Yaskawa VS626-MTII won't decel and faults

[amtekinc]

Ok. This is an old timer and probably should be replaced, but, the customer has a spare unit (broken) and wants me to make a good one from two broken drives. The one to be fixed is working with one exception and that is it will not decelerate or stop without faulting with an overvoltage fault. Parameters are set to pre-failure settings. The JPAC board was replaced as well as one of the power transistors at one time. I believe they were able to operate the unit and motor, but, were not able to decel or stop even after these items were replaced. The power transistor that was replaced has some different resistance measurements than the two that were not, although, they are the same part numbers. Ideas? Oops, it's a VS626MTIII. Dropped an I. Sorry.

[mike_Kilroy]

no takers on this for far?? OK, I will give my 2cents even tho we dont mess with Yaskawa vfds. but the circuit is basically the same.... here is list of stuff to check from the top down:

1) next, they often have a parameter to turn this feature on or off; check that it is ON or it simply coasts to a stop as you have.

2) the db or regen if u prefer, circuit is simply a comparator op am that has DC bus voltage input as one and a 'setpoint' voltage as the other' sometimes the vfd lets u vary this as a parameter - if so, see if it is set correctly. on a 230v drive it is typically 390vdc turn on, 460 drive typicallly 750v.

3) next you need to have a resistor hooked up; u did not say size of this unit or if resistor is built in or not; if external, is one hooked up? if not, it will coast. dont have one? get 10pc 100w lite bults and wire them in series and hook that up - gives nice visual indication if the circuit is working

4) find the IGBT transistor that the comparator turns on; it is usually off to the side and easy to find as it is not part of the 4 output IGBTs. check it. You can ohm also to the db resistor terminals - it should wire to one db resistor terminal directly and to one dc bus on other side. the other db resistor terminal goes to the other dc bus side.

5) last thing if above does not get u going is to trace the input to the db IGBT to a circuit board and debug the comparator circuit. A failure i have see often is the input from the dc bus to the comparator: since the comparator works at like 10v and the dc bus is 300 or 600v, they need to drop this down; often done thru an obvious string of 10-20pc small 1/4 or 1/2 watt resistors - sometimes crud gets on pc board and can cause these to overheat and one smokes open.

6) If it does not get fixed and they want a new drive, I would be happy to quote you Hitachi!

BTW, the different resistance measurement on the output IGBTs is not your problem of course, but this is a bad thing; <usually> sometimes means they won't last as a set much longer.

let us know what you find!

[amtekinc]

Thanks, Mike!
I did find the problem. It was one of the two power darlingtons in the regen circutry. For some reason when i was checking resistance on this device, I overlooked the B1, E1 connections and when I rechecked it, I found it open and it should have measured around 100 ohms. When I reinstalled drive, it worked for about one hour and then one of the output power transistors developed a short. 2nd one for this unit and now I believe I have a motor or motor connection problem and this is probably the root source of my problems. Thanks, again, for the sage advise!
Frank

[mike_Kilroy]

time for a megger test of the motor and motor leads? Hope you have access to one.... It may confirm motor or cable problem; or it may be my "BTW" comment below also - I'd say if you megger and all is fine it may be worth one more round of ALL output IGBT replacements so they all match.... good luck! Thanks for the feedback.