Transformer tap

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elec_eng

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I am wondering if I can use the tab connection to boost the secondary voltage.

Typically, the transformer has multiple taps (FCBN and FCAN) on the primary windings. I understand that these taps are used when the primary voltage is either below or above normal to get the proper voltage at the secondary, for example 120V.

I have a situation where I am expecting a huge voltage drop on the secondary side feeder due to the distance from the transformer to the panel. I am thinking if I set the tap to 5% FCBN with normal primary voltage, I can boost the secondary voltage by 5%, 126V for example. Then I don?t have to upsize my secondary feeder to compensate the voltage drop.

Do you see any problems with this either engineering or code perspective?
 
elec_eng said:
...Then I don?t have to upsize my secondary feeder to compensate the voltage drop.

Do you see any problems with this either engineering or code perspective?
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Not from the code. VD is an FPN, not regulatory.

From an engineering standpoint, the regulation will suffer. However, 5% isn't much, most equipment will run okay at that variation

cf
 
Cold Fusion said:
Not from the code. VD is an FPN, not regulatory.

From an engineering standpoint, the regulation will suffer. However, 5% isn't much, most equipment will run okay at that variation

cf
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I am well aware that VD is not the code requirement and that is not what I am asking here.

I am asking that if there is any issues with what I am proposing to overcome the voltage drop.

I have a panel located almost 600ft away from the transformer and I am experiencing a almost 12% voltage drop, so I am just looking for an easy and inexpensive solution.
 
What you are proposing is fine as long as the load is static. If you have no load to speak of, the voltage will be about 5% high. As the load increases, the voltage drop will lower the voltage to being under-voltage. In other words, you will have no voltage regulation and your setup will cause significant problems.

When a power company has the same type of problem, the customer sees flicker to the point of being very annoyed and will complain. If the serving electric utility is not responsive, they then will go to the public service commission. Please tell me who your customer is going to go to for remediation? In my opinion, you are setting up a very poor situation with sub-standard engineering. Again, if you have a static load or special circumstances, all of my arguments could be bogus. :)
 
charlie said:
As the load increases, the voltage drop will lower the voltage to being under-voltage. In other words, you will have no voltage regulation and your setup will cause significant problems.
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Charlie,

The panel is existing and it serves the receptacle loads for the research labs. The feeder will see about 12% vd when the most of lab equipment is up and running. Even with very minimal loads, you will still see about 3-5% vd due to the long distance.

What I am trying to do is if I booster the secondary voltage of transformer by 5%, I only have to upsize the feeder to overcome 7% of vd instead of 12%. I am trying to re-use the existing conduit if possible not to disturb the existing building so the size of new feeder conductors are big concern.

Do you still see a problem with this?
 
Would it be possible to relocate the transformer to the lab and run the 480(I assume you are going 480 to 120)with the exisitng conductors--Just a thought.
 
charlie said:
As the load increases, the voltage drop will lower the voltage to being under-voltage. In other words, you will have no voltage regulation and your setup will cause significant problems.
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Charlie, he has that now. He just wants to move the voltage swing to a higher range.

In my simple opinion, the line current will be a bit higher anyway, because of the slight change in the turns ratio, so the higher voltage under low load won't be terribly high.
 
LarryFine said:
In my simple opinion, the line current will be a bit higher anyway, because of the slight change in the turns ratio, so the higher voltage under low load won't be terribly high.
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Larry,

I don't think I am following you. Can you please explain how does it going to affect the line current? We are assuming the nominal voltage will be 480V and only changing the turns on the primary, won't the current be the same?
 
I think that what Charlie was pointing out is that much of the _problem_ with voltage 'drop' is not the absolute drop itself, but the _variation_ in voltage at the load end as the load changes. When the load is near zero, the voltage drop is also negligible; you have to have current flow to have voltage drop. When the load is near maximum, then you will have high voltage drop. When transient loads kick in (say when a motor starts), you will have a very high voltage drop on your feeder.

Kicking up the voltage at the beginning of the feeder help with the absolute voltage drop, but will do essentially nothing to help with the voltage _variation_.

If voltage variation is not much of a problem for the loads that you are supplying, either because the load is relatively constant, or because the loads are tolerant of voltage variation, then kicking up the supply voltage will be a benefit. If you have lots of constant power loads (switching power supplies, motors, etc.) then kicking up the supply voltage will actually decrease load current.

Note however that if you use the tap on the _primary_ side of the transformer, then you will be increasing the magnetization level inside the transformer core. Depending upon how close to saturation the transformer is at nominal design point, this may cause excessive magnetizing current flow, and could cause excessive transformer heating. That is an issue to take up with the transformer manufacturer. Ideally you would boost the secondary voltage using taps on the secondary side, but these are not common in small dry type transformers.

-Jon
 
Other loads served from the transformer will see a 5% higher voltage. As long as that's OK, then I don't see a problem.
 
elec_eng said:
Larry,
I don't think I am following you. Can you please explain how does it going to affect the line current? We are assuming the nominal voltage will be 480V and only changing the turns on the primary, won't the current be the same?
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When, say, a buck-boost transformer is used to boost voltage, the primary current increases by the ratio of the voltage boost. If you need a 10% voltage boost, the line current increases 10% plus losses.

For example, let's say you have a load that, at 120v, has a 10a current usage, and you intend to see that it actually receives that 120v. That means you'll neet to supply it with 1200va.

Now, if you feed a 480v-to-120v transformer with 480v (and ignoring losses), at the same 1200va, the primary current will be 2.5a. (120v x 10a = 1200va. 1200va / 480v = 2.5a.)

That's a 4:1 voltage ratio. Notice that the line current is inversely proportionate, meaning you'd have a 1:4 current ratio, which is why we move power at the highest practical voltage.

If, however, your primary voltage at the load end of the line is, say, 440v, for the 120v load to still receive 10a (i.e., 1200va), the primary current would be 2.73a (1200 / 440) instead of 2.5a (1200 / 480).


Added:
Remember, the whole reason for using a voltage-boosting tap is because you can't assure the 480v at the remote primary. When you boost with taps, you're altering the turns ratio, which alters the voltage ratio, which alters the current ratio.
 
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Using taps to adjust will work just fine as long as the voltage at no load is not too high. Transformers are designed with full KVA or MVA capacity taps.

What your wanting to do is done quite regularly. It's just that there are many folks that simply don't realize the benefits of using taps. As a general rule, it is best to try and make your original design so you don't need them. But during start-up you can then make fine tune adjustments using the taps. For existing installations it is very practical.
 
I would like to add one more thing: when possible, it's better to boost the voltage at the source end of a long run than the load end. The higher voltage means less current for a given load VA.
 
kingpb said:
Using taps to adjust will work just fine as long as the voltage at no load is not too high. Transformers are designed with full KVA or MVA capacity taps.
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Kingpb,

Does that mean that changing the tab will not overexcite the core? That was what I thought but seems like everyone else think that this will over excite the core and will lead to xfmr early failure. I talked to the sq. D sales rep, but I don't think he knows as well.

Thanks for your input.
 
LarryFine said:
I would like to add one more thing: when possible, it's better to boost the voltage at the source end of a long run than the load end. The higher voltage means less current for a given load VA.
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Larry,

Well noted. Thank you very much for your thought on this. I appreciate it.
 
LarryFine said:
When, say, a buck-boost transformer is used to boost voltage, the primary current increases by the ratio of the voltage boost. If you need a 10% voltage boost, the line current increases 10% plus losses.

For example, let's say you have a load that, at 120v, has a 10a current usage, and you intend to see that it actually receives that 120v. That means you'll neet to supply it with 1200va.

Now, if you feed a 480v-to-120v transformer with 480v (and ignoring losses), at the same 1200va, the primary current will be 2.5a. (120v x 10a = 1200va. 1200va / 480v = 2.5a.)

That's a 4:1 voltage ratio. Notice that the line current is inversely proportionate, meaning you'd have a 1:4 current ratio, which is why we move power at the highest practical voltage.

If, however, your primary voltage at the load end of the line is, say, 440v, for the 120v load to still receive 10a (i.e., 1200va), the primary current would be 2.73a (1200 / 440) instead of 2.5a (1200 / 480).


Added:
Remember, the whole reason for using a voltage-boosting tap is because you can't assure the 480v at the remote primary. When you boost with taps, you're altering the turns ratio, which alters the voltage ratio, which alters the current ratio.
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It makes sense now. Thank you for your kindly explanation. But like kingpb stated, I thought xfmr was designed to handle this tap connections.
 
elec_eng said:
Does that mean that changing the tab will not overexcite the core? That was what I thought but seems like everyone else think that this will over excite
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I think that I was the only person to mention core saturation.

When you change the tap on the primary side, you are changing the number of turns connected to the primary circuit, and thus changing the volts per turn of the primary circuit.

If you use the primary tap to increase output voltage, this will increase the primary volts per turn and of necessity _increase_ core flux density.

Whether or not this _over_ saturates the core is a question of the transformer design and the applied load.

This change _will_ increase the magnetizing current consumed by the transformer, and will increase core heating. But both may remain in acceptable ranges.

-Jon
 
According to IEEE C57.12.00 taps shall be full capacity, unless specified otherwise, and the transformer shall be capable of operating on thoses taps without seeing an increase in the specified temeprature rise.

The information regarding type, number, and kva capacity of taps should be on the nameplate of transformer.
 
kingpb said:
According to IEEE C57.12.00 taps shall be full capacity, unless specified otherwise, and the transformer shall be capable of operating on thoses taps without seeing an increase in the specified temeprature rise.
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Meaning that you should get the full KVA of the transformer if you connect 504V to the 504V tap, 480V to the 480V tap, or 456V to the 456V tap. But that doesn't tell us what happens if 480V is supplied to the 456V tap.

-Jon
 
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