This question is very common. Many people are puzzled when they first encounter transformers: "Transformers don't generate electricity, so why use such a strange unit?" In fact, using kVA is because it is more scientific and better meets practical needs. Let's start from the working scenario and explain it simply.
1. What exactly does a transformer transmit?
First of all, a transformer is a "mover." Its task is to transport electrical energy from the high-voltage side to the low-voltage side, or vice versa. The problem is that electrical energy does not have only one "form"; it is divided into three types:
1. Active Power (kW): This is the "working power," for example, making motors turn, bulbs light up, and air conditioners cool down; this is its role.
2. Reactive Power (kvar): Although it does not "do work," it maintains the magnetic field of motors and the electromagnetic conversion of transformers, which is also essential.
3. Apparent Power (kVA): The "sum" of the two above, which is the "total power" actually flowing in the power system.
In other words, the transformer is responsible for transmitting active power and must also carry reactive power, just like a factory owner must pay salaries (active) and rent (reactive). Therefore, using kVA to indicate capacity means it represents the total energy it can "move."
2. Is it okay to use kW to indicate capacity?
No! Why? Because kW only accounts for the "working part" and does not consider reactive power at all. However, in practical work, reactive power is unavoidable, especially for motor equipment, such as:
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Motors: They draw a lot of reactive power when starting; otherwise, they won't turn.
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Inverters: When working, the power factor is not high, and they also require a lot of reactive support.
If the capacity of the transformer is marked in kW, it would be like telling someone, "This truck can carry 3 tons of bricks," but not mentioning "whether it can carry foam boxes, straw mats, and other things." When someone actually loads it, they only realize something is wrong when it is overloaded.
3. The impact of power factor on transformers
Pure electric heating devices: The power factor is close to 1, and it is basically all active power; the power factor (cos𝜙) commonly mentioned by electricians is simply the ratio of active power to apparent power. Different devices have very different power factors, for example:
Inductive loads (motors): The power factor is low, possibly only 0.7 or even lower.
If the power factor is 0.8, then 20% of the apparent power is reactive power. At this time, a 100kVA transformer can only provide 80kW of active power. Therefore, marking it in kW completely fails to clarify the actual total power it can carry.
4. Why is using kVA more scientific?
The heating of the transformer is mainly caused by current, which is proportional to apparent power. By marking in kVA, it can intuitively reflect the current limit that the transformer can carry, regardless of whether the load is more "working" or more "magnetic field consuming."
For example: You have a 100kVA transformer, and when the power factor is 0.8, it can output 80kW;
If the power factor changes to 0.6, it can only output 60kW. No matter how it changes, the limit of 100kVA will not change. As long as you do not exceed this "total power," the transformer will not be overloaded.
5. A relatable understanding: Why not kW?
You can think of the transformer as a truck:
- kVA is the total load of the truck (including bricks, useful cargo, and packaging foam).
- kW only counts the weight of the bricks (the working part).
- Power factor is the "proportion of bricks"; if there is more foam, there will be fewer bricks.
Using kVA is more comprehensive, informing users how much this transformer can actually carry, rather than just focusing on the bricks.
Conclusion
The capacity of transformers is indicated in kVA rather than kW because:
1. It carries "total power," which includes both useful and non-useful (but necessary) power.
2. kVA is directly linked to current, reflecting the heating limit of the transformer and protecting equipment safety.
3. Using kW is unscientific, which can lead to underestimating the true capacity of the transformer, even causing misuse and overload.
Transformers are like "movers," using kVA to mark capacity is both scientific and professional! This is the recognized rule in the industry.