VFD, Inverter Bypass, PLC, & Sizing

I was wondering if anyone has used a PLC to control a VFD Inverter Bupass Panel.

I was thinking of nothing fancy, rather than three multi-position switches, just pushbuttons for ON, RUN, BYPASS, TEST, RESET, STOP, and REVERSE? Just simple logic. Any other applications?

Also, anyone use an Inverter a size or two larger than the motor. Other than overkill, I would expect the inverter to probably run cooler, and can't think or any problems.

regards.......casey

Don't know what you mean by "bupass panel" (bypass panel?) but if you can run it with relay logic then you can run it with a PLC.

As for the inverter, it is usually not a problem to use a larger inverter than the motor. Most modern inverters allow you to select the size of motor.

One place I worked had around 40 motors controlled by inverter, ranging in size from 0.75 kw to 3.3 kw, with most at 1.5 or 2.2 kw. We used only one size of inverter, rated at 4kw. We had 2 spares and ran 24/7. This made spares an replacements very easy. What we paid extra for the initial installation, we made up on spares and down time.

Doug

1) PLC to control Bypass Panel - Yes, of course. Many many times. Some simple ones (baby-bricks with only VFD / Line) and some much more complex (Multi-VFD's, Multi-motors, Line, Soft Start).

PLC's are nice for bypasses just because they make it easier to sequence things properly. And don't fail as often as those "Orange" time-delay relays that Graham used to use all the time.

Generally, we size an inverter to be just at motor rating, or one notch above if the motor is an oddball, or falls between drive ratings. The only exception is when it is known that eventually a larger motor will be installed.

Cost is still high for upsizing, and if you are worried about heat, just put a good fan or heat exchanger on the enclosure. Larger inverters run cooler because they have larger heatsinks (not by much) and larger fans. The overall dissapation in the switching devices is about the same.

Rather than upsizing inverters, I'd try to choose QUALITY inverters, rather than just any inverter. There are a few very good ones, and a whole lot of very bad ones. (but I'm not stupid enough to mention names here :O )

rdrast said:

... I'd try to choose QUALITY inverters, rather than just any inverter. There are a few very good ones, and a whole lot of very bad ones. (but I'm not stupid enough to mention names here :O )

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The rest of us would certainly like to know if you had bad experiences with certain VFD's. If you had problems, you do have the rights to tell about it.

JesperMP said:

The rest of us would certainly like to know if you had bad experiences with certain VFD's. If you had problems, you do have the rights to tell about it.

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<smile> now THAT puts me in a tough position, but I suppose the 'Politically Correct' way of expressing an opinion would be to list several drives that I have had very good experiences with.

Be aware that I haven't used every one out there, but have used and worked on a huge number over the past 15 years or so...

Anyway, the top 4 that if given a choice, I will spec for an application (again, being PC, the list is alphabetical)...

AB's Powerflex series. Extremely capable, fairly easy to apply, and if used with ControlNet, one of the best communications interfaces I've ever seen. Everything on the drive is completely exposed easily over controlnet. Very good configuration software available. Things I don't like? The door. Those stupid hinges and locking mechanism are a royal pain.

ABB's Drives. Excellent systems drives, reliable and flexible. Good configuration software, but could be improved to be easier to use and more universal. I do not like their options much though; often, I need firmware updates if I use them with any technology options.

Eurotherm (Now SSD again) 690/690+. Will work anywhere, and do just about anything. Lots of communications options, good configuration software, and the source/destination type tag mapping is very nice, especially if you have to backtrack. Things I don't like - The keypad display's seem to fail more often than other manufacturers.

Siemens Master Drives. Probably the most capable drive I've ever used, highly reliable. The configuration software is good, actually almost very good, with release 5.2. Things I don't like - Way too capable; with over 2000 parameters, they can be a hassle to set up unless you draw out a schematic of your desired software flow. Also, not very servicable. If the drive does fail, it's easier to just replace it outright than fix it.


Hrm... I've listed my favorite systems drives above, I guess for simple fan/pump applications, I should also list Toshiba's and Magneteks (Yoskowa).

It's true that all of the above are on the more expensive end of the VFD spectrum, but the overall reliability and repeatability of them are far and above all others that I have had had experience with.

I hope, Jesper, that I've sort of answered your question, while dodging it at the same time

I find your commentary about the "good" VFDs very interesting.

I can testify to the Siemens drives being a bit difficult to get into. A 300 page manual with parameters only ! And each parameter isnt explained that thoroughly. It seems to be the problem with anything with "Siemens" stamped on it.

Now I can only guess if a VFD is one of the "bad" or the "ugly", if its not one of the four you mentioned.

Jesper, I'm sure one thing you noted about my preferences, is that the system-level drives I prefer all have very flexible communications.

With the way things are going, this is more and more important, as more and more, I find myself doing all control functions externally (as in, a plc), and sending the drives simple speed and/or torque commands.

I definately believe that Siemens needs to either re-think their idea of 'More Parameters is Better', or come out with some powerful software for configuring the drives. I also agree that the manuals are seriously lacking in describing what any particular setting does. And 300 pages doesn't even begin to cover the MasterDrives /sigh.

As far as 'Bad and Ugly' drives, my selection criteria is usually based on the following:

1 - The drive should be reliable. It should not fail in normal, and sometimes a bit abnormal use. That means everything, from power devices, to terminal-clamp connections, to oxidation/corrosion of circuit traces. All of the above meet that requirement.

2 - The drive should do WHAT I COMMAND IT to do. Drives used to be a part of a system that required lots of adjustment, and speed matching, and tweaking to get them to do what was desired. That often required monitoring things like actual motor speed, or load, through a seperate channel. This is no longer acceptable. A drive should run at the commanded speed, or torque, period. If it deviates from a setpoint, then the drive should report back that a deviation exists. All of the above do that.

3 - The drive should include some available logic blocks for performing special, or odd functions. Brake sequencing, sometimes DC Injection braking, etc.

4 - The drive should be easily replacable. One important thing there, is that it's A/D and D/A channels should be accurate. Another, it shouldn't have potentiometers for setting it up Pop in a replacement, download a configuration, it should run as it did before.

5 - (Vector Drives) If I'm applying a drive in a pure torque mode, it damn well better follow my torque command exactly. All of the above do. Several other's that I've seen do not. Some actually create a third outer-loop regulator, wrapped around the speed and inner torque/current loops to achieve pseudo torque control. This is bad news.

One thing I Wish to All The Gods that Be for drives, would be for them to store all parameter/configuration data on a memory stick. I don't care what kind. MMC, CompactFlash, even a USB dongle (why don't industrial devices directly support USB? it really is a good interface).

The ones I've listed show up very infrequently in our service branch. Those not listed come in daily, for problems ranging from input bridge failures, power device failures, trace-failures on boards, and other similar failures that really shouldn't happen. Yes, there is the occasional infant mortality failure on everything, but once a drive has been in service for a month or so, I don't ever want to think about it again.

<laugh> I can be overly opinionated (and sometimes passionate) about some things. Always take my comments with a grain (or truckload) of salt!

I don't see any poblem with using a PLC to control drive byps. Just make sure that the main contactors for drive/bypass power are electrically (and preferably also mechanically) interlocked. You don't want to count on the PLC alone for that, just like a reversing contactor.

Oversizing won't necessarily get you as much extra life as you would think. You also have a down side. Maybe Dick Dv can chime in, but I think drive eficiency will drop off a little. And, of course, there is the money.

I agree with Rdast - you would be better off buying a top quality VFD properly sized.

One common reason for oversizing the drive with respect to the motor is to take advantage of more of the short-term overload capabilities of the motor. A typical NEMA B motor is capable of 220% overload and a normal-duty drive is good for only 110%. A heavy-duty rated drive will be good for only 150%. By oversizing the drive, you can go further into the motor overload range without running out of drive first.

As for Jesper's comments on drive features, he's right on, as far as I am concerned. And his comments about making drive parameters portable with a "memory stick" or equivalent are good too.

I would suggest Jesper look at ABB's system of making memory portable. On every one of their models, the detachable keypad has its own EPROM which is capable of storing the drive software. Uploading the software into the keypad should be a standard part of every drive startup. With that done, if the drive should fail, the original keypad can be inserted into the new drive and the software downloaded simply and accurately. Or, if you wish to clone multiple drives, just program one and use the keypad to clone the rest.

Of course, most of you out there know my brand affiliation so I'll just stay away from any comment on "best" or "worst" drives. But I am generally pleased with the comments from the rest of you!

DickDV said:

... As for Jesper's comments on drive features, he's right on, as far as I am concerned. And his comments about making drive parameters portable with a "memory stick" or equivalent are good too.
I would suggest Jesper look at ABB's system of making memory portable ...

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I am happy to be quoted to be spot on. But you must be refering to rdrast's clever postings.

edit: I do think that other than ABB have the feature of storing the drive settings in the detachable display (AB PowerFlex comes to mind).

Powerflex and siemens also allow storing parameters on a detachable display, yes. BUT, that is still a problem, especially in cases where multiple drives are installed, and they don't all have a 'storage' display.

Even if they all do, since most require an 'upload', you can never be truly certain that the drive parameters are what resides in the keypad/display.

I'd even like to go back to socketed EEPROMS, at least you could move them from one place to another; but a simple CF card would be the best. Always current, very reliable, easily portable. Properly designed (software wise), they could even be plugged into a newer drive, and still run, hedging how fast these things become obsolete.

Siemens is the most confused on storage. On their new PLC's, they've gone to MMC cards for program storage. Great. On the DC-Master Drives, the Parameter EEPROM is located on the backplane of the electronics cage, not on the processor card. On the AC-Master Drives, the EEPROM is located on the processor card.

Go figure.

Whoops! sorry rdrast, those were your comments!

Since I have very little success getting customers, let alone technicians, to wear anti-static equipment, I am not at all keen on having them handling IC's or the equivalent. Memory in the keypad is just fine with me.

One other thought on the original post. It mentions REVERSE which will be extremely tricky to implement. Also, an absolute necessity as far as I am concerned are electrically and mechanically interlocked drive output and bypass contactors.

Another problem I see regularly on bypass systems is the load will not run properly at bypass (60hz) speed. This is because the power train between the motor and the load is chosen so the drive outputs a max of 60hz. Much better would be to change the power train reduction ratio so, on bypass, the motor runs the load acceptably and the drive takes the motor over 60hz for short-term transient or catch-up excursions of higher speed. I almost never see this done.

The AB powerflex drives are really nice. Configuration is too easy, the first time you turn the drive on you answer a few questions and you are basically done. I recently setup a pflex 700 with closed loop (encoder) vector control in about 20 minutes.

Siemens drives are bulletrproof, never had to replace one and I've worked with lots of them. Setup is painful.

ABB, Very nice reliable drive and there DTC control is (a proprietarty flavour of flux vector control) works quite well.

Two brands I have had serious problems with as of late are Reliance (DC and AC) and SEW eurodrive.

We blew up the rectifiers on the eurodrives on such a frequent basis, the electrical department looked like a NASCAR pit crew changing these things out.

As for the original post, not sure exactly what you are trying to do but with the programming and I/O capabilities of some of the newer drives you might not even need a PLC, you might be able to do it all in the drive (drivelogix?)

Casey,
Old school, if your inverter has output phase or "loss of load" protection that would be your only gotcha! Many times working for Modicon and SQD we had to disable the output protection with a smaller load. The darn drive thought it was a failure.

bitmore

I am very impressed with the responses.

For those that have never heard of a bypass panel, here is a quick rundown.

The purpose is to feed power through the inverter to the drive motor. Should the inverter fail, power is bypassed around the inverter to the motor, where it acts like a normal 60 cycle motor. A test switch is installed, to supply power input to the inverter for test and troubleshooting, without being hooked up to the motor. When the inverter is bypassed, the output is interlocked off, there is no "double feeding the drive".

Panel typicaly has

1). Disconnect and overload protection.
2). Starter that goes to drive line input.
3). A pair of starters, whose common output feeds to the drive motor. One starter's input is fed from the inverter output. The other starter is fed from the line voltage after the disconnect.

4). Ocasionally, overload protection (integral or otherwise) directly after the pair of starters.

As Tom mentioned, the pair of starters should be interconnected. I used to use a reversing starter, with rhe input jumpers removed.

Some applications are for pumps, conveyors, and assembly lines, in order to keep them running somewhat, if the inverter fails.

I developed a product line for one of GE's customers years ago, and had a lot of problems with the end customers not understanding the switches, on-off, inverter/bypass, and test/normal.

So an idiot-proof approach with pushbuttons seems logical to me. Just wondering if anyone else has tried it.

It is so simple, a PLC isn't normally used, and no relays are usually required. But, with pushbuttons, PLC logic begins to make sense.

My AC Drive experience is mostly with Locomotives and Mining trucks. Experience with industrial drives is somewhat limited. I used to oversee a lot of machines with Reliance 3000's, 1/2-1 horse ratings. No problems with them, at the time (1996). Being conservative by nature, I would probably oversize the drive nowdays. Never gave thought to drive quality. Would have probably gone with another 3000, or called AD.

If anyone has experience with Fuji Drives or Marathon Motors, please post or send me a PM.

thanks all.....casey