Transformer Sizing
Posted by Anup Mohan on Thursday, November 3, 2011 Under: Transformers
Any transformer is rated based on their losses. The losses of the transformer i.e. the Iron loss depend on the voltage and the copper loss depends on the current. Since the loss does not depend on the power factor, a transformer is rated in terms of kVA (kilo-Volt-Ampere).
A transformer is sized based on the load connected to it. There are two types of load, viz. Continuous load and Intermittent load. As the name suggests, continuous load will always be connected with to secondary of the transformer while the intermittent loads are the one that is not intended to run at all time.
Simply saying, in a factory all the motors and other such machines are intended to run all the time except for some maintenance purpose. This is the connected load. While the lighting loads and EOT cranes are the intermittent loads, because it is not going to be used all the time.
The transformer is to be sized considering the total load that is to be connected on the transformer.
The standard ratings of the transformers available in market are 5kVA, 30kVA, 45kVA, 75kVA, 112.5kVA, 150kVA, 225kVA, 300kVA, 500kVA and 750kVA etc. The total load on the transformer is calculated and the nearest standard rating is to considered as the required transformer rating.
In order to explain this context in a more practical way, consider that we are having a panel board serving a load, say air conditioners of 1.85 kW ( a 1.5 tonne AC consumes approximately 1.85 kW/hr ) and some power receptacles with a load of 1.5 KW. Let us consider this to be the continuous load. A watering pump of 2kW as the intermittent load on our transformer. For calculating the rating of the transformer, only 50% of the intermittent load is considered.
The load in kilo-watt is converted into kilo-volt-ampere by dividing it with the product of efficiency and the power factor.
From the above example, we have a continuous load of 3.35 kW which is 4.2 kVA ( assuming and efficiency of 80 and power factor of 0.8 ). For this a transformer of rating of 5kVA can be chosen.
Similarly, in case of large demand such as in industries, a 10% to 20% margin is considered for the purpose of meeting the starting VA requirements of the motor with largest rating.
A transformer is sized based on the load connected to it. There are two types of load, viz. Continuous load and Intermittent load. As the name suggests, continuous load will always be connected with to secondary of the transformer while the intermittent loads are the one that is not intended to run at all time.
Simply saying, in a factory all the motors and other such machines are intended to run all the time except for some maintenance purpose. This is the connected load. While the lighting loads and EOT cranes are the intermittent loads, because it is not going to be used all the time.
The transformer is to be sized considering the total load that is to be connected on the transformer.
The standard ratings of the transformers available in market are 5kVA, 30kVA, 45kVA, 75kVA, 112.5kVA, 150kVA, 225kVA, 300kVA, 500kVA and 750kVA etc. The total load on the transformer is calculated and the nearest standard rating is to considered as the required transformer rating.
In order to explain this context in a more practical way, consider that we are having a panel board serving a load, say air conditioners of 1.85 kW ( a 1.5 tonne AC consumes approximately 1.85 kW/hr ) and some power receptacles with a load of 1.5 KW. Let us consider this to be the continuous load. A watering pump of 2kW as the intermittent load on our transformer. For calculating the rating of the transformer, only 50% of the intermittent load is considered.
The load in kilo-watt is converted into kilo-volt-ampere by dividing it with the product of efficiency and the power factor.
From the above example, we have a continuous load of 3.35 kW which is 4.2 kVA ( assuming and efficiency of 80 and power factor of 0.8 ). For this a transformer of rating of 5kVA can be chosen.
Similarly, in case of large demand such as in industries, a 10% to 20% margin is considered for the purpose of meeting the starting VA requirements of the motor with largest rating.
In : Transformers
Tags: 'rating' 'sizing' 'kva'
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