POLARITY TEST IN TRANSFORMER

If the transformers are used in the parallel operation or used in the polyphase circuit, the polarities of primary and secondary terminals must be known at any instant. Due to the potential difference between two points, the current can from the high voltage point to low voltage point. The polarity is nothing but it describes the direction of the current flow. The dot convention is used to identify the polarity of two windings.

- If a current enters to the dotted terminal of one winding, the polarity of the voltage on the secondary winding will be positive at the dotted terminal.

- If current leaves to the dotted terminal of one winding, the polarity of the voltage on the secondary winding will be negative at the dotted terminal.

Assume the primary side of the transformer at a particular instant of time the polarity is A1 which is positive with respect to A2. In this case, if the voltmeter reading V = E1 + E2 means additive polarity, then B1 is negative and B2 is positive at any instant of time A1 is positive and A2 is negative.

Replace dot marks at either positive polarity terminals or negative polarity terminals on both sides.

During this polarity test, if the voltmeter reads sum of two EMFs, the secondary side terminals should assign as opposite polarity with respect to assumed primary polarity.

If voltmeter reading is V = E1 – E2 means negative polarity (difference of two EMFs), then the secondary side terminal should be assigned with the same polarity as assumed primary side.

This test can be easily implemented on a low voltage range of the transformer and cannot be used for a high voltage range of transformer.

DC kick test:

This method is used for low voltage range transformer as well as a high voltage transformer. In this method, the transformer is excited with DC supply. This excitation is not continuously. It is momentarily excitation.

If you give continuous excitation of DC supply than it is possible to damage the transformer because very high current will flow in the primary. So, never excite transformer with DC supply. If you want to excite transformer with DC, excite HV winding because it has a high resistance compared to LV winding.

When DC voltage is applied to the primary winding terminal A1 which is positive with respect to A2. If the switch is closed and opened and momentary supply is given to primary winding of the transformer. In this case, the galvanometer observed a kick.

If the kick is in the forward direction, secondary side terminals should be assigned as the same polarity of primary winding terminals. If dot notation is to be applied, a pair of similar terminals should be assigned with dot marks.

If the kick is in the backward direction, secondary side terminals should be assigned as the opposite polarity of primary winding terminals. If dot notation is to be applied, a pair of dissimilar terminals should be assigned with dot marks.