The core of a transformer is made of laminated sheets of magnetic material, typically silicon steel. It serves to carry the magnetic flux generated by the alternating current in the primary winding, which induces voltage in the secondary winding. Without the core, efficient magnetic coupling would not occur.
What is the primary purpose of using a laminated core in transformers?
A Increase efficiency
B Improve voltage regulation
C Increase magnetic loss
D Reduce eddy currents
Laminating the core helps reduce eddy current losses in a transformer. The thin layers of insulation between the laminations prevent the formation of large circulating currents, which would otherwise cause energy loss and heat generation, reducing the transformer’s efficiency.
In a transformer, what determines the voltage ratio between the primary and secondary windings?
A Number of turns
B Resistance
C Current
D Frequency
The voltage ratio in a transformer is determined by the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. This is described by the formula: V1/V2 = N1/N2, where V1 and V2 are the primary and secondary voltages, and N1 and N2 are the number of turns.
What type of transformer is typically used in residential electrical systems?
A Autotransformer
B Step-up transformer
C Distribution transformer
D Isolation transformer
A distribution transformer is used in residential electrical systems to step down high-voltage electricity from power lines to a lower voltage suitable for household use, ensuring safe distribution of power for everyday appliances and lighting.
What happens to the efficiency of a transformer as the load decreases?
A Efficiency stays constant
B Efficiency decreases
C No effect on efficiency
D Efficiency increases
Transformer efficiency tends to decrease as the load decreases. When the transformer operates under light load conditions, core losses (which are constant) become a larger percentage of the total losses, resulting in a lower efficiency. Efficiency is highest at or near the rated load.
In a transformer, what causes the magnetizing current?
A Magnetic flux
B Core losses
C Induced voltage
D Voltage imbalance
The magnetizing current is the small current required to establish the magnetic field in the transformer’s core. It is primarily influenced by the voltage applied and the properties of the core material, ensuring the transformer operates correctly by producing the required magnetic flux.
What is the main function of a conservator in an oil-filled transformer?
A Prevent short-circuit
B Increase insulation
C Regulate oil temperature
D Remove impurities
The conservator in an oil-filled transformer holds extra transformer oil and ensures that the oil remains at a stable level and temperature. As the transformer operates and heats up, the oil expands and flows into the conservator, preventing over-pressurization and aiding cooling.
What type of transformer is used to change the voltage without changing the frequency?
A Isolation transformer
B Both step-up and step-down transformers
C Step-down transformer
D Step-up transformer
Both step-up and step-down transformers are used to change the voltage in an AC circuit without altering the frequency. The frequency remains the same; only the voltage is increased or decreased depending on the transformer’s winding ratio.
What occurs when a transformer operates below its rated capacity?
A Higher efficiency
B Overheating
C Lower efficiency
D Reduced core losses
When a transformer operates below its rated capacity, core losses (which are constant) become a smaller fraction of the total losses. Although the efficiency may drop under low load, core losses are minimized in comparison to copper losses.
What type of transformer is used to reduce harmonics in power systems?
A Phase-shifting transformer
B Auto-transformer
C Step-up transformer
D Isolation transformer
A phase-shifting transformer is used to reduce harmonics in power systems. By shifting the phase angle between voltage waveforms, this transformer helps cancel out harmonic components and improve the power quality in the system.
What determines the current-carrying capacity of a transformer’s windings?
A Frequency
B Voltage rating
C Cross-sectional area
D Number of turns
The current-carrying capacity of a transformer’s windings is determined by the cross-sectional area of the conductor. Larger areas allow more current to pass through without excessive heating or voltage drop, ensuring safe operation under load conditions.
What is the most common material used for transformer windings?
A Gold
B Copper
C Aluminum
D Steel
Copper is the most commonly used material for transformer windings due to its high electrical conductivity, allowing efficient current flow with minimal losses. Copper windings are durable and provide the best combination of efficiency and cost for most transformer applications.
What is the primary purpose of using a tap changer in transformers?
A Control efficiency
B Adjust current
C Protect the transformer
D Adjust voltage
A tap changer is used to adjust the voltage in a transformer by selecting different turns ratios within the winding. This is particularly important for maintaining voltage stability during varying load conditions and ensuring that the voltage stays within desired limits.
What is the main disadvantage of using an autotransformer?
A High cost
B High efficiency
C No isolation
D Limited voltage adjustment
An autotransformer provides no electrical isolation between the primary and secondary windings, which can be a safety concern in certain applications. While it is more efficient and compact, the lack of isolation makes it unsuitable for sensitive or high-risk electrical systems.
What test is used to measure the short-circuit impedance of a transformer?
A Short-circuit test
B Impedance test
C Load test
D Open-circuit test
A short-circuit test is conducted by applying a reduced voltage to the primary winding and measuring the current, while the secondary winding is short-circuited. This test helps determine the transformer’s short-circuit impedance, which is essential for fault analysis and protection.