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Old September 14th, 2018, 05:50 AM   #16
JesperMP
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Regenerative braking as per your link means that I need drives that can send the energy back to the line, rather than bleeding it off in a brake resistor.
So theoretically I could change the PM240 for PM250.
That would be a huge expense compared to just connecting the DC bus together.
And it would be a big job since the dimensions of PM240 and PM250 are different for same kW rating.
And ... if connecting the DC-bus together is possible, then that also gives me regenerative braking. Only the power is sent to the other drive rather than back to the line.
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Old September 14th, 2018, 07:06 AM   #17
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So that is a definite 'No' to the regenerative braking then. I will keep plugging away, but the law of averages I must eventually have a good idea.
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Old September 14th, 2018, 07:26 AM   #18
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Originally Posted by JesperMP View Post
The schematics in the AB PDF just says the back-to-back diodes are needed. I cannot see what the illustrated parallel diodes in opposing directions actually do.
I'm with you... seems to me like opposing diodes effectively do nothing except for waste energy and generate heat?

Unless somehow they want to prevent current flow between the buses when their voltages are very close? < (2 * Vdiode) ?

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Old September 14th, 2018, 07:31 AM   #19
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I have seen this done before and it does allow the generated power to be used by the other motor. It was in effect a huge rectifier. And then ,common DC bus to 8 "1/2drives" as I called them as they just varied the frequency out from the DC link. Was all Rockwell MCC and drives. Worked well in this situation but 1 failure on the front end equipment and all 8 drives are down... As others have said
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Old September 14th, 2018, 09:51 AM   #20
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The schematics in the AB PDF just says the back-to-back diodes are needed. I cannot see what the illustrated parallel diodes in opposing directions actually do.
The schematics in the bonitron pdf page 15 is different to AB, but it makes sense to me.
I am guessing that bonitrons description is the correct one.

edit: No wait, the bonitron illustration does not apply to my case.
I have several VFDs as in the AB PDF page 54, not with socalled "regen modules" as in the bonitron PDF page 15.
I am still mystified as to what this diode module does, and how I should apply it in my application, and if I even need it.
I did some more reading and searching last night. I noticed that if you have line reactors installed, you don't need the diode modules, and vice versa. It seems that the big issue is preventing circulating currents between the VFD's. I also noticed that if you use this setup, only one of the VFD's should be setup to use regenerative braking, typically the largent drive, and it handles dumping power to the braking resister as needed.
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Old September 14th, 2018, 10:34 AM   #21
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It is 2 drives and they are of the same size.

I have found a Siemens instructions for MM4 with some useful information.
The secret word to search for is "zwischenkreiskopplung".
Now you know

I am going to contact a Siemens drives expert to get advice for G120.
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Old September 14th, 2018, 10:52 AM   #22
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Jesper

The app note for the MM4 is very similar to the G120.
Its not a published doc but the MM4 was revised for the G120.
Your correct contacting your local Siemens Rep.

What your trying to do will work- I started up a similar system with 2 drives this week.
Just fuse the bus with the correct type and amperage.
Turn off the vdc controller on both drives. Three phase supply will go to each drive. (since they are identical)
Make sure the ready bit is On for both before enabling
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Old September 14th, 2018, 10:54 AM   #23
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Thanks.

Any advice on semiconductor fusing ?
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Old September 14th, 2018, 11:06 AM   #24
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What you are asking for is called common buss and is possible and has been done many times.
But id must be done very carefully and is drive specific Yaskawa has a very good document on doing just what you want. They recommend the just one drive supply the buss power differences in the power components it may cause problems when charging the buss It is also recommended that you inductors between the buss and the VFD.
Or as simpler solution you could just use a Yaskawa U1000 drive on each hey are line regenerative vfd's so all the breaking energy will be pushed back into the ac supply line
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Old September 14th, 2018, 11:14 AM   #25
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The extra diodes are there because when you tie the DC busses together of multiple drives, with each having their own rectifiers and bus caps, differences in the capacitor series resistances can cause circulating currents to flow around and around between them, eventually causing them to over heat and fail. This is especially bad when the drives are of different sizes but even if they are the same, the diodes halo to balance the loading in the caps. The diodes stop those circulating currents, making the busses into one bus system sharing between multiple inverters.

There is a difference between LINE REGENERATIVE braking and regenerative braking in a COMMON DC BUS. You can do a Common DC Bus arrangement with almost any two or more drives, so long as the total load that is MOTORING is using more power than the total load that is BRAKING. If not, you can still do it by utilizing DB braking transistor and resistor, usually on the largest drive or collectively on all of the drives, or using a totally independent DB unit like a Bonitron. You can ALSO get rid of the excess energy with a LINE REGEN option, but if the amount of braking is always going to be lower than the amount of motoring, you don’t need Line Regen. There are plenty of ways to skin this cat, you just need to pay attention to the details.

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Old September 14th, 2018, 02:57 PM   #26
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Here you go
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Old September 14th, 2018, 03:46 PM   #27
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Quote:
Originally Posted by JesperMP View Post
Hi !

I have an application where a vibratory conveyor is driven by 2 agitators.
In order to change the transport direction slightly (for sorting the material on the conveyor), the agitators are driven by VFDs that drive the motors at a slightly different frequency (f.ex. 49.5 Hz and 50.5 Hz). That cause the agitators to run at the same speed (due to the mechanical coupling), but a difference in the angle, which in turn causes a slight angle of the conveyors transporting direction.
Since one agitator is essentially driving and the other braking, the braking VFD has to send a lot of energy to a brake resistor.
This all works OK, but the brake resistor has to be much larger than normal, since it has to bleed off energy constantly rather than only when stopping.
This in turn also means I do not have much room for adjusting the frequency, since the resistor may overheat (despite being dimensioned much greater).

So I am pondering if it could pay off to connect the DC-bus of the 2 VFDs.
The idea being that the braking VFD sends the energy to the driving VFD, and thus do not have to bleed it off in the resistor.
I have heard of this principle, but I have no experience, and I would like to hear from anyone with experience or links to instructions regarding this.
Anything to be aware of ?
The VFDs are Siemens G120. Do some parameters have to be adapted when connecting the DC-bus ?
Wow - this seems like a popular topic!

I have direct experience with only one application. It was a pair of 400 HP 600 VAC drives in a supply fan/return fan application. The semiconductor fuses were sized for 1.5X rated current on the drives. One fan was 'pushing' and always took power. The other was sometimes motoring and sometimes generating. I don't remember enough about the process as to why this was.

Mitsubishi A500 drives. Both fed by breakers normally. They were side-by-side and connected by 1/2 inch by 2 inch buss bars with one semiconductor fuse bolted for + and one for -. I don't remember any diodes back to back. It's been a few years - I have not heard of any issues. It commissioned OK and I was called back for other things that went wrong .. nothing after 6 months though.

Using drives with an active front end to regen back to the AC distribution was much more expensive, but I don't see why it would not work.
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Old September 14th, 2018, 05:07 PM   #28
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This sounds similar to a centrifuge in the wastewater system at work. The outer screen and inside auger are on two PowerFlex 755 drives that operate at slightly different speeds. Mains power is supplied to the larger screen drive. The smaller auger drive is powered from the DC bus of the screen drive. Thereís some made to order Bussman fuses in the DC feed between the drives. The only time so far one of these blew, we didnít have a spare and I tried feeding the auger drive with the mains. As the system approached operating speed, the auger drive faulted overvoltage. The screen drive consumes the regenerative energy dissipated by the auger drive as it is actually braking the auger at speed.

I could post more details next week when Iím there. If only I could figure a way to run the screen entirely from the auger energy, we could save electricity with a perpetual motion machine.
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