Yes, the auto-generated links will take you right to the definitions. Here the summary:
If it was only a resistive load, KW would be the only number you would need to deal with. Unfortunately, resistive loads (Power Factor 1) are getting to be the minority. Although kVA may sound like the same thing since "V" volts times "A" amps equals watts, that's not what we are looking at. Any device that has a Power Factor of less than 1 (HID lamp ballasts, Dimmers, Computer power supplies, etc) draws its maximum current in a non-liner way. Because of this, they draw a disproportionate amount of current (amps) to the amount of voltage on the line. Lets say we had a 1000 watt load with a less than 1 power factor. It may be drawing 10 amps on a 120 volt line. Even though its KW = 1, its kVA would be 1.2
Even though it is only consuming 1000 watts, it is drawing that current out-of-phase with the voltage waveform. Since the generator windings produce heat based on amperage, we would need one that could handle the kVA rating as compared to the KW rating.
Yes, the auto-generated links will take you right to the definitions. Here the summary:
If it was only a resistive load, KW would be the only number you would need to deal with. Unfortunately, resistive loads (Power Factor 1) are getting to be the minority. Although kVA may sound like the same thing since "V" volts times "A" amps equals watts, that's not what we are looking at. Any device that has a Power Factor of less than 1 (HID lamp ballasts, Dimmers, Computer power supplies, etc) draws its maximum current in a non-liner way. Because of this, they draw a disproportionate amount of current (amps) to the amount of voltage on the line. Lets say we had a 1000 watt load with a less than 1 power factor. It may be drawing 10 amps on a 120 volt line. Even though its KW = 1, its kVA would be 1.2
Even though it is only consuming 1000 watts, it is drawing that current out-of-phase with the voltage waveform. Since the generator windings produce heat based on amperage, we would need one that could handle the kVA rating as compared to the KW rating.
Are you sure that you dont have this backwards? KVA does not calculate power factor, KW does. V*A = KVA. V*A*1.73(for 3 phase)*PF/1000 = KW. Most commercial generators are rated in KW not KVA and most are rated at a .8PF.
While it is a number between 1 and 0, it is also expressed as a leading pf or lagging.Since Power Factor is a number between 0 and 1,
KW is still used as a standard in the generator world. For example, I had a customer that had a 100Kw generator that was powering an elevator (among other things). The elevator had a .5PF on start up which is the same as your above example and is very extreme. If you were to design the above circuit you would correct the PF because why would you pull 20a when you can pull 8a? To date the worst I have seen was a .47. This was throwing the generator into all kinds of issues mainly the transfer switch would not go closed transition. The solution was to put a 300Kw back end on it to give it better in rush handeling and change the name plate. It was a 100Kw 300KVA generator. As opposed to 100Kw 120KVA.We are moving into a world where kVA is more important than KW.
KW is still used as a standard in the generator world.
Also Kw is related more to engine BHP in generators. With my above example, the generator was not suddenly capable of more than 100Kw. And imagine the customer sold that generator and based on your example of KVA trumping Kw.
I'm not sure I entirely agree with that from what I have seen in this part of the world. It seems to me that smaller gensets are rated in terms of their apparent power rating (kVA), while when we start talking about about large generators such as those found in power stations (I'm a power engineering student) we discuss them in terms of their real power rating (MW), although you of course have to pay attention to their reactive power capabilities especially as they are often under/over excited to provide reactive power compensation to the network.
In essence what I am saying is that the two ratings are not the same thing, but both of them can be very important in calculating the parameters of a system.
And that's the exact paradox. KW relates more to the work being done, so having a low PF does not require a bigger engine, but simply heavier windings in the gene to handle the higher current. Most low pf systems can achieve some correction. The best example is the HID ballast, where a capacitor can bring the current phase closer to the voltage phase. Other low pf systems are harder to correct. Computer power supplies pull all their current near the peak, and draw little on the lead or lag. In this case, the center of the current waveform is in phase with the voltage waveform, but the current waveform is more of a square wave as current only flows when the supplies diodes are in forward conduction with the line voltage higher then the supply capacitor's current state.
In my 50 volt bulb example, obviously going with a transformer would bring it close to 1, and adding a cap would help move the demand closer to the phase, but, the concept was a worst case scenario. Dimmers are also a big problem as the phase varies with the dimmer setting, so a fixed value correction would not work. Dimmers are also the one where you can end up with PF's well below 50 when the setting is below 10%. Conversely, at 100% the almost achieve a 1.
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