VFD AC/DC supplied

[electrolitic]

Sirs
Can I connect two 5,0kW VFD (diode front end) to a selectable (open transition) AC 3ph or a DC from battery bank?
Fuse protection and blocking diodes at DC entry. If yes, which should be the DC voltage compared to AC?
Could the built-in precharge be used for both supplies? Control power to be adequately selected.
Thanks

 

[jraef]

Many VFDs allow for a direct input to the DC bus, as long as you understand the ramifications. The DC voltage input must be at the PEAK voltage of the AC line, not the RMS voltage. So either read the specs of the VFD for the maximum DC bus voltage, or use the AC RMS / .707 as a minimum DC level, i.e. if you have a 230V VFD input, the DC input must be 325VDC minimum. Then you have to remember that with DC battery supplies, the voltage level is immediately diminishing as you use it, so your VFD must be able to adapt the output for that condition, i.e. automatically lower the output speed reference at full speed command to maintain the correct V/Hz ratio on the output. That of course means having a process that can be adapted to that eventuality. Then you must be careful of the AC VFD design; some take their control power from the AC side with a separate SMPS, others take it from the DC bus with a DC-DC converter. You will need to know which one your drive is using to accommodate that.

Maybe if you fully described what you want to accomplish and what you have to work with, we can help further.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[electrolitic]

Thanks Jraef
Direct input to DC bus is well known, some VFD models are designed to receive it.Others, don't. Existing installations would require modifications of standard equipment.
The specific question it was related to the possibility of DC connection directly to 2 AC entry phases. The rectifier diodes (two of them) would be overloaded if output load was kept but with load reduction (speed, for example) maybe it would be possible an emergency power supply from swgr battery banks to drive important motorized valves, flaps, safety doors, etc during AC shutdown. Some details as control power, alarms, interlocks would be considered. It makes sense? Are my assumptions correct?
Thanks again.

 

[itsmoked]

Yes that should be doable. The issues are just what you suggested - only two diodes would be supplying the power. If they're insanely sized, like they could be, then that might be a problem at full load.

Another issue is if the VFD monitors the supply for phase loss or something.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mikekilroy]

all but one of your assumptions is correct: using only 2 diodes will overload them: probably not a problem.

reason is the the diodes are of course current rated: you will be putting 1.4x LESS current thru those 2 diodes for the same output rating since your input voltage is 1.4x higher, even tho you lost the 3ph sharing of 1.73, so current will only be 1.73/1.414 = 22% higher.


Example: 10a ac rms *1.73*230v rms rated input = 3979 watts in. put in the 330vdc instead and it will be 3979/330vdc = 12 amps

Hence, you should be fine on current rating on any but the cheapest Chinese import drives.

 

[jraef]

Depending on the size, many VFDs under 3kW are already designed to accept single phase AC input, so their diodes are already incrementally larger, just so you know.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[jraef]

I should have added 240VAC to that statement.

Damned lack of edit function vexes me here....

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[electrolitic]

Thank you all
It is just a cheap alternative power supply idea without large modification in existing equipments.
I will prepare it for some few special equipments.
Maybe it can be used in an emergency situation. When you agree as in this case, I feel more confident.
Thanks again.