The polarity of the transformer is important when connecting transformers in parallel to amplify power or connecting multiple single-phase transformers to obtain a three-phase one.
The polarity icons show connections where the input and output voltages are of the same polarity. At this point in time, this is important when connecting current transformers for relay protection and measurement.
You can easily check the polarity of the transformer by using an undervoltage source to energize the primary winding. First move the H1 terminal to the X1 terminal of the transformer. Then connect a voltmeter between terminal H2 and X2. Apply the reduced voltage across H1 and H2 and record the voltage measured at the meter.
Caution: Use the lowest AC voltage capable of energizing the winding to reduce the risk of electric shock. To maintain the minimum test voltage, it is recommended to use a regulated AC voltage source (LATR type).
If the voltage value is equal to the sum of the step-up and step-down windings, it is considered that the polarity of the transformer is additional (additive). Otherwise, if the meter reading is less than the applied voltage, the polarity is subtractive (subtractive). schema
Another rule of thumb for determining the polarity of a transformer comes from ANSI (American National Standards Institute) notation. According to these standards, if you come across the low voltage side of a single phase transformer (the side labeled X1, X2), the H1 connection will always be on your left.
If the pin marked X1 is also on the left, it is subtractive polarity. If the X1 pin is to your right, this is additional polarity.
Think about the polarity of the transformer in terms of the direction of the current. Whenever current flows through the polarity-marked terminal on the primary winding, the current exiting the secondary will move in the same direction, leaving the terminal with the same polarity markings.
Whenever current flows through the terminal with the indicated polarity on the primary winding, the current exiting the secondary winding will move in the same direction, leaving the terminal with the same polarity marking.
Additive polarity is typically found in small distribution transformers. Most high-power transformers have subtractive polarity.
The placement of the terminals in a three-phase transformer is also standardized. The high voltage terminals are located H3, H2, H1 and H0 from left to right when facing the high voltage side of the transformer (See diagram).
In three-phase transformers, the low voltage terminals X0, X1, X2 and X3 are located from left to right
from the low voltage side. The terms "additive polarity" and "subtractive polarity" do not apply to three-phase transformers.
The placement of the terminals in a three-phase transformer is standardized. When handling three-phase transformers on the low voltage side, the low voltage terminals are located XO, X1, X2 and X3 from left to right. The high voltage terminals are located H3, H2, H1 and HO from left to right when facing the high voltage side of the transformer.