Balancing loads

Status
Not open for further replies.
M

mikehughes8

Senior Member
Location
NC
Coming off of a delta-wye 3 phase transformer with single phase loads (A-N, B-N, C-N) I understand that the load should be balanced between the phases. Using a Power Quality Meter I have seen the results if the load isn't balanced. Un-balanced loads will result in current in the nuetral. There is always some sort of im-balance even if on paper the loads are perfectly balanced.

Why is it so important to balance the loads if all applicable ratings aren't exceeded? What's to stop me from loading only (A-N) on a three phase XFMR. Does this damage the transformer?

I know the loads should be balanced I just want to be able to give a good reason as to why.

Thanks for the help
 
I like to see a balanced panel to to efficently utilize the electrical system,from the panel itself, to the conductors and keeping a good balance,the transformer has good voltage output regulation.This also helps your neighbor from having unnecessary low voltage.

Bob O.84,Pa.15330
 
From the Sola Hevi Duty FAQ page.

Q. What is balanced loading and why is it important?

A. Balancing transformer loads means being sure the transformer winding directly feeding a load is not overloaded beyond its capacity. Most single phase transformer applications involve secondary windings rated for 120/240 Volts. These are frequently connected for three wire service. Since the transformer has two 120 volt secondary windings, each one is capable of supplying only one-half of the transformer's rated KVA capacity. If care is not taken, it is possible to apply a combination of 120 and 240 volt loads that will, while not exceeding the total nameplate rating, exceed the rating of one of the 120 Volt windings.

The same is true of three phase transformers, especially those with 208Y/120 Volt or 480Y/277 Volt secondaries. Remember, each of the three secondary windings of a three phase transformer has a maximum capacity of one-third the nameplate KVA rating. It is always necessary to distribute the single and three phase loads as evenly as possible across the three secondary windings without exceeding their capacity.
 
brian john said:
From the Sola Hevi Duty FAQ page.

Q. What is balanced loading and why is it important?

A. Balancing transformer loads means being sure the transformer winding directly feeding a load is not overloaded beyond its capacity. Most single phase transformer applications involve secondary windings rated for 120/240 Volts. These are frequently connected for three wire service. Since the transformer has two 120 volt secondary windings, each one is capable of supplying only one-half of the transformer's rated KVA capacity. If care is not taken, it is possible to apply a combination of 120 and 240 volt loads that will, while not exceeding the total nameplate rating, exceed the rating of one of the 120 Volt windings.

The same is true of three phase transformers, especially those with 208Y/120 Volt or 480Y/277 Volt secondaries. Remember, each of the three secondary windings of a three phase transformer has a maximum capacity of one-third the nameplate KVA rating. It is always necessary to distribute the single and three phase loads as evenly as possible across the three secondary windings without exceeding their capacity.
Click to expand...

I appreciate the information. Correct me if I'm wrong, according to this information the sole purpose of balancing a load is to ensure the rating of each winding is not exceeded. So hypothetically, if I have a 1000KVA XMFR I can load phase A-N up to 333KVA. If I don't place load on any other phase I haven't violated the rating of the transformer. This is of course assuming I'm operating in an enviroment that doesn't lend itself to deration factors. At full load on one winding I should still maintain nameplate voltage regulation thus there shouldn't be any problems with this application. Except of course that I wasted money buying a 1000KVA 3-phase XFMR instead of a 333KVA single phase XFMR.

Does this sound right?
 
Mike voltage regulation refers to changes in voltage from no load to full load, IE. no load 120 VAC Full load 116 VAC, additionally there is voltage drop on the primary conductors. Lower input voltage and lower out put volatge, and as someone mentioned above if you feed a 3 phase load; espically a motor load from this transformer the imbalance voltage will result in significant imbalance in the motor currents and can result in increase heating and resulting motor damage.

A 100% load on one winding leads to heating of that winding, if the load could be split between the others windings the heat would be lessened and this is better for the transformer.

So well you could run a 3-phase delta wye transformer at 100% load on just one phase it is not a recommended practice, for a variety of reasons.


Thanks to Sola again

http://www.sola-hevi-duty.com/support/faq.html#Q30

Q. What is regulation?

A. Under no load, a transformer is not providing voltage to the output. When a load is applied, the voltage will drop slightly. The difference in the output voltage under load vs. unloaded is referred to as the transformer's output regulation and is normally expressed as a percentage. If under no load a transformer had an output voltage of 240 Volts but under load the output voltage was 230 Volts, the difference would be 10 Volts and the regulation would be 10/240 or 4.17%. The power factor of the load can impact the transformer's regulation. General purpose transformers can be used with a variety of loads, the most common being inductive motor loads and resistive loads. For that reason, it is common to express transformer regulation at 100% power factor and also at 80% power factor.
 
balancing the loads will ensure the efficient operation of a 3 phase motor, should you ever operate one.

most of the customers in our utility apply for a 3 phase service if they have a three phase motor they want to operate for a business. if they have none, a single phase service is sufficient.

the cost between having a three phase service and a single phase service is great, specially if no three phase service exists in the area.
 
brian john said:
...A. Under no load, a transformer is not providing voltage to the output...
Click to expand...
Is that something like if a tree falls in the forest and no one is there to hear it fall, does it make a sound?

Any typical attempt to measure voltage adds a load to the transformer, so how does one know if "Under no load, a transformer is not providing voltage to the output" is a true statement? My belief is to the contrary. Many would say since current is 0 amperes there has to be 0 voltage as a result of the application of Ohm's Law. However, in absolute truth, there is a current, just not one discernably measureable through conventional means. One electron transmitted into and from the surrounding air constitutes a current. Using Ohm's Law again, calculating with near-infinitely-small current and near-infinitely-great resistance, the output has voltage.
 
Smart $ said:
Is that something like if a tree falls in the forest and no one is there to hear it fall, does it make a sound?
Click to expand...

I think Schroedinger's Cat is tired of being killed.

I think it's a safe bet that the output voltage of a transformer with a 120v secondary is 120v. If you plot the voltage from 100% to 0% load, the value approaches 120v as the load approaches 0. Another way of measuring the voltage would be to measure the electric field strength. Since we know than an electric field exists even if no load is present, we know that the voltage must be non-zero.

You can't apply Ohm's Law to this situation.
 
If only one phase of a three pahse XFMR is loaded that will produce a large amount of current in the neutral as a result of the imbalance. Can a large enough current in the nuetral cuase heating issues in the transformer? Can it cuase nuisance tripping?
 
Status
Not open for further replies.
Top