Open Delta Transformer 1

[boereseun]

Have a facility which derives 3 phase 460 volt from single phase supply on open delta system according to FPL (Florida Power and Light), 3 phase pump motors are burning out from this supply due to severe amperage imbalance.
Advise..is fitting variable frequency drives a solution to this problem?

 

[burnt2x]

I cannot visualize what your details tell. "derive 3 phase 460 volt from single phase supply on open delta system according to FPL (Florida Power and Light)"! I don't know if the FPL guys can do that! Do you mean the single phase supply is from an open delta bank of tx? Or do you mean a single phase line is the nearest supply point and you installed a tx bank to give you 3-phase?
You can derive 3-phase from single phase thru the use of converters, and VFD is one of them.

 

[wareagle]

An open delta bank consists of 2 transformers fed from 2 phases of primary conductors. You are able to provide a 120/240 volt 3 phase service for smaller 3 phase motors.
What size is your motors?

 

[wareagle]

Sorry. Should have said 480 volt 3 phase system. All though
I do not think I have seen a 480 volt open delta installation.

 

[davidbeach]

A drive is more likely to accept the out of place phase angles that are causing your current imbalances than the motor is, but you are likely to need a larger drive than the motor size alone might indicate.

 

[oldfieldguy]

Open delta requires three phases of incoming line. The savings is that it provides a three-phase step-down source with only two transformers.

One of my stations has a three-phase open delta utility feed.

old field guy

 

[cuky2000]

Enclosed is an open-Y, open Delta configuration with 3 single phase transformers bank with 3-phase (4-wires secondary) connected to a single-phase (2-wires primary).

http://cuky2000.250free.com/Wye_Open_Delta_Open_Rev_b.jpg

 

[stevenal]

Cuky,

I count three primary wires on your diagram, A,B, and N. The primary is open wye, not single phase and not three phase.

Any advantage to this three can version over the normal two can one?

 

[pwrengrds]

The drives should stop the motors from burning up. As David Beach points out the drives should be oversized to take hit on the incomming voltage problem. Talk to the drive rep about how much it bigger it needs to be.

 

[waross]

Cuky2000's diagram is not an open delta connection.
It is a connection used to derive three phases at 120/208 volts, or 240/416 volts from two primary phases and a neutral.
The advantage:
Open delta with two transformers will supply either a three wire or four wire system. The four wire system may be 120/240 with a wild leg.
Cuky2000's diagram will supply a 120/208, four wire system.
Take your pick.
respectfuly

 

[davidbeach]

I agree, the diagram is not an open wye-open delta connection.

 

[boereseun]

Thanks for the input. Am doing research on what the primary set up is. Pumps are 2.75 hp, 2/10 hp,1/25 hp and various pull pumps also on 3 phase.

 

[cuky2000]

You are right, the neutral in this case should be counted since is a current carrying conductor the above diagram was originated by a special case requested by a colleague in this forum. I do not see any advantage with two can version as show below.

http://cuky2000.250free.com/Wye Ope...-phase 4 wires sec - 2 phase 3 wires Prim.jpg

This sketch with two transformers provides more information including connection and vector diagrams showing the electrical phases.

 

[boereseun]

Thank you cuky2000. My problem has been solved thanks to your diagram.

 

[waross]

Hi Cuky2000;
Can you find a vector diagram for your first post with three transformers? It is a unique way of deriving a 120/208 volt wye system from two primary phases and a neutral.
Respectfully

 

[cuky2000]

Hi Waross,

I could not find easily a wiring diagram. I guess that we need to create one. This may be fun for all of us.

 

[waross]

Hi Cuky;
Note that transformer 1 and transformer 3 are wired for 120 volts.
Transformer 2 is wired for 120:240 volts.

To sketch the vector diagram, draw a "Z" All three lines are equal length and the included angles are 60 deg.
Label the nodes in order, 1,2,3 and 4.
Now extend the center line past node three a distance equal to the original length. Label the end 5.
This is your vector diagram. Nodes 1, 4, and 5 are the three lines and node 3 is the neutral.
1-2 is transformer 1.
2-5 is transformer 2.
3-4 is transformer 3.
Transformer 1 and transformer 3 are both fed from the same primary phase and their respective vectors are parallel.
A 120/208 volt system derived fro two primary phases and a neutral.
Disclamer: I originally learned this connection from a fellow poster on Eng-Tips. I mentioned it to a utility engineer who knew it as a Winnipeg connection.
respectfully