Which of the following is a primary function of a step-down transformer?
A To increase the voltage
B To decrease the voltage
C To reduce the current
D To regulate the frequency
A step-down transformer is used to reduce the voltage from the primary side to a lower level on the secondary side. The number of turns in the secondary winding is fewer than that in the primary winding, which reduces the voltage while maintaining the same frequency.
What does a transformer’s impedance mainly consist of?
A Resistance and capacitance
B Resistance and reactance
C Resistance and voltage
D Voltage and current
The impedance of a transformer is the total opposition to alternating current, which includes both resistance (the real part) and reactance (the imaginary part). Reactance is caused by inductance and capacitance within the transformer windings, while resistance is the opposition to current flow due to the material of the windings.
Which type of transformer is used to maintain voltage stability and prevent power loss in long-distance transmission?
A Step-up transformer
B Step-down transformer
C Isolation transformer
D Autotransformer
Step-up transformers are used in power transmission to increase the voltage and decrease the current, which reduces power losses during long-distance transmission. Higher voltage allows for the use of thinner conductors, reducing energy losses due to the resistance of the transmission line.
What is the primary purpose of the cooling system in oil-filled transformers?
A To increase the insulation resistance
B To prevent the transformer from overheating
C To enhance the transformer’s magnetic field
D To regulate the voltage
In oil-filled transformers, the oil serves as both an insulating material and a coolant. It absorbs the heat generated during the transformer’s operation and dissipates it into the surrounding environment. This prevents the transformer from overheating and ensures efficient operation.
Which of the following is the best insulating material for transformers?
A Wood
B Oil
C Air
D Water
Oil is commonly used as an insulating material in transformers because it has excellent dielectric properties and can efficiently dissipate heat. This makes it ideal for maintaining the electrical insulation and cooling of the transformer during operation.
What type of transformer is typically used to convert high-voltage AC to low-voltage AC for household appliances?
A Distribution transformer
B Isolation transformer
C Power transformer
D Autotransformer
A distribution transformer is commonly used to step down high voltage from power lines to the low voltage required for household appliances. These transformers are typically found in electrical substations and on utility poles.
Which test is used to evaluate the copper losses in a transformer?
A Open-circuit test
B Short-circuit test
C Load test
D Insulation test
The short-circuit test is performed to determine the copper losses in a transformer. During this test, the secondary winding is shorted, and the primary winding is supplied with a reduced voltage. The power loss measured during this test corresponds to the copper losses due to the resistance of the windings.
What is the purpose of an earthing transformer?
A To increase the voltage
B To provide a neutral point in an electrical system
C To step down the voltage
D To protect the transformer from overcurrent
An earthing transformer is used in systems that lack a neutral point, such as a delta-connected system. It creates a neutral point to safely ground the system, preventing dangerous floating voltages and ensuring safe operation.
Which type of transformer is commonly used for protection against electrical shocks in critical systems?
A Step-up transformer
B Isolation transformer
C Power transformer
D Autotransformer
Isolation transformers provide electrical isolation between the primary and secondary windings. They are commonly used in critical applications, such as medical equipment or sensitive electronics, to prevent electric shocks and ensure safety by isolating the connected equipment from the main power supply.
What is the typical frequency of operation for most transformers?
A 20 Hz
B 50 Hz or 60 Hz
C 100 Hz
D 200 Hz
Most transformers operate at a frequency of 50 Hz or 60 Hz, depending on the country or region. These are the standard frequencies for alternating current (AC) in most electrical power systems worldwide.
What is the main advantage of using a dry-type transformer over an oil-filled transformer?
A Higher efficiency
B Reduced risk of fire
C Smaller size
D Easier installation
Dry-type transformers use air for cooling and do not contain flammable oils, making them safer in environments where fire risk is a concern. While they may have slightly lower efficiency than oil-filled transformers, they are commonly used in residential, commercial, and industrial applications where safety is a priority.
What is the main function of transformer protection devices?
A To reduce energy losses
B To protect against overloading, short circuits, and faults
C To regulate voltage
D To increase transformer efficiency
Transformer protection devices, such as fuses, circuit breakers, and relays, are used to safeguard the transformer from damage due to overloading, short circuits, and other electrical faults. These devices disconnect the transformer from the circuit when dangerous conditions are detected, preventing further damage.
Which of the following losses occur in the core of a transformer?
A Copper loss
B Core loss (hysteresis and eddy current loss)
C Dielectric loss
D All of the above
Core losses in a transformer are mainly due to hysteresis (energy lost due to the magnetization and demagnetization of the core material) and eddy currents (currents induced within the core material). These losses occur regardless of the load and are typically reduced using laminated core designs.
What happens when the transformer experiences inrush current during startup?
A The voltage increases significantly
B The transformer experiences a brief surge of high current
C The transformer becomes inefficient
D The transformer becomes overloaded
Inrush current is the initial surge of current that occurs when a transformer is first energized. This surge can be several times higher than the normal operating current and can cause damage if not properly managed. Inrush current is typically limited using devices like inrush current limiters.
What is the purpose of load testing a transformer?
A To determine the transformer’s core losses
B To determine the efficiency and temperature rise under load conditions
C To measure the insulation strength
D To measure the impedance of the transformer
Load testing is conducted to evaluate a transformer’s performance under normal operating conditions when supplying power to a load. It checks parameters like voltage regulation, efficiency, and temperature rise. This test helps ensure that the transformer operates safely and efficiently under typical load conditions.
What is the role of harmonics in transformer operation?
A Harmonics improve the transformer’s efficiency
B Harmonics increase transformer losses and cause overheating
C Harmonics reduce voltage regulation
D Harmonics have no effect on transformer performance
Harmonics in the supply voltage can distort the waveform, increasing losses in the transformer and causing overheating. Harmonics are often generated by non-linear loads, such as variable-speed drives, and may require filtering to ensure transformer health and efficient operation.
Which of the following transformers is commonly used in industrial applications to isolate different circuits?
A Power transformer
B Isolation transformer
C Step-up transformer
D Autotransformer
Isolation transformers are used in industrial applications to isolate electrical circuits. They provide safety by preventing direct electrical connection between the primary and secondary circuits, thus reducing the risk of electric shock and providing protection from power surges.
What does the “no-load loss” of a transformer refer to?
A Losses due to the resistance of the windings when no load is connected
B Losses caused by core magnetization and eddy currents when no load is connected
C Losses that occur only during load operation
D Losses due to the heating of the oil in oil-filled transformers
No-load loss, also known as core loss, occurs when the transformer is energized but not supplying any load. These losses are mainly caused by the magnetization of the core material and the eddy currents induced within it. They are independent of the load and are present whenever the transformer is operating.
What is the role of insulation in transformers?
A To increase the efficiency of the transformer
B To provide safety by preventing electrical faults
C To store electrical energy
D To reduce the size of the transformer
Insulation in transformers is essential to prevent electrical faults such as short circuits between windings. It ensures that the windings are electrically separated from each other and from the core, preventing arcing and reducing the risk of electrical shock.
What is a common fault in transformers?
A Overvoltage
B Overheating
C Low current
D Short circuit
Overheating is a common fault in transformers, often caused by overloading, poor cooling, or excessive losses. Overheating can damage the insulation and windings, leading to transformer failure. Proper cooling and load management are essential to prevent overheating.
What is the primary function of a transformer?
A To change the frequency of electrical power
B To change the voltage of electrical power
C To store electrical energy
D To convert AC to DC
A transformer is used to either increase (step-up) or decrease (step-down) the voltage in an AC circuit. It operates on the principle of electromagnetic induction and does not change the frequency of the power.
What is a single-phase transformer used for?
A To step up the voltage for industrial use
B To convert DC to AC
C To step down the voltage for residential use
D To power three-phase systems
Single-phase transformers are primarily used to step down the voltage from high transmission levels to the lower levels suitable for residential and commercial use. They are commonly found in areas where the load is relatively small.
What is the most common core material used in transformers?
A Copper
B Silicon steel
C Aluminium
D Lead
Silicon steel is commonly used for transformer cores because of its high magnetic permeability and low hysteresis loss, which makes it efficient at handling magnetic flux with minimal energy losses.
What is the primary difference between a single-phase and a three-phase transformer?
A The number of windings in the transformer
B The number of phases used for electricity supply
C The efficiency of the transformer
D The core material used in construction
The primary difference between a single-phase and a three-phase transformer lies in the number of phases the transformer is designed to handle. A single-phase transformer is used for single-phase electrical supply, while a three-phase transformer handles three-phase supply, which is typically used in industrial and large-scale applications.
What is the main cause of transformer losses?
A Eddy currents and hysteresis losses
B Transformer cooling failures
C Harmonics generated by the load
D Voltage regulation issues
Transformer losses mainly occur due to eddy currents and hysteresis losses in the core material. Eddy currents are induced within the core, while hysteresis losses occur due to the magnetization and demagnetization of the core material.
What is voltage regulation in a transformer?
A The ability to change the current according to load
B The percentage difference between no-load and full-load voltage
C The efficiency of the transformer
D The maximum voltage that the transformer can handle
Voltage regulation refers to the ability of a transformer to maintain a constant output voltage despite changes in load. It is measured as the percentage difference between the no-load voltage and the full-load voltage.
Which of the following methods is commonly used for cooling oil-filled transformers?
A Air cooling
B Water cooling
C Oil cooling
D Liquid nitrogen cooling
Oil-filled transformers use oil for both cooling and insulation purposes. The oil helps to dissipate heat generated by the transformer’s operation, maintaining the transformer’s temperature within safe operating limits.
Which test is used to determine the core losses in a transformer?
A Open-circuit test
B Short-circuit test
C Load test
D Impedance test
The open-circuit test is used to measure core losses (also called no-load losses) in a transformer. In this test, the secondary winding is open, and the primary winding is supplied with rated voltage to measure the losses due to hysteresis and eddy currents in the core.
What is the function of the secondary winding in a transformer?
A To generate the magnetic flux
B To supply power to the load
C To maintain the core flux constant
D To regulate the voltage output
The secondary winding of a transformer receives induced voltage from the primary winding and supplies power to the load. The voltage and current in the secondary are determined by the turns ratio of the transformer.
What is the role of a transformer’s primary winding?
A To regulate the output voltage
B To generate magnetic flux in the core
C To maintain insulation between windings
D To supply current to the load
The primary winding of a transformer is connected to the input voltage source and generates magnetic flux in the core. This flux induces voltage in the secondary winding, which powers the load.
What is the typical efficiency of a modern transformer?
A 70-80%
B 80-90%
C 90-98%
D 100%
Modern transformers typically have efficiencies ranging from 90% to 98%. This efficiency depends on factors such as the core material, winding resistance, and cooling methods. Transformers are designed to minimize energy losses such as core and copper losses.
What does the term “step-up transformer” refer to?
A A transformer that increases the current
B A transformer that increases the voltage
C A transformer that decreases the voltage
D A transformer that steps up both voltage and current
A step-up transformer increases the voltage from the primary side to the secondary side. It has more turns in the secondary winding than in the primary, leading to a higher output voltage than the input voltage.
What is the typical voltage range for a distribution transformer?
A 5-10 kV
B 10-25 kV
C 25-50 kV
D 50-100 kV
Distribution transformers typically operate in the voltage range of 10-25 kV. These transformers step down the voltage from high transmission levels to the low voltage required for residential and commercial use.
Which of the following is a key characteristic of a dry-type transformer?
A It uses oil for cooling
B It does not require external cooling
C It uses air for cooling
D It operates only in high-voltage applications
Dry-type transformers use air as a cooling medium instead of oil. These transformers are commonly used in indoor applications and areas with a high fire risk, as they do not involve the use of flammable oil.
What is an autotransformer?
A A transformer with a common winding for primary and secondary circuits
B A transformer that can work with both AC and DC
C A transformer that regulates voltage automatically
D A transformer that is only used in high-voltage applications
An autotransformer is a type of transformer where the primary and secondary windings share part of the same winding. This design makes the autotransformer more compact and efficient than conventional transformers for voltage regulation.
What is the typical construction material used for transformer windings?
A Iron
B Copper
C Aluminium
D Silver
Copper is commonly used for transformer windings due to its excellent electrical conductivity, which minimizes energy losses. In some cases, aluminum may also be used for the windings in low-cost transformers, although it has higher resistance than copper.
Which of the following losses occur in the core of a transformer?
A Copper loss
B Core loss (hysteresis and eddy current loss)
C Dielectric loss
D All of the above
Core losses in a transformer are caused by hysteresis and eddy currents in the core material. These losses are present regardless of the load and are typically reduced using laminated core designs.
What happens when the transformer experiences inrush current during startup?
A The voltage increases significantly
B The transformer experiences a brief surge of high current
C The transformer becomes inefficient
D The transformer becomes overloaded
Inrush current is the initial surge of current that occurs when a transformer is first energized. This surge can be several times higher than the normal operating current and can cause damage if not properly managed.
What is the purpose of using a three-phase transformer over a single-phase transformer in industrial applications?
A Higher voltage regulation
B Higher power capacity and efficiency
C Lower cost
D Simplicity in design
Three-phase transformers are used in industrial applications because they can handle larger loads, provide more efficient power transmission, and offer better power distribution. They provide continuous power, unlike single-phase systems, which experience power dips.
What is the oil in oil-filled transformers primarily used for?
A To store electrical energy
B To provide electrical insulation
C To enhance voltage regulation
D To increase the transformer’s efficiency
The oil in oil-filled transformers serves as both an insulating material and a coolant. It prevents arcing between windings and helps dissipate the heat generated during operation, ensuring the transformer operates safely and efficiently.
What is the primary purpose of a transformer?
A To store energy
B To convert AC to DC
C To step up or step down voltage
D To regulate frequency
Transformers are used to either increase (step-up) or decrease (step-down) the voltage in alternating current (AC) circuits. They can either step up the voltage (increase it) or step down the voltage (decrease it) without changing the frequency.
Which of the following is a common type of transformer used for low-voltage residential applications?
A Power transformer
B Distribution transformer
C Autotransformer
D Isolation transformer
Distribution transformers are used to step down the voltage from transmission levels to levels suitable for residential and commercial use. They are typically found in substations and on utility poles.
What does the turns ratio of a transformer determine?
A The current in the primary winding
B The voltage ratio between the primary and secondary windings
C The power loss in the transformer
D The size of the transformer
The turns ratio determines the relationship between the primary and secondary voltages. If the number of turns in the secondary winding is greater than the primary, the transformer steps up the voltage; if it’s less, it steps down the voltage.
What is the purpose of the core in a transformer?
A To store electrical energy
B To provide a path for the magnetic flux
C To regulate the voltage
D To provide insulation
The core in a transformer provides a low-reluctance path for the magnetic flux generated by the primary winding. This flux induces voltage in the secondary winding. It is typically made from silicon steel to minimize losses.
Which of the following is a characteristic of an oil-filled transformer?
A It uses air as a cooling medium
B The oil acts as an insulating and cooling medium
C It operates at low voltages
D It is typically used in dry climates
Oil-filled transformers use oil both as an insulating material and as a coolant. The oil helps to dissipate heat generated during operation and also prevents electrical arcing between the windings.
What is the main function of an autotransformer?
A To step up voltage
B To share the same winding for both primary and secondary circuits
C To convert AC to DC
D To provide electrical isolation
An autotransformer shares part of its winding for both the primary and secondary circuits, making it more efficient and compact than a traditional transformer. It is often used for voltage regulation.
What does voltage regulation in a transformer refer to?
A The efficiency of the transformer under varying loads
B The ability to keep the secondary voltage constant despite load changes
C The losses in the transformer
D The process of changing the voltage from AC to DC
Voltage regulation measures the ability of a transformer to maintain a constant secondary voltage under varying load conditions. Good voltage regulation is crucial for ensuring the stability of connected equipment.
Which of the following tests is used to measure the core loss in a transformer?
A Open-circuit test
B Short-circuit test
C Load test
D Insulation resistance test
The open-circuit test is used to measure core losses (no-load losses) in a transformer. This test involves keeping the secondary winding open and applying rated voltage to the primary winding.
What is the typical purpose of a step-up transformer?
A To decrease voltage
B To increase voltage
C To maintain a constant voltage
D To reduce current
A step-up transformer increases the voltage from the primary to the secondary winding. This is commonly used in power transmission systems to increase the voltage for long-distance transmission, minimizing power loss.
What happens to the efficiency of a transformer as load increases?
A Efficiency increases
B Efficiency decreases
C Efficiency remains constant
D Efficiency fluctuates
The efficiency of a transformer generally increases as the load increases, provided the transformer operates within its rated capacity. Efficiency is highest when the transformer is operating at full load and decreases at very light loads.