How to mitigate regenerative power from VFD running on a rotary phase converter? 1

[fastline12]

First of all, this system is currently functioning fine but has come to attention that VFDs with regenerative braking as opposed to using a braking resistor, may not be able to sync power back to the line if using a rotary phase converter?

Testing revealed a DC buss on a VFD with a nominal voltage of 330VDC climbs to around 385VDC under braking. The VFD has not alarmed out and does lots of braking. However, it might be a good preventative to at least determine if we have any options to protect the system or if there is a reasonable amount of voltage rise that can be considered normal?

On this system, I highly doubt there are provisions to add a resistor as the system was designed to be regen. As well, there is no current option to get true 3P power to the device at this time.

 

[waross]

Description?
Model number?
Supply voltage?
????

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

 

[itsmoked]

fastline12; While all VFDs will 'regen' there are actually two different cases. ALL will stuff regenerative power back onto their DC Bus capacitors. There the VFD monitors its own DC Bus and when the voltage there gets high enough will start sending pulses (PWM) of the excess to the DC braking resistor to 'waste' it into the local air as a lot of heat. Most smaller VFDs under 10HP or so all have built in DC 'Braking resistors' capable of limiting the DC Bus voltage to something.... undamaging.

These built-in resistors are more than adequate for about 80% of VFD applications where there isn't large inertia involved nor any overrunning load like a gravity load. If the user sets things up with enough of either of the aforementioned regen loads causing more power to be dumped into the built-in braking resistor than it can handle then the drive will protect itself at the moment the voltage climbs above a hard limit on the DC Bus. The drive will trip disconnecting itself from the motor and leaving the motor and load to "run away" to their combined fate. Usually it's something like a lathe that would've stopped in 2 seconds instead coasting down for an unbraked 60 seconds.

The other regen and what tags a particular VFD as a "Regenerative Drive" or "Regenerative VFD" is a VFD that has a power inverter built into it that completely replaces the the braking resistor scheme described above. The power inverter takes the excess voltage (Energy) piling up on the DC Bus and synthesizes it into synchronous AC power that is injected back into the power mains that are supplying the VFD with its grid connection. This allows the inertially provided energy to be removed from the vicinity to do more useful work than otherwise provided by a local braking resistor.

Since all drives regenerate onto their DC Bus's the term "Regenerative Drive" DOES NOT APPLY TO THEM FOR THIS FUNCTION but only for the latter type described above that includes the built-in inverter.

Regenerative drives are very rare.
Regenerative drives cost twice as much as regular drives.
Regenerative drives are often 50% larger or include a second regenerative module to provide the function.

Long story-short; Are you sure you have a regenerative drive?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

When you say it is "functioning fine", is that with a Line regenerative drive? Because all of the Active Front End drives I am aware of on the market are designed for 3 phase input only, and I can't imagine how they would function on a single phase input.


" We are all here on earth to help others; what on earth the others are here for I don't know.
" -- W. H. Auden

 

[fastline12]

Yes, this is a line regen system. Take a look at Okuma drives. Built specific for their own machine tools but do have line regen.

We have the decel rate pulled back but I think we need to look at other ideas.

 

[itsmoked]

OK.

If you have any other machine tools running on the 3P side of your RConverter then I would never expect you to have any issue with the brief regen energy available out of your tools. It will be absorbed by the other loads faster than it can be regened.

If you find yourself running a single machine and no other aux equipment like a compressor, chip conveyor, or coolant pumps then you could end up with surplus. IF this is really the case..

Some machine tools use "regen" but it's not mains regen. They tie all the DC Buses together and regened energy is then equally distributed back to ALL the drives in the machine. This causes a much slower rise on the bus since there's three or four times more capacitance to charge up and everything else is consuming. While the spindle is decelerating other axis are consuming to hold against gravity or axis motion is continuing or the tool changer is indexing. Since no machine tool can regen much energy like a falling payload or long down hill can the amount returned is always seriously limited. It is so limited that the tool maker usually goes with only the DC Bus tie-together and never synthesized AC back to the mains. That doesn't stop them from crowing about "REGEN" it's just not the expensive kind. Are you sure this isn't the case?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The rotary phase converter is interesting. If this is mains regen, then the VFD will be driving the phase converter motor. It will be back-feeding into two lines but I can't (and don't really want to) visualize what it will do to the third line and to the caps. Maybe it's ok?


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

 

[fastline12]

I cannot seem to remember what values we had when we tested the AC line during braking.


I have attached a manual of a drive that we also need to run on the line, but appears it may have line regen? I don't have the general manual, but on page 24 you can clearly see there are inverter and converter IGBTs. There is also mention in the alarms page of having separate over volt alarms for them too.

So it appears this drive, dated back to 25yrs had line regen?

 

[itsmoked]

Unexpected..

Nothing I read in the manual leads me to believe there is line regen BUT. The wiring diagram shows no braking resistor at all and high performance machine tools always need a braking dump of some sort. Sooo they must be dumping back to the mains. I'd get someone with a scope to hook up to your RC output (someone who understands the hazards) and see what happens with fast stops. You should be able to carefully stop faster and faster and see if things start to go off the rails before you'd damage anything. It could well be the RC absorbs the energy just fine. Or, maybe not.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[fastline12]

Look at page 24, you will notice two IGBT sets.

Also, look on page 34. There are alarms for the inverter and converter sections.


It is interesting.

Yes, I figure the options would be to slow down decel, or find another way to sync power! I have heard if you have other 3P motors on the line, you are likely fine but a big lathe slowing down from 4000rpm to 0 in 3sec has to dump a good amount of power and I can't ensure there will always be something else running.

 

[waross]

I see that the unit has the following alarms/warnings:
Phase loss.
Low voltage.
High voltage.
It would appear that the unit is able to push enough current/power back through the rotary phase converter to function properly.

Testing revealed a DC buss on a VFD with a nominal voltage of 330VDC climbs to around 385VDC under braking. The VFD has not alarmed out and does lots of braking. However, it might be a good preventative to at least determine if we have any options to protect the system or if there is a reasonable amount of voltage rise that can be considered normal?

Remember that the motor losses dissipate part of the regen energy.
The drive should self protect by keeping the regeneration within safe limits.
It may be self protecting now. The first indication may be slightly longer than set deceleration times.
When the self limiting is active the deceleration will be reduced to prevent the DC bus voltage from exceeding the safe limit.
As the deceleration time is set shorter, the DC bus voltage will rise under braking.
A point will be reached where shortening the decel time setting no longer shortens the decel time nor increases the DC bus voltage.

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

 

[LionelHutz]

A DC bus voltage rise before regen is normal and it's working now. So, I don't currently see any problem.

 

[bull0082]

To comment on OPs post, I have worked with a different line regen product that at times can be connected to a building generator. If there are no other loads on the generator to absorb the regen then the DC bus will continue to rise.

In these cases, we typically have a smallish braking resistor connected to the VFDs brake chopper for emergency operation on the generator. The activation level for the braking resistor is higher than the regen unit so it will turn on in the case the DC bus gets too high. If you have access to the DC bus terminals I suppose you might be able to add a chopper module and resistor.

http://www.kebblog.com