In the hope that this will help others


Senior Member
I had a couple of unusual experiences recently with VFD's. Thought I would share what I learned.

The first was a 125 HP chill water pump. Original drive was an older Altivar with drive output and bypass contactors and HOA switch. Drive was recently replaced with a newer Altivar but all the contactors/switches were kept. Every time the drive was reset, and it would ramp up past 10 hertz, the contactor would drop out. Drive fault was output phase fault. But the drive would self restart several times in a row, sometimes the output contactor would "chatter".

In the end it turned out some plastic pieces that secure the coil in place in the LC1F185 contactor had deteriorated over time. I am having no luck finding them as they actually attach to the laminated armature/pole. Looks like I have to fork out the bucks for a new contactor. All over a four plastic clips, one of which has a yellow pushbutton on it which is depressed to slide the coil out. This took some time to locate.

The second was on some new AHU's that happen to have two fan motors and two VFD's. Seems when one drive trips out but the other is still running, the motor on the off-line drive spins backwards and dumps back into the drive. You will notice faults like high DC bus voltage, over current, and/or short circuit faults. You will get these faults before the drive even takes off. Just shut down all drives, reset as needed, and fire them all up at the same time and you will be fine This might be self evident to many here but I wasn't used to such setups in AHU's and wasted some time with a megger and amp probe:)


New User
Illinois, USA
In regards to the first experience, would you be able to share the schematic, or explain it a little more? I'm curious as we've had a grounding and phase faults before on Altivar drives, but never resolved the issue it just went away. This was on a 75HP motor however, and we don't have a contactor on the output of the drive.

Thanks in advance.


Staff member
San Francisco Bay Area, CA, USA
Electrical Engineer
Thanks for sharing that. It helps other people to realize that there is ALWAYS a reason for something happening, it's just hard to find it sometimes. The broken contactor pieces is a new one even for me, and I have seen some bizarre "stuff" out there... :happyyes:


Senior Member
The Altivars especially the newer 300/600/900 series require some extra setup in a bypass configuration. Specifically you need to turn off the error for line and load bus loss (phase loss). Otherwise you will get a spurious error when the contactors open that will drive you crazy. Like the line side can open and since run command drops too the drive sits there with the DC bus energy being consumed only by the control board. Then if you run again the contactor closes in but the drive is still faulted and not reset. So you are stuck with the spurious trip.

The other “gotcha” is level vs. transition run mode. In transition mode usually with a bypass contactor setup the line side contactor and the run signal are asserted simultaneously. Then the drive boots up. By the time the drive has booted it sees the run input immediately. In level mode it starts. In transition mode though because it doesn’t see a stop/run transition it won’t run. The reason is because in a typical manufacturing plant situation we don’t want a machine to suddenly move right after a power loss. It’s a safety feature. But in a bypass contactor condition that doesn’t apply. So level mode is the correct setting.

Not a big deal really just something you need to do when setting them up. Sort of like the problem if temporarily jumpering out the bypass logic long enough to power up and program the drive and leave the motor closed in for tuning purposes.

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Senior Member
Rflotho, my drive was part of a factory wired MCC. Model 6 IIRC, 12,000 amp bus, etc. So I can't really give you any info on a schematic (and I doubt it would matter). Other than that, it drives a chill water pump. A building automation system looks at a remote differential pressure transducer, compares it to set point, and outputs a 4-20 ma signal to make drive match it. I will add this, when the drive made it clear it was going to have to be replaced those "grounding and phases faults" that I could find a way to clear turned into things I couldn't work around. Parts are hard to come by but be advised there is a "rating plug" that plugs into a socket on a motherboard that determines the drive's HP rating. Had I known that earlier I might have been able to save the drive. You will have to manually re-enter every drive parameter so make sure to get them off the old drive while you can.

Oldsparky52, what I loved about my old drives were that they were made for virtually everything to be field replaceable/repairable. They were huge in size and real tanks. I had replaced fans, diodes, and capacitor banks over the years. Never had to do an IGBT but those too were made easy to replace. I have only had the new drives for a couple of years so can't comment on them other than to make sure you order them with the DC bus choke. IMHO they look like "throw away" devices, drives they don't expect you to change out diodes, caps, and IGBT's on.

Jraef, I like seeing some of the new forums added. Sometimes a lot of the troubleshooting game is knowing a particulars piece of gears idiosyncrasies. The more we share our experiences the easier our jobs become. Not like the old days where nobody shared because they viewed it as "job security". IMHO there is enough work to go around when you deal with customers who "must" keep older stuff on-line and simply can't easily retrofit to newer stuff. Besides, I will always have a fond spot for oil filled dashpots and electro-mechanical relay logic. I'd wager you have a few select vendors that are your go to guys. I'll throw out satin america for low/medium voltage breaker repair. CBS sales has also been of help, but more for retrofit. Astro controls for reconditioned older style breakers. For pump panels I like But I'm a little out of date as current ownership doesn't believe in in-house solutions:)