Inverter Problems - Confounded by reading compared to results

[SparkRay]

Good morning to all!

I really need some help. We have a blower run by a 15hp motor. It's a Baldor SuperE model. It's rated at 17.5A and 3400 rpm. Yesterday the inverter (TB Woods 4000 series WFCht) stopped working. This was after getting faults of a phase imbalance. (I was not there. This is based on another tech's report) When I came in It would light up for about twenty seconds and then go dead. In a pinch we replaced it with a Yaskawa P7 inverter. We started the blower. It began to run and the we got a error "Output Phase Loss". We would start it and I measured the load on the three phases and they would be different by about 30% max. I thought the motor was bad so we replace it with a new motor. We are still getting the same errors. I've meggered the wires and they are good. The motor is going in the right direction. The system has a bypass built in where the motor can be run directly off the line feed. At that point the motor runs full speed drawing about 18 Amps per phase and not more than 3% imbalance. As soon as we use the inverter we get the faults. Also, if the motor is allowed to run at low speeds it's okay. As soon as we hit about 30% output we get the fault.

I've checked all the parameters and they seem okay. This inverter was pulled from a running system that has virtually as virtually identical set up.

My co-worker just told me though, that at times last summer he would have to reset this inverter but he does not remember the fault.

Any thoughts or ideas would be appreciated. I could post a bunch of details about voltages and parameters but please know that I've exhausted everything I can check regarding phase voltages across the contactor.

Thanks in advance.

Ray

 

[waross]

Check all the wiring for bad connections.
That is,
Connections on the switch or contactor feeding the circuit.
Connections at any splice points.
Connections at the inverter, both input and output.
Connections at any isolating or disconnect switches.
Hint:
First check the connections at the inverter and the bypass.
When checking switches and contactors, check the devices themselves for high resistance or open contacts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SparkRay]

Thanks for the reply.

We did all that. In fact, more than one of us did all that.

The drive is now working normally. I truly suspect the VSD is bad. It's fairly old and has been used in an area where the ambient temperature is frequently over 100 deg F especially in the Summer. Right now the motor is tied directly to the drive bypassing the contactor. The contactor is on the output of the VSD. We DID try this yesterday with the previous results (faults).

 

[ScottyUK]

Generally contactors on the output of a VSD is a really bad idea. If the contactor ever opened while the drive was running then it might well have caused damage to the drive. If you need an electro-mechanical switching device for E-stop then put it ahead of the drive.

 

[itsmoked]

I bet it's the ill placed contactor's contacts. Never used since it would lunch the VFD anyway, the contacts slowly oxidize until one presents a discernible resistance. Then the VFD detects an imbalance and tilt!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[SparkRay]

I measured less than a volt across the contacts on the contactor. That leads me to believe there is no voltage loss across the contacts themselves. Electricity is fun until it's not.

 

[waross]

I am starting to suspect that a VFD with an intermittent fault was replaced with a VFD with an intermittent fault.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I doubt that it is two completely different brands of VFD, one relatively new, that would have the EXACT same fault.

My first suspicion too would have been the output contactor. You said that you checked resistance across the contacts. Did you do so when the coil was energized? The reason I ask is that typically the output isolation contactor will be mechanically interlocked with the bypass contactor. On several brands of contactors, that interlock mechanism can wear and cause the contacts to hang up, sometimes on only one side because it causes the armature to skew. If you checked resistance across the contacts with power off by manually pushing in the armature, you are not recreating the conditions as they exist. But now I think you said that you have wired around that contactor directly to the drive terminals and you get the same faults on the new drive, correct?

So if that isn't it, my next best guess is the only remaining thing that was not changed, the output conductors. You say that you megged the conductors, but how did you do that? The typical method is to tie all three together and energize them with the meggar to ground. That eliminates a leakage to ground, but not phase to phase. In the output conductors of a VFD, ESPECIALLY if you used THHN as most people do, there is a risk that standing waves will create enough corona discharge effect to burn tiny pin holes between phases where the conductors touch each other in the conduit, but if the conduit is dry, that doesn't show as a leakage to ground with a meggar. If there is leakage between phases, that shows up as an imbalance on the output current. Eventually it gets worse until the imbalance is so severe that the drive interprets it as a phase loss.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

My suspicion was based on this comment:

My co-worker just told me though, that at times last summer he would have to reset this inverter but he does not remember the fault.

However, your suggestions may be more valid.
Does anyone else remember the old Square "D" interlocked contactors that used a heart shaped cam?
They were so prone to failure that we called them the "Broken Heart".
In that plant down time was very expensive. The mechanical interlocks were often removed. We had one or less incident a year with contactors without mechanical interlocks. Generally resolved by replacing fuses and possibly by a change in the control scheme.
We had about one incident a month with a "Broken Heart" jamming and halting production.
The bottom line cost of down time and repairs was much more with the mechanical interlocks than without.
Management would not OK the purchase of parts or replacement of the "Broken Hearts". Cheaper to throw them away and go without.
One poor design in an otherwise excellent product line.
Another really bad design was used by another maker on manual transfer switches. Some of these were jammed and inoperative, New In The Box.
Many of us have encountered sticking mechanical interlocks of other types also, but the "Broken Heart" may have been the worst on contactors.
On the possibility of pin holes, what test do you recommend to check for this condition?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

To test for pin holes between conductors that are not going to ground, you test phase-to-phase with a PI (Polarization Index) test at 1000 to 2000VDC in steps, or a DAR test (Dielectric Absobtion Ratio) that keeps energy on the conductors for something like 10 minutes. Good modern insulation resistance testers usually have instructions on how to perform these tests. The other issue is the voltage I mentioned above. Simpler older muggers are typically 500VDC, the pulse voltage on a 480V PWM drive is around 690VDC and standing waves can get to as high as 2000VDC. If you only test to 500V, you might miss the problem.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[SparkRay]

Thanks guys.

It's been running for the last 24 hours with the wires directly from the drive to the blower.

I did check the contactor while energized. I've been bitten by the "push it in with a screwdriver" test too many times.

This one really has me befuddled. Although I am intrigued by jreaf's comments regarding the THHN failures phase to phase. At this point the wires are separated and hanging in the air in the panel whereas before they were running together through the panduit trough and rubbing together.
I don't really have any way to apply high freq, high voltage to test them the way you described.

I did get a new A1000 today but have not put it in yet.

A second co-worker, who was not on shift the last two days, also commented on how he would have to reset this P7 periodically last summer when he heard that we swapped it the day before.

The contactor on the output is the original design. I understand why they did it. It's to allow the use of a bypass contactor that simply puts full voltage on the motor allowing it to run full speed. Not sure why they thought that necessary but I wasn't part of the design team.

Our place is very hot during the summer. Easily many drives are exposed to humid, hot air (100+ def F) all day long. The majority of them are located near plastic extruders and it's HOT. I will typically replace three to four drives, older models averaging ten years old, each summer due to failures. Motors, too. There is no chance of air conditioning. I am actually surprised they last as long as they do.

Thanks for all the tips and ideas. You guys are pretty thorough and I appreciate it much. I'll keep this thread informed when I install the new drive. They are behind on that line and did not wish to shut it down to install the drive.

BTW, I have looked all over for the quote function on this thread but can't find it. Could you point me to it? Thanks, again.

 

[waross]

Five or six icons to the left of the preview button. A small torso with an empty text bubble. Just to the right of the Omega.
The first screen gives you an opportunity to attribute the quote to a name. eg: Jim, OP, etc. You can click through to leave the quote unattributed.
I first copy the text that I want to quote. Then click on the quote option. One more click and the quote code is posted and the cursor is in between the quote codes. Just paste without moving the cursor and you are done.
Or you may enter the code manually as shown below.

paste text here.

paste text here.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SparkRay]

Good afternoon,

I installed the Yaskawa A1000. Upon power up I noticed that there was PG (pulse generator) fault. I thought that was odd since we don't have any encoder on this. I hooked up Yaksawa Drive Wizard and started going through the parameters to find out why I had this error. The drive was already programmed and had the Pg-X3 (encoder input) card installed. Ooops! This was supposed to be a brand new drive.

I reprogrammed for our purpose and tested the drive. It worked okay. I reconnected the wiring to the contactor and it still worked. I've had it running for several hours now and it's stable and quiet. The load imbalance is about 3 %. I remember hearing the previous motor occasionally making a pretty loud noise. Not a bearing noise but some sort of noise that now I attribute to the load imbalance or it could have been the bearings. Without putting the motor on a test stand I can't really say if it's bad or what's wrong with it.

I first copy the text that I want to quote. Then click on the quote option. One more click and the quote code is posted and the cursor is in between the quote codes. Just paste without moving the cursor and you are done.

That wasn't so hard.


Thanks for all the help.

Ray

General Hammond: "O'Neil, what the hell are you doing?" O'Neil: "Right in the middle of my backswing?!?!"