Have you ever noticed that Transformers, generators have power rating in kVA but Induction motors and Home appliances have power ratings in kW? Before answering this question “why transformer is rated in kVA not in kW”, we have to first learn that what is kVA and kW.
What is power rating?
In electrical terms we can say that, power rating of a particular electrical machine denotes the voltage at which the appliance is designed to do work and the current consumption at that voltage.
In simple words, the power rating of an appliance is related to the power it consumes. Power rating is given on the rating plate of that device.
Types of power rating
- Power rating in Volt-Amperes. (ex. Iron, hair dryer, etc.). It is also known as Apparent Power.
- Power rating in Watts. (ex. Transformers, generators etc.). It is also known as Active power or Real power.
Volt-ampere = V *A
Watts = V * A * cosɸ
Here, V = Voltage in volts
A = current in amperes
cosɸ = Power factor
We can write 1000VA as 1kVA and 1000W as 1kW. (here ‘k’ is for kilo.)
We can clearly see that, there is a difference of cosɸ between both formulas.
Now, first we will understand that why induction motors are rated in kW?
Why induction motors are rated in kW?
We already know that Induction motors are widely used motors in homes and Industries. You might have seen a fan motor or a cooler motor. If you read the name plate of a fan then you will find its rating is written in kW.
Actually, power rating of an appliance can be given in two ways only, either it will be in kVA or in kW.
- To give the rating of a machine in kVA, we need its maximum Voltage and current through its windings at that voltage.
- To give the rating in kW, along with voltage and current we need one more thing which is Power factor of the load.
Types of load :
- Resistive load
- Capacitive load
- Inductive load
All three loads have different power factor. Power factor of a Resistive load is Unity. For capacitive and inductive load it is leading and lagging respectively. To learn about power factor in detail, you can read our article, click here.
When a manufacturer is designing the Induction motor, He knows that what is the maximum voltage of this motor and what maximum current can easily pass through its windings.
He also knows that where the particular motor is going to be used and which load is going to be connected with it. Like, a fan motor is going to be used in a fan only. In this way, the manufacturer also knows the power factor.
Now, rating can be given in kW because all three parameters are known which are necessary to find watts. (Watts = V * A * cosɸ)
That’s why induction motors are rated in kW. I hope you clearly understood this topic.
Now we will go to the main question that Why transformer is rated in kVA not in kW?
Why Transformer is rated in kVA not in kW?
Transformers are basically used to change the voltage level from primary side to secondary side. They do not consume any Power. Basically we can say that, transformers are used to transfer Electrical energy.
When a manufacturer designs a Transformer, He knows about the voltage rating of both Primary and secondary side and he also knows the Current through its windings. But he doesn’t have any knowledge about power factor of the Load.
This is because, we do not know that which load is going to connect with the transformer and what will be its power factor.
For example, a transformer can use to supply an Inductive load like fan. And it can also be use to supply a resistive load like Iron or heater.
We know that, both types of load have different power factor and we can not determine that which particular load is going to connect with the transformer.
So, manufacturer can not predict the type of load and hence he doesn’t know anything about power factor of the load. So, he can not give the rating in kW (as power factor cosɸ is needed to find power rating in Watts).
In this way, Power rating of transformer is given in kVA not in kW.
This is the only correct reason for this question.
How to know power rating of a device without seeing its Rating plate?
To know that which devices are rated in kVA and which are rated in kW. This is the Simple way to find it.
- The devices which give electrical energy in their output are generally rated in kVA.
Example: transformers, generators, transmission lines, UPS, stabilizer etc.
- The devices which do not give electrical energy in their output, but give some other form energy like Mechanical energy or heat energy etc. are generally rated in kW.
Example: fan, geyser, heater, iron, refrigerator, washing machine etc.
You can also watch Video of this topic in Hindi.
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