EETC Transformers 3 — Questions and Answers
Question 1: A transformer's no-load test (open-circuit test) is performed to determine:
- Core losses and magnetizing current (Correct answer)
- Copper losses and winding resistance
- Short-circuit impedance
- Efficiency at full load
Correct answer: Core losses and magnetizing current
The open-circuit test is conducted at rated voltage with the secondary open to measure core (iron) losses and the magnetizing branch parameters.
Question 2: What is the effect of connecting transformers with different voltage ratios in parallel?
- Circulating currents flow between the transformers even at no load (Correct answer)
- They share load in proportion to their kVA ratings
- The combined voltage ratio equals the average of both ratios
- No adverse effect as long as impedances match
Correct answer: Circulating currents flow between the transformers even at no load
Mismatched voltage ratios create a voltage difference that drives circulating currents through the low-impedance winding loops, causing overheating.
Question 3: Hysteresis loss in a transformer core is minimized by using core material with:
- A narrow B-H hysteresis loop (high permeability, low coercive force) (Correct answer)
- A wide B-H loop and high residual magnetism
- High resistivity and thick laminations
- Low permeability and high saturation flux density
Correct answer: A narrow B-H hysteresis loop (high permeability, low coercive force)
A narrow B-H loop means less energy is lost per magnetization cycle; silicon steel is used because it has this property.
Question 4: Which transformer connection is most susceptible to third-harmonic voltage distortion on the secondary side?
- Wye-wye (Y-Y) with ungrounded neutrals (Correct answer)
- Delta-wye (Δ-Y)
- Delta-delta (Δ-Δ)
- Wye-delta (Y-Δ)
Correct answer: Wye-wye (Y-Y) with ungrounded neutrals
An ungrounded Y-Y transformer cannot circulate third-harmonic currents, so they appear as third-harmonic voltages on both windings.
Question 5: The regulation of a transformer is defined as the change in secondary terminal voltage from:
- Full load to no load, expressed as a percentage of full-load voltage (Correct answer)
- No load to full load, expressed as a percentage of rated kVA
- Primary to secondary, expressed as the turns ratio
- Open circuit to short circuit conditions
Correct answer: Full load to no load, expressed as a percentage of full-load voltage
Voltage regulation = (V_no-load − V_full-load) / V_full-load × 100%, and a lower value indicates better performance.
Question 6: A step-up autotransformer has a common winding of 120V and a series winding of 80V. The input is 120V. What is the approximate output voltage?
- 200V (Correct answer)
- 160V
- 80V
- 240V
Correct answer: 200V
In a step-up autotransformer, Vout = V_common + V_series = 120V + 80V = 200V.
Question 7: What does the polarity marking 'H1' and 'X1' on a single-phase distribution transformer indicate?
- These terminals have the same instantaneous voltage polarity (Correct answer)
- H1 is always positive and X1 is always negative
- H1 is the high-voltage terminal and X1 is the low-voltage terminal only
- They indicate terminals with opposite polarity
Correct answer: These terminals have the same instantaneous voltage polarity
ANSI/IEEE standards define H1 and X1 as 'like' polarity terminals — when H1 is positive, X1 is also positive.
EETC Transformers 3
A transformer's no-load test (open-circuit test) is performed to determine: