EETC Transformers 3

Question 1

A transformer's no-load test (open-circuit test) is performed to determine:

Accepted Answer

Core losses and magnetizing current

Answer

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?

Accepted Answer

Circulating currents flow between the transformers even at no load

Answer

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:

Accepted Answer

A narrow B-H hysteresis loop (high permeability, low coercive force)

Answer

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?

Accepted Answer

Wye-wye (Y-Y) with ungrounded neutrals

Answer

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:

Accepted Answer

Full load to no load, expressed as a percentage of full-load voltage

Answer

Voltage regulation = (V_no-load − V_full-load) / V_full-load × 100%, and a lower value indicates better performance.

(EETC) Electrical Technician Certification Practice Test