Why is the Transformer Rating in kVA Instead of kW?
Transformers play an important role in several industries. From manufacturing and electrical contracting to health care and higher education, several industries use them for their constant and efficient operations. Transformers are significant for large facilities that require substantial amounts of power. However, transformer users always face confusion about why transformer rating in kVA. This blog will address this confusion. However, before that, let’s look at what a transformer is.
What is a Transformer?
A transformer is a crucial device used in the power transmission of electric energy. It is usually used for AC transmission current. The primary role of a transformer is to increase or decrease the voltage supply without altering the AC frequency between circuits. The basic working principles of a transformer include mutual induction and electromagnetic induction.
Understanding the rating of a transformer is crucial if you plan to get one or use it already. One common question that will come to your mind, like several others, is- Why transformer rating in kVA? Continue reading to know an appropriate answer to this and several other questions related to the rating of a transformer.
How is Transformer Rated?
A transformer is first sized by considering the following factors.
- Load demand
- Voltage levels
- Power factor
- Efficiency
- Overload capacity
Once the size of the transformer is determined, it gets rated according to the capacity needed for it to handle power in the electrical system it will serve. The rating is usually expressed in kVA, which depicts the maximum amount of power that a transformer can handle.
What Does kVA mean?
kVA is a measuring unit for apparent power. It measures the total amount of power used by a system. But what does kVA stand for? It stands for Kilo-volt-amperes. One kVA equals 1,000 Volt-Amps. The term apparent power is the product of volts and amps.
Why Transformer Rating in kVA?
The high-temperature loss in a transformer is largely driven by the current, specifically, iron losses on the applied voltage and copper losses. This shows the complete heat loss in a transformer is dependent on the apparent power, measured in volt-amperes (VA), instead of on the angle between the voltage and the current. To put it simply, the energy element does not impact heat loss. That is exactly why transformer ratings are provided in kilovolt amperes (kVA) rather than kilowatts (kW).
Transformers are ranked in kilo-volt-amperes (kVA) rather than kilowatts (kW) since their primary role is usually to transport electric power with no regard to the energy factor of the tons they serve. The kVA rating provides the transformer’s capacity to deal with the all-around apparent power; this includes both energetic power (reactive power and kW) (kVAR). By using kVA, companies make sure that transformers are created to operate reliably under varying load circumstances, regardless of whether the energy element is leading, lagging, or unity. This particular rating approach allows transformers to be robust and flexible in an assortment of electric systems.
Choosing the correct kVA rating for transformers is essential to maintaining efficient and reliable operation, especially in locations with several load demands and fluctuating power factors. But why are transformers rated in kVA rather than kW? This method provides for a correct plus pragmatic analysis of a transformer’s capability to do across a broad range of uses, accounting for equally obvious power and variants in power factor.
Below are the two reasons behind rating transformers in kVA instead of kW.
Power loss and kVA
Transformers are quite efficient in their functionality. However, they are still not perfect. They experience two kinds of losses- copper and iron losses. Both these losses result in the heating of a transformer. Eventually, the heating leads to exceeding thermal limits, which can damage the transformer. These losses are independent of the connected loads’ power factors. Now, kVA is used because it represents the product of current and voltage. It shows the apparent power transferred. The copper and iron losses are determined by current and voltage. Therefore, rating a transformer in kVA helps us determine the maximum capacity for the bearability of these losses while maintaining safe operating temperatures.
Flexibility for Varying Loads
Transformers are used for varying loads with a diverse range of power factors. kW only represents active power. Therefore, choosing kW to rate a transformer would not tell us all the information. This is why kVA is used for transformer rating.
How to Size Transformer kVA?
Figuring out the right kVA size is crucial when buying a transformer. You must start by understanding the terminology or abbreviations related to it, as explained above. Now, as you delve into the world of transformers, you will find small-sized transformers with ratings in VA units. Here VA stands for Volt-Amperes. If you pick a transformer with a 100 VA rating, it will handle 100 volts at one ampere of current.
The large-sized transformers have ratings in kVA. Here 1 kilovolt-amperes represents 1000 volt-amperes. If you pick a transformer with one kVA, it can handle 100 volts at ten amps of current. Follow the steps below to find a transformer with a suitable rating.
- Multiply the secondary amps by low voltage.
- Multiply the resulting number by the square root of three.
- Now, divide the result by 1000 to find the required rating in kVA.
- Choose a transformer with a slightly higher rating than required.
Bottom Line
kVA is the most suitable unit for rating a transformer as it offers a realistic measure of its capability. Also, it is a more versatile option. It can handle the combined influence of active and reactive power. We hope you understand why transformer rating in kVA. Now, calculate the appropriate rating and size for your requirements and shop from Schneider Electric’s e-shop for the best quality.
FAQ
Why is transformer rated in kVA instead of kW?
Hint: Iron loss on voltage and copper loss of a transformer depends on current. Hence, total transformer heat loss depends on volt-ampere (VA) and independent of phase angle among voltage and current i.e., it is independent of power factor. That is why the rating of the transformer is in kVA and not in kW
Why is kVA more than kW?
kW denotes the unit of real power and kVA denotes the unit of apparent power. The power factor, which is defined and known, is, therefore, an approximate value (typically 0.8), and in the kW to kVA calculations, kVA value will always be more than the kW value
Why are generators rated in kVA and not in kW?
Generators are rated in kVA instead of kW because they measure different aspects of the power output. kW is the real power that a generator can supply based on its engine’s horsepower. kVA is the apparent power that a generator can supply if it was 100% efficient
Why are motors rated in kW not in kVA?
Since the power factor is unknown both generator and transformer are rated in kVA. But in the case of a motor, it has a fixed power factor. That is why the power factor is included in the nameplate data. Hence, Electric Motors are rated in kW and not in kVA
Is 1 kVA equal to 1 kW?
The easiest way to think of it is that 1KVA is equal to 0.8 or 0.75KW (0.8 is usual). See explanation below. Most electronics are rated in KW (or W), which denotes how much electricity the appliance will actually consume over time. Which is why your power meter is in KWh
Why are inverters rated in kVA not kW?
Because the AC output may have the voltage and current with a phase difference. Only at zero phase the KVA is equals real KW, otherwise VA is only the apparent power
