Identify Transformer Primary Secondary High Low Voltage Side


There is a lot of confusion came when you going to read about a transformer first time. The main confusion is which side of the transformer is primary, which side is secondary, how to identify the primary side of a transformer, how to identify the low voltage and high voltage side of a transformer. So let's go to know about Transformer Primary Secondary and High Low Voltage Side. In simple word, a transformer is an electrical machine which transfers electrical energy by stepping up or down the voltage but keeping constant the power and frequency. An electrical transformer is the very simplest electrical machine and it works in the principle of electromagnetic induction.


Transformer Primary Side


The input side of the transformer is called Primary Side means, in which side of the transformer is connected to the power supply is called the primary side. Here you can see the below diagram to understand the primary side of a transformer.

identify Transformer primary side, primary side of transformer


  • On the primary side of the transformer, the power supply is connected.
  • In a step-up transformer, the primary side carries low voltage and high current.
  • In a step-down transformer, the primary side carries a high voltage and low current.
  • In a distribution transformer, the primary side consists of winding in a delta connection.
  • In a generation transformer, the primary side consists of winding in star connection.


Transformer Secondary Side


The output side of the transformer is called the secondary side means in which side of the transformer is connected to the load is called the secondary side of the transformer. Here you can see the below diagram to understand the secondary side of a transformer.

identify Transformer secondary side, secondary side of transformer


  • On the secondary side of the transformer, the load is connected.
  • In a step-up transformer, the secondary side carries high voltage and low current.
  • In a step-down transformer, the secondary side carries a low voltage and high current.
  • In a distribution transformer, the secondary side consists of winding in star connection.
  • In a generation transformer, the secondary side consists of winding in the delta connection.


How to identify the Primary and Secondary Sides of a Transformer?


There are various methods to identify transformer primary and secondary sides, it depends on the type of transformer also.

  1. In a large step-down power or transmission transformer, you can identify the primary side by observing the bushing. You cannot get help from the nameplate to identify the primary or secondary side. So the side with bigger bushing is the primary side.
  2. In a medium-size distribution transformer that hangs on the poles, the top side of the transformer which holds the bushing can be the primary side.
  3. In a small transformer used in electrical or electronic devices primary side always carries only two wires, if you observe one side carries more than two wires then this is the secondary side and there are more than two wires because of tappings.
  4. Some transformers are marked with 'H' and 'X'. So remember that 'H' always be the primary side.
  5. You can also identify by the color and thickness of the wires. Generally, the primary side always carries red wires only, and if it is a step-down transformer then the primary side wires will be thinner than the secondary side because the secondary side carries more current.


Transformer High Voltage(HV) Side


The side of a transformer which consists of high voltage winding is called the high voltage side. See the below diagram to understand the high voltage side of a transformer.

identify Transformer High Voltage (HV) Side, high voltage side of transformer


  • The transformer high voltage side carries the high voltage or HV windings.
  • The transformer's high voltage side carries high voltage and low current.
  • The transformer high voltage side winding having more turns than the low voltage side.
  • The conductors of the high voltage windings have a lower thickness than the low voltage side.
  • The resistance of the high voltage winding is always more than the low voltage winding.
  • The high voltage winding of the transformer highly insulated than the low voltage winding.
  • For large transformers, high voltage windings are generally delta connected.


Transformer Low Voltage(LV) Side


The side of a transformer which consists of low voltage winding is called the low voltage side. See the below diagram to understand the low voltage side of a transformer.

identify Transformer Low Voltage (LV) Side, low voltage side of transformer


  • The transformer low voltage side carries the low voltage or LV windings.
  • The transformer's low voltage side carries low voltage and high current.
  • The transformer low voltage side winding having fewer turns than the high voltage side.
  • The conductors of the low voltage windings having a higher thickness than the high voltage side.
  • The resistance of the low voltage winding is always lesser than the high voltage winding.
  • The low voltage winding of the transformer having less insulation than the high voltage winding.
  • For large transformers, low voltage windings are generally star-connected.


How to identify the High Voltage(HV) and Low Voltage(LV) sides of a Transformer?


There are also some methods to identify the high voltage and low voltage sides of the transformer.
  • As we know that, the transformer high voltage side having high resistance than the low voltage side so check with a multimeter of both winding resistance.
  • For large transformers, you can identify the high voltage and low voltage sides by observing the bushing. Generally, in the large step-down transformers, the bushing is used on the high voltage side or primary side of the transformer. And the thicker cables or busbar are used in the secondary side or low voltage side. If any transformer has bushings on both sides, then the long length and thinner size bushing are connected to the high voltage side, and the short length and much thicker size bushing are connected to the low voltage side.
  • By observing the cross-sectional area of the winding we can identify the primary and secondary sides of a transformer. Generally, the LV winding carries more current so the cross-sectional area of the LV winding will be more. So we can easily observe the cross-sectional area of the transformer winding externally of any large power or distribution transformer.
  • For, smaller size tapping transformers used in electrical or electronic devices, we can identify the LV and HV sides by observing the terminals. Generally, the HV sides contain only two terminals whereas the LV side contains more than two terminals.


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Identify Transformer Primary Secondary High Low Voltage Side Identify Transformer Primary Secondary High Low Voltage Side Reviewed by ETechnoG on 2/23/2021 Rating: 5
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