Testing a transformer involves several steps to ensure its functionality and safety. The process starts with visual inspections for any physical damage or signs of wear and tear. Next, the insulation resistance is checked using a megohmmeter to ensure there are no insulation failures. Turns ratio testing follows, which compares the primary and secondary winding ratios to expected values. Excitation current testing is also performed to verify that the current drawn by the primary winding when energized is within acceptable limits. Finally, performing a winding resistance test using an ohmmeter can identify issues such as open or short circuits within the windings.
There are several methods of transformer testing, each serving different purposes. The insulation resistance test measures the quality of insulation between windings and ground. The turns ratio test ensures the correct ratio between primary and secondary windings. The winding resistance test detects issues within the windings, while the excitation current test checks the magnetizing current. Other methods include dielectric tests such as the applied voltage test and induced voltage test to ensure the transformer’s ability to withstand operational and over-voltage conditions. Oil analysis and dissolved gas analysis can also be conducted for transformers with oil insulation to monitor the condition of the insulating oil.
To determine if a transformer is bad, look for signs such as unusual noises, excessive heat, and physical damage. Conduct electrical tests such as insulation resistance, turns ratio, and winding resistance tests. Abnormal results in these tests can indicate issues like short circuits, open circuits, or insulation breakdown. Additionally, monitor for unusual voltage drops or rises under normal load conditions. If the transformer fails to maintain the expected voltage levels, it may be defective. Overheating and unusual smells, especially in oil-filled transformers, can also signal problems.
Testing a current transformer (CT) with a multimeter involves several steps. First, ensure the CT is de-energized and isolated from the circuit. Set the multimeter to measure resistance and connect it to the secondary terminals of the CT. A reading within the manufacturer’s specified range indicates a good winding; a significantly higher or lower reading suggests a fault. For further testing, apply a known primary current and measure the secondary current to verify the CT’s accuracy. The ratio of primary to secondary current should match the CT’s specified ratio.
Testing a three-phase transformer involves a sequence of checks for each phase. Start with a visual inspection for physical damage. Perform insulation resistance tests between each winding and the ground, as well as between windings. Conduct turns ratio tests for each phase to ensure proper ratios. Winding resistance tests for each winding can reveal issues such as short circuits or open circuits. Finally, apply a balanced three-phase voltage and measure the output to ensure the transformer operates correctly under load conditions.