Transformer turns ratio tester

Data records

• A turns ratio tester is verified against a transformer ratio standard, a precision reference with four-terminal connections that supplies known step-down and step-up ratios, used as a laboratory calibration standard or as a field check when a ratio result on a complex transformer is unexpected. Source: Megger product page, web page edition. Verified 2026-10-27. Record instr.transformer-turns-ratio-tester.calibration-ratio-standard
• A transformer turns ratio tester applies a low alternating voltage to one winding of an unloaded transformer, measures the voltage induced in the other winding, and reports the ratio of the two together with the phase relationship between them and the excitation current drawn by the energized winding. Source: Electrical Testing Reference editorial taxonomy, 2026-10 edition. Verified 2026-10-27. Record instr.transformer-turns-ratio-tester.definition
• The excitation current reading, though taken at a lower voltage than a dedicated excitation test, is interpreted by pattern: on a three-phase core the two outer phases draw similar currents and the center phase the lowest, and a departure from that pattern or from earlier results at the same voltage points to core or winding defects such as shorted turns. Source: Megger product page, web page edition. Verified 2026-10-27. Record instr.transformer-turns-ratio-tester.excitation-current-pattern
• The phase deviation between the high and low windings of a sound transformer is very small; a significant deviation reflects an inefficient core, such as shorts between laminations, and phase deviation together with ratio error serves as a low-cost check of the accuracy class of instrument transformers at zero burden. Source: Megger product page, web page edition. Verified 2026-10-27. Record instr.transformer-turns-ratio-tester.phase-deviation
• A change in measured turns ratio from the nameplate value or from earlier results points to shorted turns, open turns, a tap changer malfunction, a core problem, or an incorrect or improper winding connection. Source: Megger product page, web page edition. Verified 2026-10-27. Record instr.transformer-turns-ratio-tester.ratio-deviation-meaning
• The tester reports three quantities for each measurement: the measured ratio, the excitation current, and the phase deviation between the applied and the induced voltage; given the nameplate voltages it also computes the percentage deviation of the measured ratio from the expected ratio. Source: Megger product page, web page edition. Verified 2026-10-27. Record instr.transformer-turns-ratio-tester.readout
• A single-phase tester measures a three-phase transformer one phase at a time, which loses accuracy on a delta-connected low-voltage winding because the winding under test is loaded by the other two legs of the delta; a three-phase source excites all three phases at once, gives a more uniform flux distribution, and makes the result less sensitive to the excitation voltage. Source: Megger product page, web page edition. Verified 2026-10-27. Record instr.transformer-turns-ratio-tester.single-phase-versus-three-phase
• Three-phase turns ratio test sets detect the three-phase transformer connection type automatically from a catalog of types covering the ANSI, IEC and Australian conventions, display winding polarity alongside ratio, phase angle and excitation current, and provide control of an on-load tap changer. Source: Doble (Vanguard Instruments) product page, web page edition. Verified 2026-10-27. Record instr.transformer-turns-ratio-tester.vector-group-detection
• What the instrument measures is the voltage ratio of the transformer; with no load on the secondary that voltage ratio is taken as the turns ratio, and for a three-phase transformer a correction factor that depends on the vector group of the windings may need to be applied before the result is compared with the nameplate. Source: Megger product page, web page edition. Verified 2026-10-27. Record instr.transformer-turns-ratio-tester.voltage-ratio-assumption