Transformer KW
- Thread starter Mike01
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zog
Senior Member
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- Charlotte, NC
There are three ways of expressing power in an AC circuit:
?Real power (P) expressed in watts (W) is the power that is required to perform work.
?Apparent power (S) expressed in volt-amperes (VA) is the total power flowing is the circuit. Transformers are rated in their apparent power expression using units in kVA (kilovolt-amperes), as this value is proportional to the total heating in the windings
?Reactive power (Q) expressed in volt-amperes reactive (VAR) is the power which does no useful work but is required to assist in performing work, such as setting up magnetic fields in motors and transformers. It is often called ?imaginary power.?
A transformer does no work, it just transforms one form of electrical energy to another form of electrical energy. Things that do work transfer electrical energy into other types of energy (Or vise-versa) like heaters (Heat), motors (Rated in Hp but DP to watts, to mechanical energy), generators, lights, etc.
So the answers to your questions are zero, no, and you cant.
The loads that are connected to the SYSTEM the transformer are in will determine the PF and therefore the relationship between KVA and KW (Or P and Q)
?Real power (P) expressed in watts (W) is the power that is required to perform work.
?Apparent power (S) expressed in volt-amperes (VA) is the total power flowing is the circuit. Transformers are rated in their apparent power expression using units in kVA (kilovolt-amperes), as this value is proportional to the total heating in the windings
?Reactive power (Q) expressed in volt-amperes reactive (VAR) is the power which does no useful work but is required to assist in performing work, such as setting up magnetic fields in motors and transformers. It is often called ?imaginary power.?
A transformer does no work, it just transforms one form of electrical energy to another form of electrical energy. Things that do work transfer electrical energy into other types of energy (Or vise-versa) like heaters (Heat), motors (Rated in Hp but DP to watts, to mechanical energy), generators, lights, etc.
So the answers to your questions are zero, no, and you cant.
The loads that are connected to the SYSTEM the transformer are in will determine the PF and therefore the relationship between KVA and KW (Or P and Q)
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Power factor, almost by definition, is related to work being done; it's the relationship of real power (work being done) to reactive power (what is necessary to facilitate the work being done). A transformer doesn't do any work, it just transforms.
How kW of a load relates to transformer kVA is that the transformer has to supply all of the apparent power, both the real and reactive. So if the work being done down stream is done with inductive devices (i.e. motors) which have a lower power factor, the kW (real work output) of the motor will always be lower than the kVA that the transformer is required to deliver to it. But if the load on the transformer is not inductive, i.e. resistive heaters, then the load on the transformer is the same as the load consumed by the heaters. Sure the heaters are measured in kW as well, but the power factor is 1, so the kW load is the same as the kVA. In reality though, loads are rarely all inductive or all resistive, there is usually a mixture.
So since we can't differentiate the load on a transformer in advance, it makes no sense to size the transformer by kW. The only safe way to do it is to size it by kVA, that way you know that it can deliver what is necessary regardless of the power factor of the load behind it.
LOL, crossed in cyberspace with zog. Good to see we agree
How kW of a load relates to transformer kVA is that the transformer has to supply all of the apparent power, both the real and reactive. So if the work being done down stream is done with inductive devices (i.e. motors) which have a lower power factor, the kW (real work output) of the motor will always be lower than the kVA that the transformer is required to deliver to it. But if the load on the transformer is not inductive, i.e. resistive heaters, then the load on the transformer is the same as the load consumed by the heaters. Sure the heaters are measured in kW as well, but the power factor is 1, so the kW load is the same as the kVA. In reality though, loads are rarely all inductive or all resistive, there is usually a mixture.
So since we can't differentiate the load on a transformer in advance, it makes no sense to size the transformer by kW. The only safe way to do it is to size it by kVA, that way you know that it can deliver what is necessary regardless of the power factor of the load behind it.
LOL, crossed in cyberspace with zog. Good to see we agree
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zog
Senior Member
- Location
- Charlotte, NC
Ha! Beat you to the punch this time Weird, basicly the same post.
Weird, basicly the same post.
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- Lockport, IL
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- Semi-Retired Electrical Engineer
I would answer the question a bit differently. A transformer is itself an inductive device. Because of this fact, if you connect it to a purely resistive circuit, the overall power factor will not be exactly 1.0. But the power factor associated with a system is determined primarily by the types of loads that the transformer is seeing.
1. The transformer?s equivalent rating, in units of KW, is any value of KW such that, at the operating power factor, does not drive the transformer?s KVA output above its rating. In other words, it is a useless rating, so we generally don?t talk about it.
2 and 3. It does. If you were to short out the transformer terminals, first turning down the primary voltage so that you don?t get an explosion on the secondary, you could measure a real power and a reactive power, and combine them to calculate a value of power factor. This method for performing a ?power factor test? is sometimes used on motors. But I don?t know if it is used on transformers.
1. The transformer?s equivalent rating, in units of KW, is any value of KW such that, at the operating power factor, does not drive the transformer?s KVA output above its rating. In other words, it is a useless rating, so we generally don?t talk about it.
2 and 3. It does. If you were to short out the transformer terminals, first turning down the primary voltage so that you don?t get an explosion on the secondary, you could measure a real power and a reactive power, and combine them to calculate a value of power factor. This method for performing a ?power factor test? is sometimes used on motors. But I don?t know if it is used on transformers.
zog
Senior Member
- Location
- Charlotte, NC
charlie b said:
But not until you add a "load". If I put a 500kVA transformer with nothing attached to the secondary as a "Load" on a generator how many kW does it draw? (Lets assume no losses)
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
The question was about ratings, not about installations. I can determine the transformer's power factor, using the method I described. We generally don't, because it is of no value.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Now that I think this through again, the answer to the OP's second question is a clear "yes." The ratings given for a transformer include the "X/R Ratio." You can calculate the equivalent power factor by taking the cosine of the arctangent of that ratio.
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Thanks for all the replies, what I was doing was looking at how people view transformers transformers are rated in KVA however most of the panel schedules I see are totaled in watts / va / or kw with that being said if you have 30kw of load I always see a 45KVA transformer? I am trying to understand the relationship, The responses I get from most people are in a typical installation for lighting receptacles, etc what is stated is acceptable, however when you start getting into installations like power houses where the majority of the load 70-80% are motors than this can become an issue with substation transformers and you need to find the kva of the equipment apply the correct power factors to determine the KVA and then size the transformer accordingly, I was just trying to get a better grasp on the concept of KVA rating of transformers and the loads served.
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Thanks for all the replies, what I was doing was looking at how people view transformers transformers are rated in KVA however most of the panel schedules I see are totaled in watts / va / or kw with that being said if you have 30kw of load I always see a 45KVA transformer? I am trying to understand the relationship, The responses I get from most people are in a typical installation for lighting receptacles, etc what is stated is acceptable, however when you start getting into installations like power houses where the majority of the load 70-80% are motors than this can become an issue with substation transformers and you need to find the kva of the equipment apply the correct power factors to determine the KVA and then size the transformer accordingly, I was just trying to get a better grasp on the concept of KVA rating of transformers and the loads served.
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