CEA Transformers and Distribution Systems 2 — Questions and Answers
Question 1: What advantage does a delta-wye transformer secondary provide over a delta-delta secondary?
- Higher output voltage only
- A neutral point allowing both single-phase and three-phase loads (Correct answer)
- Smaller physical size for the same KVA rating
- Complete elimination of harmonic currents
Correct answer: A neutral point allowing both single-phase and three-phase loads
The wye secondary's neutral point allows the system to supply both single-phase line-to-neutral loads and three-phase loads from the same transformer bank.
Question 2: What standard secondary voltage does a center-tapped single-phase distribution transformer supply for typical US residential service?
- 480V single-phase
- 208V single-phase
- 120/240V single-phase (Correct answer)
- 277V single-phase
Correct answer: 120/240V single-phase
The standard US residential distribution transformer provides 120/240V single-phase service via a center-tapped secondary, delivering both 120V and 240V.
Question 3: In a three-phase delta transformer connection, how does the line voltage compare to the phase (winding) voltage?
- Line voltage = phase voltage × 1.732
- Line voltage = phase voltage ÷ 1.732
- Line voltage equals phase voltage (Correct answer)
- Line voltage = phase voltage × 2
Correct answer: Line voltage equals phase voltage
In a delta connection, each winding is connected directly between two line conductors, so line voltage equals phase voltage.
Question 4: What is the primary purpose of voltage taps on a distribution transformer?
- To connect the transformer mechanically to its mounting pad
- To adjust the turns ratio and compensate for supply voltage variations (Correct answer)
- To measure the transformer's secondary output current
- To provide the required grounding connection per the NEC
Correct answer: To adjust the turns ratio and compensate for supply voltage variations
Transformer taps provide multiple connection points on the primary winding, allowing the turns ratio to be adjusted to maintain proper secondary voltage despite supply variations.
Question 5: A 480V to 208Y/120V transformer is best described as which winding configuration?
- Delta-delta
- Wye-wye
- Delta-wye (Correct answer)
- Wye-delta
Correct answer: Delta-wye
The 480V primary has no neutral and is connected in delta; the 208Y/120V secondary with a neutral is connected in wye, making it a delta-wye transformer.
Question 6: What does transformer polarity (indicated by dot notation on a nameplate) describe?
- The direction of magnetic flux through the core
- The instantaneous voltage relationship between primary and secondary terminals (Correct answer)
- The direction of current flow in the primary winding only
- The phase angle displacement between primary and secondary voltages
Correct answer: The instantaneous voltage relationship between primary and secondary terminals
Transformer polarity indicates the instantaneous voltage relationship between primary and secondary terminals, which is critical when connecting transformers in parallel or in a bank.
Question 7: Which three-phase transformer connection is most commonly used for commercial and industrial power distribution in the United States?
- Delta-delta
- Wye-wye
- Delta-wye (Correct answer)
- Open-delta
Correct answer: Delta-wye
The delta-wye connection dominates US commercial and industrial distribution because the wye secondary provides a neutral for single-phase 120V loads alongside three-phase service.
CEA Transformers and Distribution Systems 2
What advantage does a delta-wye transformer secondary provide over a delta-delta secondary?