VFD Cyclic Regeneration for a Dummy...

Ok, first time poster on this forum. I checked the archives for a similar situation and found this. http://www.plctalk.net/qanda/showthread.php?t=12816

My situation is this: We are putting a 20 HP VFD on a large laboratory transportation shaker table. This table moves up and down with eccentric shafts driven by the motor. This motion causes the VFD to trip the bus overvolt fault. The table will run up to 20 hz, then trips. Watching the live bus voltage option on the drive i can see the voltage spike whenever the table is in the downward stroke, then it drops back when the table is one the upsroke.

Can i use a AB dynamic brake, for a 10 HP motor. They are rated at a 20% duty cycle. The thread i listed above about the fish mixer stated to use one about a 1/4 the size of the motor. But this shaker has a 8 x 10' platform and a payload of 8000lbs. Is this motion of an ecentric shaft with gravity helping some of the time a %50 duty cycle? The only other option i saw for %50 duty cycle at 20 HP is to use a brake chopper and a resistor pack, for a sum of about $3500. That is a bit too much.
Thanks for looking!

Specs
L.A.B Transportation shaker with a 20 Hp 460 motor
AB 1336f plus II AC drive
Drive trips bus overvolt fault code above 20 hz

Ideally in your situation you would add bus capacitance but I don't think you can do that with AB drives. You may want to check into that, though, as it is the most efficient solution and it is passive. Another good idea would be this:

http://www.bonitron.com/drive_site/line_regen.htm
but if you don't like $3500, then $2900 isn't much better. Although you may want to do an energy audit and see if you can get some payback. This may be a more cost-effective solution for you if initial proice is your main concern.

Your regen energy will probably go up with the square of speed so at 20Hz you are only about 11% of the way there. Granted you have some regen energy simply due to gravity so you are probably a little better than that.

Basically you need to get rig of the energy that the table is tossing back at the drive one way or another. Since the 1336 Plus doesn't have a built-in way to get rid of it you will need something to do it for you.

Keith

This post strikes me as kind of odd. The 1336 is old news. Is this an existing system, or did you try to retrofit an old drive on a new machine?

If the system is existing, what has changed?

I believe you can add a simple braking resistor module to your 1336. One would think that in a shaker it would have to be sized about 35% of your load (I'm pulling numbers out of the air, call AB), it depends on the DC bus cap's.

You could just get a new drive with regen capablilty. I use SSD.

http://www.ssddrives.com/usa/Products/AcDrives/690-series.php

No, I don't work for them. I just like the product and the support.

My situation is this: We are putting a 20 HP VFD on a large laboratory transportation shaker table. This table moves up and down with eccentric shafts driven by the motor.****
Is the VFD actually physically attached to the vibrating platform?
I didnt have time to look up the +2 vibration specs but they may be being exceeded. ( You dont want them bus bars flexing and touching nothing )

This motion causes the VFD to trip the bus overvolt fault. The table will run up to 20 hz, then trips. Watching the live bus voltage option on the drive i can see the voltage spike whenever the table is in the downward stroke, then it drops back when the table is one the upsroke.
*******
I'm thinking its going to depend on the length of the stroke whether regen is occuring or just noise... moving tables are hard to ground properly also...
Can i use a AB dynamic brake,*******

Dynamic brake is for fast stops... not typically bus overvolt snubbing... ( if on a sustained bus overvolt it will heat up real fast and any over 15% is way high)

You need to ID whats really happening here before deciding on best course of action.

If you don't need really stiff speed regulation, you might cheat the system on this and cut the maximum output voltage on the motor. This reduces the motor's ability to make torque but also reduces the braking energy which comes backward to the drive. Since their is a net input of energy on each complete cycle, you might be able to find a point where the motoring gets done with some of the regen energy thus limiting the runup of DC bus voltage.

I've done this with 1336's on stamping presses successfully.

I had exactly the same application (vibratory conveyor driven by excentric shaft) and exactly the same problem (DC bus overvoltage every half cycle).
I retrofitted a chopper with a brake resistor to the drive, and when that turned out to be too small (resistor thermo switch tripped) I added a bigger resistor to the pack. This solved the problem.

3500 USD for a chopper+resistor kit for a 20 HP drive sounds a bit stiff. I would have guessed it to be less than half of that price. Then again, I dont shop for such drive options every other day.

How does the conveyor operate at 60Hz ? What happens if you start at 60 Hz and then try to reduce the frequency, rather than increasing from a low value ?. I bet the harmonic resonance is somewhere between 20 Hz and 60 Hz. In my case the conveyor didnt create regen between 45 and 55 Hz, but below 45 Hz I started to get the problem.

DickDV's tweak sounds interesting too.

JesperMP said:

3500 USD for a chopper+resistor kit for a 20 HP drive sounds a bit stiff. I would have guessed it to be less than half of that price. Then again, I dont shop for such drive options every other day.
DickDV's tweak sounds interesting too.

Click to expand...

More than a bit stiff. Out of curiosity I priced a new 20HP Pflex 70 with a braking kit (still need to buy the resistor) on AB's web site and the list price is only $3000.00 so expect to pay only about $2500.00

The 1336 plus II is on the verge of obsolesence if it isn't discontinued already, you really shouldn't be buying and installing these in new instalations.

If it was just laying around and you decided to use it then that's a different story.

It sounds to me like the cycling of the machine causes a disturbance in the DC bus that happens faster than the drive can react to it. The DC bus voltage probably ripples and accumulates too suddenly.

External dynamic braking can help, but I would bet that you'll need more braking capacity than you'd expect for this application.

The 20% duty cycle means that the drive can only be regenerating 20% of the time or the resistor(s) will overheat. If the down stroke is half of the cycle, you may need to calculate for a braking system that can handle at least 50% duty cycle.

Be sure you use a brake with a thermoswitch to disable the drive if it overheats.

I always wished that you could adjust the voltage at which regeneration begins, but that is not the case with those older A/B drives.

The newer powerflex 700 drives that I have used recently have a lot more range in their bus voltage tolerance. Might be worth consideration, but you still may need external braking.

The brake should be either fused or fed through a circuit breaker. I had a drive where the braking transistor failed on with no protection. The idiot engineer that designed the system mounted the breaking resistor on top of the machines control cabinet. The heat actually warped the cabinet and melted the top of the SLC cards underneath. A very bad day.....


The OP seems to have dissapeared so I won't waste anymore breath.

I've seen lots of braking resistors mounted on the tops of the cabinets that the drives are located in. Braking resistor packages generally include thermal switches to prevent the sort of thing that allscott described. The "idiot engineer's" mistake was not in mounting the braking rsistor on top of the cabinet; it was in not utilizing the available protection. It's also possible that some maintenance technician got tired of having to reset drive faults due to overheated brake resistors and jumpered out the thermal switch.

I've seen lots of braking resistors mounted on top of control cabinets too, that doesn't make it right. Most control cabinets have some sort of cooling be it fans or AC or whatever. Why would you mount a device that does nothing but generate heat and mount it right on the cabinet? If there is no other logical place to mount them I usually build a little stand out of square tubing to that the resistors are not directly transfering heat into the cabinet.

The "idiot" engineer in question earned his title by doing far worse things than this though...

allscott said:

I've seen lots of braking resistors mounted on top of control cabinets too, that doesn't make it right. Most control cabinets have some sort of cooling be it fans or AC or whatever. Why would you mount a device that does nothing but generate heat and mount it right on the cabinet? If there is no other logical place to mount them I usually build a little stand out of square tubing to that the resistors are not directly transfering heat into the cabinet.

The "idiot" engineer in question earned his title by doing far worse things than this though...

Click to expand...

Let me step in because I THINK, which is rare, that there may be a difference in "TOP OF CABINET". What I hate to see is when the braking resistors are mounted to the top of cabinet ON THE INSIDE.

I have never had a problem when mounted on top or side, OUTSIDE the cabinet in their own vented panel or covering that prevents contact.

I've never seen a braking resistor mounted inside a cabinet, that would be really bad. Mounting them flush on the outside of a cabinet to me is just poor practice. Heat is the enemy in control cabinets. Lifting them a couple of inches above the cabinet so that they are not in direct contact doesn't take a lot of effort.

The top of the cabinet that I was talking about was completely covered in braking resistors. They were about 400W each with a relatively high duty cycle (I think there were 6 or 8 of them). They definately were heating the cabinet up.

I wasn't trying to imply that anyone who mounts one on a cabinet is an idiot. I'm sure that most of the time it isn't a problem. After that experience I decided that mounting a heater on top of a cabinet was counter productive and I don't do it any more.

Also, these units had thermal switches in them to disable the drive. The drive was disabled but the braking transistor had failed (shorted). Therfore even though the drive wasn't running, the DC bus was continually dumping on the resistor and thus the tremendous amount of heat. I have only ever seen this once so I don't think it's a common problem but because of this I don't rely on the thermal switches anymore and install either fusing or a CB on the resistor circuit to disconnect it if there is a problem.

As far as the idiot engineer. I wish I had taken pictures of some of the things he had done, it would have been a good entry in the bad wiring thread that was here awhile ago.

Thanks for the insight....

Ok, everyone thanks for the responces.
1) NO, the drive is not mounted on, in or near the shaker machine whatsoever.

2) We are installing this used Ab drive just because we have it laying around and we needed to replace the bent variable pitch pulley that use to be the speed regulating device on the machine. ALthough the shaker is an older unit, it is in very good condition mechanically and one this size is rare. So we are putting some updated controls on the machine (i know the 1336 is old, but it is better than manually turning a handle that operates a movable motor base that operates a variable pitch pulley).

3) When i called AB, they said i need dynamic braking and gave me part numbers of a chopper and a resistor pack. The chopper was a AB 1336-WB035 and the resistor pack was IPC 445-11A. From AB the chopper is about $1200 and resistor pack from IPC is about $1800. I guess i exaggerated the price when i said 3500.

4) I JUST NOTICED. The rated Nominal bus voltage is 648 V . With the drive on but not running the bus voltage is 684 V. The drive input is rated at 380 - 480 VAC. We have about 475v, maybe this is accounting for the high nominal voltage, thus creating a bus overvolt during operation. Maybe lowering the input voltage and adding a small dynamic brake will work.


5) I think i will also try what dickdv suggested, reducing the max output voltage to keep the Dc bus voltage down.