Transformer Connections
Turns Ratio, Polarity and Dual-Voltage Windings, Three-Phase Banks (Delta-Delta, Delta-Wye, Wye-Delta, Wye-Wye, Open Delta), Parallel Operation, Autotransformers and Instrument Transformers
Theory from NEETS Module 2 (NAVEDTRA 14174A) chapter 5 and Module 5 (NAVEDTRA 14177A) 3.12, read 2026-09-20 (read-log-neets.md); NFPA 70-2023 Articles 450 and 250 Part IX as read in read-log-2023.md · Reviewed 2026-09-21
The PSI transformer block (7 items Class I, 21 items Class II — the Class II candidate is tested hard here) is a list of connection tasks: dual-voltage windings, autotransformers, three-phase banks in every combination, open delta, parallel operation, current and potential transformers, load testing and harmonics. Article 450 (lesson nec-07-02) covers protection and installation; this lesson covers how the windings are connected and what comes out. Theory is paraphrased from the public-domain Navy NEETS course.
1. Turns ratio, voltage, current and power (NEETS 2-5.5, 5.6)
- Voltage ratio = turns ratio. A secondary with twice the primary's turns delivers twice the voltage; with half the turns, half the voltage. Current is the inverse: a 6:1 step-down transformer has one-sixth the voltage and six times the current in the secondary. Primary power ≈ secondary power, less the copper (I²R), eddy-current and hysteresis losses; large power transformers exceed 98 percent efficiency.
- kVA is the same on both sides: I = kVA × 1000 ÷ V for single-phase; I = kVA × 1000 ÷ (1.73 × V) for three-phase line current.
- Polarity: the H1/X1 markings show which ends of the windings are instantaneously of the same polarity. Subtractive polarity (H1 and X1 on the same side — ANSI standard for distribution transformers) and additive polarity (X1 diagonally opposite H1) matter when windings are paralleled or banked: connecting like-polarity ends together puts winding voltages in phase; a polarity mistake in a bank or a parallel connection is a short circuit through the windings.
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