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Discussion Starter · #1 ·
We have a client with chronic low voltage problems.
Right out of the vault we have just about 205 volts.
The POCO insists that it is within what they are allowed to provide so, that about all we are going to get from them.

They have lost a couple of three phase heat pump type pool heaters over the last few years and the equipment supplier said that was it for warranty unless the voltage was brought up a bit.

No problem, just a nice couple of boost transformers and we should be OK.
I went onto SquareDs website and rilled down to the Buck/Boost page and found the calculator.
I used 200 volts in with 22 KW as the load and they gave me the BOM and the wiring diagrams.

I was expecting to see two transformers but, they are giving me three.
Would I be incorrect in thinking that I could go WYE/ Delta and just use two transformers?


http://www.buckboostcalculator.com/index.cfm
 

· Donuts > Fried Eggs
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In a ∆ you can get away with two boosting transformers because the voltages are only measured between phases, so boosting one wire increases the voltage measured between it and any other wire.

In a Y you are measuring voltage against the neutral point which never changes, so if you only boost two phases there will still be a third one that measures low L-N.

If you didn't care about L-N voltage, you could get away with increasing you L-L voltage using only two transformers. But if that were the case you probably wouldn't be running a 208 Y.
 

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Discussion Starter · #4 ·
In a ∆ you can get away with two boosting transformers because the voltages are only measured between phases, so boosting one wire increases the voltage measured between it and any other wire.

In a Y you are measuring voltage against the neutral point which never changes, so if you only boost two phases there will still be a third one that measures low L-N.

If you didn't care about L-N voltage, you could get away with increasing you L-L voltage using only two transformers. But if that were the case you probably wouldn't be running a 208 Y.
The units are three phase so, no neutral needed but,
considering other loads that I could pick up, it might be best to just boost the entire equipment panel.
The last time I was just boosting a blower motor and only needed straight 3 phase.
That might have been what was stuck in my head.
 

· Bilge Rat
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In boosting a 3Ø system, 2 transformers are needed if there's no neutral. the only time 3 are needed is if the neutral is involved.

The 2 transformers are connected open ∆, not to be confused with an open ∆ as the supply. They're different.

Also, do not connect 3 buck-boost transformers closed ∆. A closed ∆ system will tend to balance voltage at the expense of current, and small transformers will have little effect on a larger supply so they'll most likely burn up.

To boost voltage to a 3Ø heat pump, I'd use 2 transformers provided the neutral is not involved.
 

· Bilge Rat
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Using 3 transformers to boost the entire panel will be ok, but the 120 will be boosted as well as the 208.

The actual figure is the 120 will be boosted by 58% of the 208. For example, if the 208 is boosted 12 volts, the 120 will go up by 7 volts.

Conversely, if the 120 is boosted by 12 volts, the 208 will go up by 21 volts.

BTW, what is the voltage imbalance? A-B, C-A, and B-C. Only the phase to phase imbalance matters, the phase to neutral imbalance doesn't matter. If it's more than 2% (4 volts difference from the 2 highest to the 2 lowest), it'll have a bad effect on a hermetic compressor.
 

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The kicker is that whenever the utility voltages goes up, so does the utilization voltage out of the boost tranformer(s). What can you do when the utility voltage gets you near 140 on L-N loads? What's needed is voltage regulation.
 

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The kicker is that whenever the utility voltages goes up, so does the utilization voltage out of the boost tranformer(s). What can you do when the utility voltage gets you near 140 on L-N loads? What's needed is voltage regulation.
I think he needs to have people install equipment rated for the voltage at the site.

If the owner wants to up the voltage to the heater then so be it. I would not be using 3 buck boost transformers and connecting 120V loads to it.
 

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Discussion Starter · #11 ·
I think he needs to have people install equipment rated for the voltage at the site.

If the owner wants to up the voltage to the heater then so be it. I would not be using 3 buck boost transformers and connecting 120V loads to it.
I think I might segregate the three phase loads into a separate loadcenter and Boost the voltage to it open delta.
Im not a big fan of leaving an open delta with a neutral somewhere a handyman might encounter it long after Im gone.
 

· Donuts > Fried Eggs
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The kicker is that whenever the utility voltages goes up, so does the utilization voltage out of the boost tranformer(s). What can you do when the utility voltage gets you near 140 on L-N loads? What's needed is voltage regulation.
Always a valid concern. The trick with using any winding adjustments for voltage control is don't change it beyond the maximum upper threshold: So if you only want to do 5% boosting, no problem, because equipment can handle 105% of nominal.

You wanna boost 24V on a 208V system? Not the best idea. If the utility ever changes taps you'll be creating an 111% overvoltage.
 

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I wonder if it would be best in the long run if the PoCo changed out the service to a closed Delta (closed because of the heavy 3-phase loads) with one winding center tapped? No voltage changing on the customer side, you have the 240V 3-phase and 120/240 single-phase.
 

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Discussion Starter · #14 ·
I wonder if it would be best in the long run if the PoCo changed out the service to a closed Delta (closed because of the heavy 3-phase loads) with one winding center tapped? No voltage changing on the customer side, you have the 240V 3-phase and 120/240 single-phase.
All of the gear and the risers for the apartments are setup for 3 phase Wye so, quite impossible.
We have asked for them to change the taps several times but they insist what they are giving them is perfectly acceptable.
 

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Discussion Starter · #15 ·
I think he needs to have people install equipment rated for the voltage at the site.

If the owner wants to up the voltage to the heater then so be it. I would not be using 3 buck boost transformers and connecting 120V loads to it.
I dont think Im going to get a production heat pump rated for 200 volts in this lifetime.
 

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Discussion Starter · #17 ·
And they are correct....205 on a 208 volt system is well within the voltage range that they are required to supply.
That and the #3 Al they ran 250' out to the pool equipment panel.
That 205 volts, going that far with about 70 amps of load on it makes me want to boost it at the MDP.
I would rather just boost the equipment panel with two transformers but, considering the long run and the not so accessable underground feeder, boosting it 12 volts WYE at the MDP and sending it underground might be the way to go.

Plus, if someone finally convinced the POCO to bring the voltage up 5 or 10% the transformers would be right next to the MDP staring them in the face.
 

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I dont think Im going to get a production heat pump rated for 200 volts in this lifetime.
Help me out please, what is a "production heat pump"? How does it differ from a regular heat pump?
 

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John, I think your mind is made up. Good luck on this project and I hope you tell us how it all works out.
 
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