Have a 3 phase synthesizer question.

[itsmoked]

I run into a lot of customers who have shelled out for one of the many single-to-three phase phase-converters. Pricey buggers they are but they put out perfect 3-phase. I have a customer who is being provided by PG&E 245V L-N-L power and the magic phase converter puts out 245V 3ph. Problem is many machine tools are Taiwan made and so are designed for 220 volts, frequently US 240V causes problems.

See any solutions not involving 2 separate buck transformers to feed the phase converters?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

There is my old favorite, Keith, the open delta auto-transformer connection.
The downside of either two transformer solution is that there is still the high line to neutral voltage on one phase,
Neutral use on the two transformer solution may be problematical.
Open delta versus buck connections:
With the same transformer, the buck connection will drop the voltage a few percent more than the open delta connection.
There is no free lunch, but there may be a daily special. grin
In larger KVA capacities it may be productive to cost out both the open delta and the wye solutions.
The KVA load per transformer is greater with the open delta connection.
You may find a case where three smaller transformers cost out less than two larger transformers.
I realize that there may be no control over the input to the inverter. (Owned by PG&E) but as a general question in the event that we are faced with a similar problem with an inverter where we have access to the primary side:
Would one auto-transformer dropping the primary voltage to 220 Volts also drop the secondary to 220 Volts?
Does PG&E have an inverter available that outputs 120/208 Volts?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

I doubt PG&E would ever offer an inverter. They are "coming to look" but I expect them to say "we can provide anything between 228-252 and 245 is fine, "deal-with-it".

I'm waiting for the guy to tell me the inverter model number. It would be trivial for the inverter front-end to be SCR based and phase controlled to set the output voltage at anything lower than the input. I doubt Phase Perfect bothered though because most those companies ring every last nickle out of their designs.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

If the phase converter is customer owned, try one buck transformer or auto-transformer on the single phase input may be the solution.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

EGads!

I thought these inverters were sophisticated. They're cheap Charlestons.

How does that make 3 phase? Yes, it has three different phases but they aren't 120° apart. How could a rectifier front-end like this?


PhasePerfect or rather PhaseImperfect states, "Want to lower the voltage? Put a single buck in front of it."

Your guess is right Bill, seeing that schematic that makes sense.




Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Food for thought:
How about three phase with one inverter?
A variation on the Scott transformation:
Given, L1, N, L2, 240 volts.
Have the inverter develop 208 Volts at 90 degrees and connect it to N.
If the inverter develops the same voltage as the supply voltage you can drive an auto-transformer with the inverter output and take your third phase from a 208 Volt tap. That way the output voltage will follow the input voltage.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

That can make a perfectly good 3-phase "delta" source with phase-phase voltages equal to the original voltage. T1 and T2 are the base of the triangle and T3 is at the top. No neutral, no ability to feed single phase loads, but it should do fine for feeding a motor.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

If you Feed a Scott transformer you may also have a three phase output with a groundable, symmetrical, neutral.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Does anyone remember the "T" transformers that were popular for feeding 120/208 Volt, 100 Amp lighting panels in plants, years ago.
The primary was typically 480 Volts, sans neutral. The "T" transformer was similar to a Scott transformer but the primary would be 480 three phase and the secondary would be 120.208 Volt wye. It used two transformers rather than three for three phase transformation.
I think that I saw a lot of them in the late 60's, and early 70's, but you know what they say about remembering the 60's. grin

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

I bet that would work Bill one would have a heck of a time finding a robust enough inverter for, say, a 5hp motor.

David;

That

Are you refering to Bill suggest or the PhaseImperfect. Or, now that I think of it, both I guess, as they are almost identical.

So you're both saying 3ph equipment is fine with 90°-90°-90° power?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

If the two horizontals are 120 then the vertical is 208.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[cranky108]

Using a delta-wye transformer is what many inverter systems I see do. The transformer also forms part of the filtering for the harminics.

 

[waross]

What David said.
That is why I suggested using an auto-transformer for the 90 degree inverter output.
240 Volts in to the auto-transformer with a tap at 208 volts.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

Well I'll be...

Still not clear on how a load sees this as okay. What is a bridge rectifier seeing? Is one diode.. um. .

So the synthesized OV point is there?

Keith Cress
kcress -

http://www.flaminsystems.com