DC to AC conversion

Hello
I am trying to retrofit a slitter from DC motors to AC motors.
I have a question for those of you that are much smarter with slitters than I.

This particular machine currently has 240 VDC 170amp motors which, during the startup process draw about 90 amps with no motor rotation. (A external brake is holding the mechanism at this point) This condition exists for approximately 15 minutes while the operator messes around with the machine adjustments.

It is how they've "always done it" so I really dont want to kick that in the head.

My first insticnt is to use a flux vector drive, if I have to upsize the AC motors this is ok, and I plan on using quality vector drive motors with external cooling, probably a TEBC design. What other considerations should I take into account?

My preferred drives are Yaskawa F7s, Ive used them in some ugly apps and come out the other side.

All adivce welcome.

Tackdriver said:

It is how they've "always done it" so I really dont want to kick that in the head.

Click to expand...

I think this is the first thing you need to kick in the head. Not only is this a waste, it is dangerous.

Can't help you with Yaskawa but someone here will.

Why get rid of the DC motors? There bought, paid for, and installed. If the drives are just in need of updating there are still a lot of good DC drives on the market. Just my opinion.

Take a look at Reliance WebPak 3000 DC drives. They contain a lot of the "smarts" to deal with variable diameter winding applications. I worked on a slitter that was upgraded using these drives and was amazed by their performance. That will allow you to overcome obsolescence (if that is the motivation for the change) without re-inventing the wheel. If your existing system is well documented, it should be straightforward to upgrade the controls and even keep the motors.

As for operators working on a slitting machine with the motors energized, I agree that sounds dangerous. I would imagine that it could be necessary to put tension on the product during setup, but you should try to find a way to make sure the moving parts are braked in a fail safe way, and remove motor power if possible.

JMHO
Paul

This falls back on the power issue, why use 90amps on standstill in this situation.

I agree, kick this aspect in the head, determine the actual HP needed for the application and possibly slightly oversize if unsure.

I AM GOING OUT ON A LIMB, I assume you are slitting metal to need that much power. That said determine needed HP and method to allow slitter to start before line to obtain appropriate cutting speed.

It depends on the situation, if slitting over an open area all you need is the slitter to be at speed before line and come to a stop a few seconds after line has stop. The stop part can be important if the slitter can "burn" the metal by still turning. It may be necessary to raise and lower the slitter into position; whether automated also depends.

My 2 cents worth for now.

OK, Lets start over.
I should make it clearer that I want nothing more to do with these DC motors.

Lets focus on the AC concept.

This is a paper slitting application.
Basically what is happening is the paper is loaded with a huge, water cooled brake energized on the unwind stand. The ops then fiddle with the adjustments, then take tension off the brake.

The machine then slits and rewinds the paper.

I like your choice of drives. Until they actually release the brake, do they really need the torque? If this is a necessity, I suggest you use closed loop/aka flux vector. Give the drive a run command and NO reference. You can get the MAGNETIZING currents flowing into the motor. This is some times called torque proving. That way when they are ready to run, a speed reference will cause actual torque to be produced and things turn. I have seen many applications, cranes, and drill rig hoisting that do just the same thing with out problems.

BTW, I have seen G3 and G5 drives power thru mechanical brakes when given both run and speed commands. That vector control will provide 150% torque right fast.

Another thing, you really do not need to increase the HP when going to a VFD. Make sure the drive and motor are matched. Usually when converting to AC folks tend to use 4 pole motors that are readily available. They fail to notice the DC motor was a low base speed motor (400 to 1200 rpm) with field weakening to get 2000 rpm. Spec your AC motor base speed close to the origianl DC motor. Usually a 6 pole motor does the job. Then you do not need to upsize the motor to get the same torque production down low.

If paper slitting then it should never use 90 amps at any time.

To offer background I left a paper plant just a while back, do not know it all but learned a few things.

I changed the drives from DC to AC for converters, slitters, and tubewinders almost everyday. The reasons were many but had to do with old sheave drives, very old DC drives, maintenance, and more. The AC drives proved to be energy efficient and more or less no maintenance compared to the VS DC drives.

With the tubewinders we oversized, as suggested, from 10HP to 15HP, no issues and documented lower maintenance and power costs to fulfill the ROI before doing the rest.

There are numerous options with AC drives that offer full torque no speed conditions, personally I do not see why it would be needed BUT not fully familiar with your situation.

The numbers provided still amaze me ...240vdc@170amps to slit paper? You slitting 3 foot rolls all at once?

RS

This machine is HUGE. It slits rolls of paper that are taller than I 5,6, or 7 wide. It can boogie too, when it runs right. I suppose I should have been a bit more clear in my OEM post.

The thing actually has two of these motors one for the front and one for the rear drum. Another thing I noticed is that when the front motor is up to speed the rear drum motor is being pulled, (regeneration). I am also investigating a better tension setup but that is another post.

Keep the notions on those AC motors coming, I am starting to get a "mental visual" of how this will fit together.

The OE motors are 50 HP, 1750 base speed/2100 field weakening.

Are you using one drive to power several slitter motors? Like Ron, I have a hard time coming to grips with such a large motor for a paper slitting application. I also have a hard time with 15 minutes of "messing around with the machine adjustments" after a new roll is loaded into the unwind. What are the adjustments that have to be made while the web is under tension? Even if you need the web to be tight, why not jog the drive until you take up the slack and then engage a holding brake on the motor along with the unwind brake?

One thing you may need to be more careful about with the AC drive than with the DC is to make sure that the web doesn't drive the slitters. If the load drives the motors, you may get DC bus overvoltage faults.

Steve
I have already figured dynamic brakeing resistors into the equation. I have some experience with multi motor printers and if I dont add them now, Im gonna add them later.

Great discussion thus far guys!!!

Problem I have is I do not see a "WEB", it looks like slitting on a roll. Using a web would probably take no longer and definitely decrease the power usage...in my opinion.

Out of my league.

Take a hard look at connecting the 2 drives to a common DC bus. That way the drive running in regen will provide power to the drive motoring. My company has done this on many applications.

To run that VFD in constant regen, your braking resistorw will need to be BIG in the wattage department for the continuous duty they will be operating under.

You can use a readily available rectifier assembly to get the common DC power. The drives can be connected directly on the terminal strip. You will need to fuse both DC leads to each drive.

The F7 in vector control can have a seperate torque control input. You can use this on both drives to control the max regen and the driving torque.

If your DC motors are 1750 base speed, I really do not believe you need to upsize the drives and motors unless you want redundancy and better heat dissapation. With the programmability of the F7's and their torque control capability over sizing them will not prove to be a problem. Just make sure to lock the key pad to keep inquiring minds out.

You are aware the F7 key pad has program storage capability?

My slitter experience with slitters is strictly steel and involves motors 300-400HP. The best one was controlled by AB 1395 DC drives. But since you don't want DC.....

I agree with your flux vector choice of AC drive. You mentioned that the unwind drive was under regen all of the time. If this is the case you should look at a regenerative drive instead of dynamic breaking resistors.

The reason is twofold. DB resistors are generally used to stop a motor under a certain duty cycle. If you dump on them all the time they will need to be really big (wattage I mean). With a regen drive you will pump the energy back onto the line, power savings $$$.

Make sure the drive you select has a good winding algorithm in it. There is no need to reinvent the wheel (or winder in this case), when there are many canned winding programs already packaged with some of the drives. A slitter seems simple when you first look at it but these can get quite involved.

good luck

Or use a common DC bus. You must type quicker than me!!

I think I need to taka a step back and review the terminology. When you use the term "slitter", are you referring to the entire machine or just to that part of it that that slits the web into narrower widths?

I'm picturing your application as having a wide, large diameter roll of paper at the unwind. On the way through your machine, the paper is slit into narrower widths and rewound into multiple rolls of paper, each of which is narrower than the roll of paper on the unwind. The rewound rolls of paper rest on a pair of driven drums. You might rewind several sets of smaller rolls of paper from a single parent roll. That machine is what I would call a winder (or rewinder). The section of the machine that slits the web into narrower widths is what I would call the slitter (or slitter section). The machine will have a front drum drive and a back drum drive as well as a slitter section drive. There will probably also be a rider roll drive, but we haven't discussed that yet.