The transformer DC resistance tester is a commonly used tool for measuring DC resistance in transformers. This specialized instrument is designed to measure the DC resistance of transformer windings and other electrical components. It typically utilizes a low-frequency AC source and a sensitive DC meter to measure the resistance of the winding, which is then displayed on a digital readout.
The following are the basic steps involved in measuring the DC resistance of a transformer:
1. Observe all safety rules.
2. Measure winding temperature.
3. Wheatstone Bridge (windings >1 Ω)
• Check the condition of the internal batteries
• Adjust the Galvanometer to reference the null mark
• Use test leads with 0.1 Ω or less
• Bridge test leads
• Make a low-resistance connection to bushing terminals
• Make temperature measurements of the winding
• If applicable, operate the tap changer several times
• Measure bridge winding plus test-lead resistance • Subtract test-lead resistance and compare with factory test data
• Record temperature and resistance data on the test form
• When disconnecting test leads, do not become in series with the test lead and winding terminal
4. Kelvin Bridge and Volt-Amp Method (windings <1 Ω)
• Check the condition of the internal dry-cell batteries, 12-V car batteries, or regulated supply
• Warm-up regulated supply
• Adjust the Galvanometer to reference the null mark
• Use adequate-size test leads with soldered connections to spade lugs and clamp-type terminals
• Connect potential leads inside current connections, closest to the unknown resistance
• Measure the resistance of the standard resistor
• Operate the tap changers several times to ensure tap contacts have low resistance
• Make low-resistance test-lead connections directly to bushing or winding terminals
• Secure leads to the bushing terminals
• Make temperature measurements of the winding
• Measure the winding resistance for each tap and each winding
• Record temperature and resistance data on the test form
• Discharge stored energy in the winding
• Disconnect test equipment