fast when bus voltage exceeds the specified limit, and this momentary situation means that the AKD-x012 will turn on it's internal regen circuit before the AKD-x024.  The internal regen resistor is typically much smaller than external resistors, so if the momentary situation continuously occurs, the AKD-x012's internal regen resistor will reach it's peak power rating before the external regen resistor does.  Hence, you get a nuisance warning of n521, Regen Over Power, in the AKD-x012.

What if you want to eliminate the n521 warning?  Simply disable the use of the AKD-x012's internal regen resistor.  This does reduce your overall regen power available, but the internal regen resistors are typically small enough that you won't care.  Here's how to do it:

1)  Disconnect the internal regen jumper.  In the above diagram, that jumper is on the AKD-x012's X3 connector, pins 1-2.

2)  Disable the AKD-x012's regen in the AKD Workbench software, as shown below.



3) Sit back and enjoy your warning free system!