Powerline Technician exam: Transformer connections

Key points

• Turns ratio equals voltage ratio. Current goes the opposite way.
• Additive and subtractive polarity change how secondary leads are marked and connected.
• In a wye bank, line voltage is 1.73 times phase voltage.
• In a delta bank, line current is 1.73 times phase current.
• Always confirm phase rotation before connecting a new bank to load.

Study tip

Ratio questions: write primary over secondary and keep it the same way up for every step.

Try these

1. A 14 400 V to 240 V transformer has a ratio of:

   1. 30:1
   2. 60:1
   3. 120:1
   4. 240:1
   Show answer

   B. 60:1 14 400 / 240 = 60.

2. In a wye system, 120 V phase-to-neutral gives line voltage of about:

   1. 120 V
   2. 208 V
   3. 240 V
   4. 347 V
   Show answer

   B. 208 V 120 x 1.73 = 208.

3. Stepping voltage down makes secondary current:

   1. Lower
   2. Higher
   3. The same
   4. Zero
   Show answer

   B. Higher Power stays roughly the same.

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