Open Delta Feeding Inverters. 2

[itsmoked]

Any problems with using open-delta via two buck xfrmrs (240V) to feed a bunch of VFD driven machine tools?
This won't cause any significant load imbalances on the rectifier front-ends?

I have to buck 236,236,239V (out of a rotary phase converter) down to 220Vx3.

The kicker is that in the near future the shop will be converted to 208V/3ph where I'll have to then boost back up to ~220V (x3).

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Really? The VFDs won't take that voltage? It seems strange that a VFD that can take 220V wouldn't be able to take 240+.

 

[itsmoked]

Naw its not that David it's the maker who will refuse to honor the warranty if the voltage is not MUCH closer to 220V.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith.
Most voltage unbalance problems with an open delta originate on the supply conductors.
The common phase has a higher current than the open ended phases. By root three or 1.73%
That can cause uneven supply voltage drops.
I have used the open delta connection many times but always on motor loads. DOL, not VFDs. An induction motor does a great job of balancing up supply voltage problems if the motor is a little oversized. Often the motors are somewhat oversized.
If your supply conductors are short enough and sized so that voltage drop is not an issue, it will probably work well.
As David mentioned, is there a reason why you cannot feed the VFD with 240 Volts and configure the output at 220 Volts?
Are you looking ahead to the time when you must boost 208 Volts up to 220 Volts?
Are these IEC machine tools?


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

 

[jraef]

Keith,
Drives tend to not like Delta services, open or not, but mostly because of component selection being based on a lower lione to ground reference voltage (although usually not an issue at 230V) and more importantly, also the fact that internal noise filters and MOVs are referenced to ground. Some drives make it simple to remove that ground reference point when using them on Delta (or HRG Wye) systems, some do not, especially Asian drives.

There are plenty of drives out there that are not so picky as to the input voltage, I can get you some that will accept 200V-15% to 240V +10% and not bat an eye. It seems a waste to go to this added expense just to use a specific drive.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

That's what I was concerned about Bill since the power never lands directly on any motor.
I'll just run with three bucks. I'll buck 16V for 236 - 16 = 220V which aught to be close enough to 220V to make even the terminally paranoid happy.

Yes Bill, thinking ahead!
Then when the shop switches to puking 208V 3ph I'll turn around and boost 16V to get them ~224V.

Thanks Jeff. This is a 'turn-key' brand new $150k CNC machining center. Ain't gonna be any 'drive replacements' as CNC drives are so heavily enmeshed in the tools that I've only seen one out of hundreds of cases where someone could hope to switch drive types and have a machine that still worked.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Is the rotary phase converter a floating or grounded source?

I've seen a VFD installed on a floating power system with a grounded motor start grounding the phases of the power system at the carrier frequency. Really not a very good thing to be happening.

The ground on one of the VFD outputs will alternately ground the positive and negative side of the bus at the carrier frequency. Then, in the 3-phase rectifier there is always one diode connecting a phase to the positive side of the bus and another diode connecting a different phase to the negative side of the bus.

 

[waross]

Grounding and the open delta auto-transformer.
If the source is grounded, such as a 120:208 Volt feed, then the output of the open delta boost (or buck) is grounded subject to the following concerns:
1> The voltage to ground of the common phase remains the same.
2> The voltage to ground of the open phases changes slightly.
3> The available fault current of the common phase remains the same, determined by the source impedance.
4> The available fault current of the open phases is limited by the impedance of the auto transformers.
5> I assume that the rotary phase convertor is fed from a 120/240 Volt panel. The three phase output will be equivalent to a four wire 120/240 Volt delta arrangement with a grounded center tap on one phase.
and
If you want or need symmetry, use an autotransformer wye connection with three transformers.
With the wye connection you will be boosting the line to neutral voltage, not the line to line voltage.
The voltages of interest are:
208 V : 120 V
220 V : 127 V
240 V : 139 V


Your target voltage will be 127 Volts so you will be dropping 139 Volts to 127 Volts and boosting 120 Volts to 127 Volts.
The star point should be connected to the source neutral.
A 120:8 Volt transformer should do well to boost 120 Volts to 128 Volts. Close enough.
If you are nervous about applying 139 Volts to a 128 Volt rated transformer,
you may consider a 120:8/16 Volt transformer.
on the 120V+16V connection the rated voltage is 136 Volts. Close enough.
At the 120V + 8V connection you will be able to extract a nominal 128 Volts, actual 131 Volts. Only about 3% high. Close enough.
KVA will have to be double as only one of two secondary windings will carry the load current.
The last option is to use two 120V:8V buck connections to drop the input to the rotary phase converter to 112.5V/225V so that your rotary convertor develops 225 Volts. You may then use the 120V:8V transformers to boost up to 128 Volts from 120 Volts.
You should be a little oversized when only two transformers are used initially, but often in the small sizes, the next available standard size is adequate for both applications.
This is probably more than you ever wanted to know about three phase auto transformers. Grin.

I hope that this does it for you.

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

 

[itsmoked]

Thanks Bill and with Lionel's point I decided instead to buck only the single phase supply to the rotary converter. It's about 244V. This means I need only one buck (larger) but I don't have to worry about the ramifications of open delta and the transformer maker's anathema about three bucks in delta, (rogue circulating currents).

Thanks all.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

If you want symmetry use three transformers wye, and boost the 120 volts instead of the 208 volts.
I agree, three buck or boost transformers in delta is bad news. It only works in a perfect world.
Things are seldom perfect enough in the field.


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