Why transformer rating is always in kva

Why is an Alternator or Generator Rated in kVA, Not in kW? As we already know that why is a √3 V L I L Cos Φ delivered by the alternator and generator for the same value of Power Factor = Cos ϕ) of the load. In other words, generators and alternators don’t deliver Watts but both of active and reactive powers which depends on the power factor of the connected load to it. Keep in mind that the conductors of an alternators are calculated for a definite current and the insulation at the magnetic system are designed for a definite For this reason In very short, a generator generates both The main factors manufacturers consider while designing electrical devices and appliances which provide So they simply design and rate the electrical device according to its maximum current output that the conductors can safely carry while they consider unity power factor (In case of pure resistive load). Related Posts: • • If we connect inductive or capacitive load (When power factor is not at least unity), The output would differ as there are For this reason, KVA is an apparent power which does not take into account the PF (Power factor) instead of KW (Real Power). Where:

The power factor at the transformer secondary and primary is equal to the combined power factor of all types of electrical loads connected to the transformer. Thus the transformer does not have fixed If transformer is rated in kW,the designer has to fixed the kVAR capacity of the transformer.However, kVAR depends on the types of loads connected to the transformer and it is not possible to fix the kVAR delivering capacity of the transformer. In other words, if at the designed power factor of the transformer. However, it is not pragmatic to operate the transformer at particular power factor or at particular kVAr because the load power factor varies and the system power factor changes with variation in power factor of the connected loads. The transformer designer does not know at what power factor the customer will operate the transformer. The designer gives the choice to operate the transformer at whatever power factor by giving the rating of transformer in apparent power (KVA or MVA). kVA ⃗= kW ⃗+ kVAr ⃗ kVA 2 = kW 2 + kVAr 2 kVA =√(kW 2 +kVAr 2 ) ——–(1)

Transformer rated in KVA/MVA. Why? Explained Transformer rated in KVA/MVA. Why? Explained https://www.theelectricalguy.in/wp-content/uploads/2020/06/Thumbnail-1-1024x576.jpg 1024 576 Gaurav J Gaurav J https://secure.gravatar.com/avatar/87a2d2e0182faacb2e003da0504ad293?s=96&d=mm&r=g June 24, 2020 June 24, 2020 Transformers are rated in KVA or MVA. Why we cannot rate them in kW or kVAR? In this tutorial, let’s understand this. Before moving on in this tutorial, I strongly recommend you to go through my tutorial on active reactive and apparent power. This will help you to understand the concept more clearly. Already done watching? Then let’s start. Transformer rated in W/KW/MW Let’s say, we have a transformer to which we only resistive load will be connected. Resistive load can be, lights, electric heaters, ovens, etc. As the load is only resistive load, we know that, it is only going to draw active power. Therefore, in such case, I can rate my transformer in W, kW or MW. The condition is load must be purely resistive, and load should not draw any reactive power. If this condition is met, then I can rate my transformer in W, KW or MW. For instance, I can rate my transformer as 100 KW, which indicates I can take 100kW of active power from the transformer. Transformer rated in VAR/KVAR/MVAR Now, let’s understand the scenario 2. In this case, let’s say, I know load on my transformer will be purely inductive. Examples of inductive loads are induction motor, fans, pumps, relay, et...

The power rating of a device is the maximum magnitude of power that can be allowed to operate by the device. Active power, Reactive power, and Apparent power are rated in watts (W), Volt-ampere reactive (VAR), and Volt-ampere (VA) units respectively. Active power = V I cos ɸ, unit is watts or W (power factor, pf = cos ɸ ) Reactive power = V I sin ɸ, the unit is volt-ampere reactive or VAR Apparent power = V I, unit is Volt-ampere or VA Apparent power is the resultant of Active and Reactive power. The apparent power value is independent of the power factor or the phase angle of the electrical system. • • Transformer power loss and rating For a transformer, its power rating is in KVA (Apparent power) not in KW (active power). The main reason for the transformer rating in kva is its power loss. A transformer has two types of power loss, copper loss, and Iron loss. Copper Loss Copper loss occurs due to the ohmic resistance in the primary and secondary winding. The copper loss varies with the amount of current through the windings. Iron Losses or Core Loss. An iron loss in the transformer core occurs due to the alternating flux. It consists of eddy current loss and hysteresis loss. The copper loss of a transformer depends on its current and the iron loss depends upon the voltage. Hence the total loss depends upon the voltage and the ampere, but it is independent of the power factor or the phase angle between them. The winding temperature and oil temperature of the transformer r...

For any type of machine, rating means the output. Also, we know, OUTPUT= INPUT – LOSSESS As the transformer is connected to an Infinity system (Electrical Grid), the INPUT is unlimited. Hence, the output is limited by the Transformer losses. The losses produce heat in the transformer and if the heat is not within limits or not dissipated in time, the transformer cores, transformer winding and the transformer oil etc. may damage. This may cause failure of the transformer. Reason 1: Transformer has mainly two losses. Iron loss and Copper loss. Iron loss (P i)= hysteresis loss + eddy current loss Hysteresis loss, P h = k hf B x m Eddy current loss , P e = k ef 2 B 2 m Both the Hysteresis and eddy Current loss depend on Max. Flux density (B m). The Max. flux density depends on (V m/f). Means, the iron loss depend on voltage. For, more details if we put the value of B m , Hysteresis loss depends only on Voltage and Eddy current loss depends on frequency and square of voltage. As the Voltage and frequency of a transformer is constant, the Iron loss is constant. Copper loss= I 2R. The copper loss depends on the current. Both the losses of transformer is depends on Voltage and Current. The power factor has no role in transformer losses. Hence, the output is depends on the product of Voltage and Current, i.e VA and dividing by 1000, we may say the losses depend on the KVA. Reason 2: At the time of design, the designer doesn’t know about the type of load on that transformer. For Exa...