1ph vs 3 ph cost savings??

[MAGTiger]

I am aware that some areas, primarily in the western US, both irrigation and downhole pumping are using single phase source to a VFD acting as a phase converter then the 3 phase pump motor. But I'm having a problem seeing the cost justification in this scheme. It seems to be mainly 5 to 70hp. But once the drive is oversized by 75 to 100%, then the cable is oversized likewise, then you compensate for the horrendous single phase current distortion with an expensive filter, can there really be that much savings in running the 3rd wire. Especially when with 3 phase the wire is smaller? Help me4 under stand the cost justification of such a system.

Neil

 

[jraef]

The reason is not savings. The reason is down hole pump technology. Above 3HP, 1 phase submersible pumps have no physical room to mount the starting capacitors. So the caps need to be above ground, but as close to the pump as possible. As the HP goes up, it usually means the pump is deeper, hence more and more distance between the caps and motor, longer cables for the caps, more weight in the cables etc. etc. Using a 3 phase motor eliminates all of that.
And by the way, the 1 phase supply cable is not necessarily going to be over sized with or without a VFD to convert it to 3 phase for the motor. The motor power remains the same, so the only difference is a few percent of VFD throughput efficiency loss in the circuit.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[MAGTiger]

I think you are misunderstanding. When I say cable obversize, I mean at the input to the vfd where single phase 2 wires will need to pass 73% more amps than 3 lines in three phase. And at the pump I understand why that is 3 phase. It makes clear sense. But why only single phase to the vfd?

Neil

 

[itsmoked]

The typically NO ONE would do this if they had three phase available. They do it only because out by some well in the middle of nowhere -electrically speaking - they may only have 1 phase available.

The only other reason is if they have a system they want to control the speed of the pump. Say, the well has seasonal flow issues and they want to draw water at the make-up rate and not cycle the pump. Then, of course, a VFD can help there.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

As itsmoked says, three phase power may be $50,000 $100,000 or more to install away out in rural areas.

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

 

[jraef]

What they said...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[MAGTiger]

So I'm understanding that in these applications, they would be far from a generating station and possibly a substation so would it be safe to consider this a soft source?

Neil

 

[waross]

It is common to run only one phase in rural areas. Some utilities use a ground return and only run one conductor in rural areas. You need at least two phases and a neutral to develop three phase. One primary phase is typically run without cross arms. You may need miles of cross arms installed as well as at least one more primary conductor.

So I'm understanding that in these applications, they would be far from a generating station and possibly a substation so would it be safe to consider this a soft source?

Although this may be true in many cases, this is not a safe assumption. Protection schemes based on assumptions may not provide the best protection.

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

 

[davidbeach]

Protection schemes based on assumptions may not provide the best protection.

How very true!