Introduction
The polarity of the transformer is crucial when paralleling transformers to increase capacity or connecting several single-phase transformers to create a three-phase bank. The polarity marks indicate the connections in which the voltages at the input and output have the same orientation. A polarity test for transformer is crucial when connecting current transformers to protect relays and measuring.
The transformer’s polarity is elaborate by the coils’ direction of rotation around the central point, clockwise or counterclockwise, and the way that leads are connected. Usually, polarity markings on transformer nameplates and transformers are represented by symbols, like dots or plus-minus patterns.
What is the meaning of testing the polarity of transformers?
Polarity refers to the direction in which the induced voltages within the primary and secondary windings of the transformer. If two transformers are connected in a parallel fashion, then the polarity needs to be identified for the correct connecting of transformers. There are two kinds of polarity. One is additive, one is Subtractive, and the other is additive.
Subtractive Polarity (Transformer)
In subtractive polarity, the transformer’s different terminals are connected on the secondary and primary sides.
Additive Polarity (Transformer)
The same terminals for a transformer’s primary and secondary winding are connected with additive polarity.
Quick procedure for Polarity testing of transformer
The short testing of polarity is possible using methods including the following:
1) It is possible to determine the degree of Polarity through Continuity Testing
It is possible to conduct a polarity test using an ohmmeter that has low resistance to perform this test. When you are constantly checking rings and final radial circuits, a portion of the process is to visually inspect the polarity of permanent equipment and the outlets connected to the socket.
2) Polarity testing through observation
By visually checking the cables that connect to the core colors, the precise operation of the cables can be determined. The polarity of the lines must be inspected visually during the entire fitting process, especially when the testing method isn’t feasible.
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3) Live testing
If either of the two methods above is not feasible because of necessity, it is possible to conduct live polarity tests using the standard GS38 voltage.
- Look for LINE terminals and NEUTRAL terminals.
- Make sure to check the NEUTRAL terminal as well as the EARTH terminal.
- The test instrument must indicate the full voltage between the neutral line conductor and the line earth conductor. The test device cannot detect any voltage between Earth and neutral.
Detailed methods for the Polarity of Transformer testing with detail
1) Dot Convention & Notation in Transformer Phasing
The Dot Convention (aka the dot notation) of a transformer technique is utilized to indicate the polarity of transformer windings.
The above figure shows two dots on the same side of the primary and secondary winds. It is because the polarity of both voltages on the ends with dots is in the same order. So that if you have a voltage that is positive in a dotted spot within the first winding, it will also be so in the secondary winding.
In figure B in figure-B, two dots appear in opposite directions. In this situation, the transformer’s windings are wrapped in opposing directions around the core. Therefore, the polarity of the voltages at a dotted line is not in phase. Thus, if you have a positive voltage on a dotted line within the primary winding, it will become negative on the secondary.
To determine the polarity of subtractive or additive, connect one terminal from the primary winding to one terminal of the secondary winding. Then connect the other terminals of secondary and primary windings with the help of a voltage meter.
Additive Polarity
In the case of additive polarity, the voltmeter measures the summation of the primary and secondary voltages. The reading on the voltmeter is known as VC. The primary and secondary voltages are indicated as VB and VA, respectively. In the case of additive polarity, the voltmeter displays:
VC = VA + VB
The circuit diagram for additive polarity can be seen in the image below.
Circuit Diagram of Additive Polarity of a Transformer
Subtractive Polarity
In subtractive polarity, an instrument reads the voltage from the subtraction of secondary and primary voltages. VC indicates the reading of the voltmeter. The equation of voltmeter readings is
VC = VA – VB The circuit diagram for subtractive polarity is illustrated in the image below.
2) ANSI Rule of Thumb for Transformer Polarity
Another method of the determination of transformer polarity is derived from ANSI designations. According to these standards, if we have a low-voltage connection to one-phase transformers (the side marked X1 or X2), The H1 connector will be to your left.
The terminal marked with X1 is located on your left, which is subtractive polarity. If the terminal marked X1 is located on your right side, it’s additive polarity.
Think of transformer polarity in terms of the direction in which current flows. When the current flows through a terminal marked with polarity at the beginning of the winding, the current exiting the secondary winding will go along the same path and leave this terminal with the identical mark of contradiction.
3) Polarity Test by DC Source (Battery)
This method for polarity tests is unsuitable for establishing the polarities familiar to winding transformers with two windings. It can be executed using a DC power source (battery), a switch, and a DC permanent magnet Voltmeter. The diagram of the connection for testing the polarity of the transformer with the dc source is displayed below.
The switch connects to the primary winding. When the switch is closed, it is connected to the primary winding, and the current flows over the primary winding. Current causes flux linkage of both windings and generates EMF on both windings.
The positive Polarity of the induced EMF on the main winding occurs at the point where you connect the battery’s negative terminal. If the voltmeter displays positive readings, the polarity is in the right direction. If the voltmeter is towards the negative, then the polarity is the opposite.
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FAQs
Do transformers have an inverse?
Transformers are described as having either an additive as well as subtractive polarity. The issue is the position of high-voltage terminals concerning the low-voltage terminals when they are removed from the transformer’s casing.
What is the polarity of voltage?
Voltage Polarity
Voltage is a level of energy that differs between two points, and the direction of the voltage shows which one is the one with the greater energy level
