Engineering technicianshave many nightmare stories when it comes to on-sitetroubleshooting. In this instance, I wascalled to helptroubleshoot a Closed Loop (Sensorless Vector)Variable Frequency Drive (VFD)on a production line that was so large, itseemed to goso far down the plant, it never ended.
Afterovercoming theintimidationassociated withthe sheersize of the line, it became a question of focusing on just theVariable Frequency Drive (VFD)causing the issue. The maintenance technicianson sitenot only had home court advantage, but were on the phone with the Original Equipment Manufacturer (OEM) technical support. Everything on the line ran great, right up to the pointwhere theVFD on thenetwork in question was supposed to start turning a set of rolls.
TheVFD was a proprietary brandthat was made in Japan with the rest of the machine. After speaking with technical supportfrom Japan,a list of discrete inputs to the Programmable Logic Controller (PLC) on thenetwork were determined that could be checked. The maintanceguys at this site were pretty sharp so they had already checked the same list twice.It is still good practice when troubleshooting these types of problems to check these discrete inputs.
The problem most have withtroubleshooting aVariable Frequency Drive (VFD)controlled by anetwork is that it seems so intangible. There are no wires going to the start terminal to throw a meter on. ThisVFD, likemost others, had a keypad on its front. These circumstances brought aphrase to mind that was often published on the cover of Love temperature controllers stating, “If all else fails, please read these instructions.” The customerhad a manual on theVFD, looking to the manual is always agood first stepto discovering the root of the issue.
When troubleshooting communications between aVFD and a PLCon thenetwork it is necessary to look at theVFD‘s “read only” parameters and look for the answers to these two important questions:
- IstheVariable Frequency Drive (VFD)getting a run comand?
- IstheVFD getting a speed referance?
After getting familiar with the keypad, the customer was requested to get the machine running again.With the information learned from the manual, through the keypadit was determined theVFD was getting a run command and was, in fact, going into run.
However, it was also discovered that the speed reference was running at0%. Normally a speed reference on aVFD will run between 0% and 100%. So in thisinstance, it was running at0%. So theVFD was running great at 0speed, but 0 speed is not moving.After discussing it over the phone withthe OEM technicalsupport,it had turned out the rung within the PLC on thenetwork turning on the speedreference was not giving the proper speed reference to theVFD. Now the customerhad something to work with and were able to resolve the issue through the PLC.
Keypads can be very useful whentroubleshooting Variable Frequency Drives (VFDs)controlled bynetworks. Sometimes it is just a question of getting familliar with the way each uniqueVFD handlesnetwork communications.