The Electrician Workbook Electrical Machines

A transformer is a device that is used to change the level of voltage in an electrical circuit. It works on the principle of electromagnetic induction, where it transfers electrical energy between two or more circuits through electromagnetic fields. By adjusting the number of turns in the primary and secondary coils, a transformer can step up or step down the voltage level. This allows for efficient transmission of electrical power over long distances and also enables voltage conversion for different devices and systems. Therefore, the basic function of a transformer is to change the level of the voltage.

A transformer is used to step up or step down the voltage because it can change the voltage level of an alternating current (AC) without changing its frequency. This is achieved through the principle of electromagnetic induction, where the primary coil induces a changing magnetic field in the core, which in turn induces a voltage in the secondary coil. By varying the number of turns in the primary and secondary coils, the transformer can either increase (step up) or decrease (step down) the voltage level according to the desired application.

The correct answer is "any of the above" because all three conditions mentioned (voltage is too low, one phase is open, connections are faulty) can prevent a 3-phase synchronous motor from starting. If the voltage is too low, the motor may not have enough power to start. If one phase is open, there will be an imbalance in the motor's operation, preventing it from starting. Similarly, if there are faulty connections, it can disrupt the flow of electricity and hinder the motor's ability to start. Therefore, any of these conditions can be the reason for the motor not starting.

Oil in a transformer serves two main functions: insulation and cooling. The oil acts as an insulating material, preventing electrical breakdown and ensuring the safe operation of the transformer. It also aids in cooling by absorbing and dissipating heat generated during operation. This helps maintain the optimal temperature for the transformer's components, preventing overheating and potential damage. Therefore, the oil in a transformer provides both insulation and cooling, making it the correct answer.

A universal motor is capable of operating on both AC and DC power sources with comparable performance. This means that it can function efficiently and effectively regardless of whether it is powered by alternating current or direct current. This versatility makes universal motors suitable for a wide range of applications, such as in household appliances like vacuum cleaners or power tools.

The correct answer is "any of the above" because all of the mentioned factors can lead to an electric motor getting over-heated. Over-loading can cause excessive strain on the motor, shorted stator winding can result in increased current flow and heat generation, worn-out or dry bearings can cause friction and heat, and low or high voltage can affect the motor's efficiency and temperature regulation. Therefore, any of these factors can contribute to the motor overheating.

An induction motor consists of two essential components: the stator and the rotor. The stator is the stationary part of the motor that contains the winding, which produces a rotating magnetic field when an alternating current is passed through it. The rotor, on the other hand, is the rotating part of the motor that is placed inside the stator and interacts with the magnetic field produced by the stator. Together, the stator and rotor work in harmony to generate the necessary electromagnetic forces for the motor to function properly.

A DC motor can be easily identified by its commutator. The commutator is a crucial component in a DC motor that helps to change the direction of the current in the armature coil. It consists of a set of copper segments that are insulated from each other and connected to the armature coil. As the armature rotates, the commutator ensures that the current flows in the correct direction, resulting in the desired rotation of the motor. Therefore, the presence of a commutator is a clear indication that the motor is a DC motor.

The emfs generated in three phases of an alternator are 120 electrical degrees apart. This means that the voltage waveforms in each phase are offset from each other by 120 degrees. This phase displacement is necessary for the proper functioning of a three-phase alternator, as it allows for a more balanced and efficient distribution of power.

In a transformer, electrical power is transferred from one circuit to another without a change in frequency. This is because transformers operate based on the principle of electromagnetic induction, where a changing magnetic field induces a voltage in a nearby coil. The primary coil in a transformer is connected to an alternating current source, which produces a changing magnetic field. This changing magnetic field induces a voltage in the secondary coil, allowing for power transfer. However, the frequency of the alternating current remains constant throughout the process. Therefore, the correct answer is frequency.

A synchronous motor is not inherently self-starting because it requires an external means to bring it up to synchronous speed before it can start running. This is typically done by using a separate starting motor or by using a special starting technique such as providing an initial rotating magnetic field. In contrast, the other three motors mentioned (3-phase induction motor, reluctance motor, and DC series motor) are inherently self-starting, meaning they can start running on their own without the need for any external assistance.

An induction motor works with AC only because it relies on the principle of electromagnetic induction to operate. AC current is necessary to create a rotating magnetic field in the stator of the motor, which induces currents in the rotor and causes it to rotate. DC current does not produce the alternating magnetic field required for the motor to function properly. Therefore, an induction motor cannot work with DC power.

Phase advancers are devices used with large induction motors to improve their power factor. Power factor is a measure of how effectively electrical power is being utilized. Induction motors often have a lagging power factor, which means they consume more reactive power and result in higher losses. By using phase advancers, the motor's power factor can be improved by compensating for the lagging current. This helps to reduce the overall power consumption and improve the efficiency of the motor.

The squirrel cage induction motor is widely used in various applications due to its simplicity, reliability, and low cost. It consists of a rotor with conductive bars that resemble a squirrel cage, which allows for efficient operation and easy starting. The absence of slip rings and brushes eliminates the need for maintenance and reduces the chances of failure. Additionally, its robust design and ability to handle high torque make it suitable for a wide range of industrial and commercial applications.

An infinite bus-bar refers to a theoretical power system with an unlimited power supply. In this scenario, the voltage remains constant as it is not affected by any load or generation changes. Additionally, the frequency of the power supply also remains constant, as it is determined by the generation source. Therefore, both options A and B are correct, as the voltage and frequency of an infinite bus-bar remain constant.

The efficiency of a power transformer refers to the ratio of output power to input power, expressed as a percentage. A higher efficiency indicates that a larger proportion of the input power is being converted into useful output power, while a lower efficiency means that more power is being wasted as heat. Therefore, a power transformer with an efficiency of 95% is considered highly efficient, as it indicates that only 5% of the input power is being lost as heat and 95% is being effectively utilized for the desired output.

A synchronous motor operates at a fixed speed determined by the frequency of the power supply. It is designed to maintain this speed regardless of the load. As the load increases, the motor adjusts its torque to match the demand, ensuring that the speed remains constant. This characteristic makes synchronous motors suitable for applications where a constant speed is required, such as in industrial machinery or power generation.

When two alternators are operating in parallel, they are synchronized and running at the same speed. If the power input to one of the alternators is cut off, it will continue to run as a synchronous motor because the other alternator will supply the necessary power to keep it running. However, since the power is no longer being generated internally, the alternator will act as a motor and rotate in the same direction as before.

An auto-transformer is a type of transformer that has a single winding which serves as both the primary and secondary winding. It can be used as a voltage regulating transformer because it allows for the adjustment of the output voltage by tapping off different points along the winding. This makes it useful in applications where precise control of voltage levels is required, such as in power distribution systems or in electrical equipment that requires a specific voltage input.

The speed of single phase induction motors can be controlled by either varying the applied voltage to the stator winding or varying the number of poles on the stator. By adjusting the voltage applied to the stator winding, the magnetic field strength can be altered, which in turn affects the motor's speed. Similarly, by changing the number of poles on the stator, the motor's speed can also be adjusted. Therefore, either option A or B can be used to control the speed of single phase induction motors.

A squirrel cage induction motor is not favored when high starting torque is the main consideration because squirrel cage motors have a lower starting torque compared to other types of motors. They are designed for applications that do not require high starting torque, such as fans, pumps, and compressors. For applications that require high starting torque, other types of motors like wound rotor motors or synchronous motors may be more suitable.

The function of the starter for a DC motor is to limit the starting current. This is important because when a DC motor starts, it draws a very high current which can damage the motor and the power supply. The starter is designed to control and limit this initial surge of current, allowing the motor to start smoothly and preventing any potential damage. By limiting the starting current, the starter ensures the motor operates within safe limits and extends its lifespan.

Transformer action requires an alternating magnetic flux because transformers work based on the principle of electromagnetic induction. When an alternating current passes through the primary coil of a transformer, it creates an alternating magnetic field. This alternating magnetic field induces an alternating voltage in the secondary coil, allowing for the transfer of electrical energy from one coil to another. Therefore, an alternating magnetic flux is necessary for the proper functioning of a transformer.

In a synchronous motor, squirrel cage winding is provided for self-starting. The squirrel cage winding creates a rotating magnetic field that interacts with the stator's magnetic field, causing the rotor to start rotating. This eliminates the need for external devices or manual assistance to start the motor, making it self-starting.

A slip-ring induction motor is recommended for applications requiring high starting torque because it has the ability to provide a high torque at startup, which is necessary for applications that require heavy loads to be moved or started. It is also recommended for variable speed operation as the slip rings allow for the control of the rotor's speed, making it suitable for applications that require different speeds. Additionally, the slip-ring induction motor is recommended for frequent starting, stopping, and reversing operations as it can handle these operations without any issues. Therefore, it is recommended for applications requiring all of the above features.

A synchronous motor can conveniently operate at both lagging and leading power factor because its field winding can be adjusted to create a leading or lagging reactive power. This flexibility allows the synchronous motor to support the power factor correction requirements of the system it is connected to. In contrast, squirrel-cage and wound rotor induction motors have fixed power factors and cannot easily adjust their reactive power. DC shunt motors do not have a reactive power component and therefore do not have a power factor.

An auto-transformer has one winding with taps taken out. This means that there is a single winding in the transformer, but there are multiple connection points or taps along the winding. These taps allow for different voltage levels to be obtained by connecting to different points along the winding. This makes the auto-transformer more versatile and efficient compared to a regular transformer, as it can step up or step down the voltage depending on the tap used.

An auto-transformer is a type of transformer that has a single winding with multiple taps. It is used to provide a small voltage transformation ratio, typically close to unity. This means that the output voltage is almost the same as the input voltage, with only a slight difference. Auto-transformers are often used in situations where a small voltage adjustment is required, such as in voltage regulators or in electrical distribution systems. They are more efficient and cost-effective than traditional transformers for voltage transformation ratios that are close to unity.

Increasing the supply frequency of an induction motor will increase its synchronous speed. The synchronous speed of an induction motor is directly proportional to the supply frequency. By increasing the supply frequency, the motor's rotor will rotate at a higher speed, resulting in an increase in its synchronous speed. This is because the synchronous speed is determined by the frequency of the rotating magnetic field produced by the stator winding, and increasing the frequency will increase the speed at which the magnetic field rotates.

The principle of operation of a 3-phase induction motor closely resembles that of a two-winding transformer with its secondary short-circuited. This is because both the induction motor and the transformer have a primary winding and a secondary winding. In the case of the transformer, the secondary winding is usually connected to a load, while in the induction motor, the secondary winding is short-circuited. This short-circuiting of the secondary winding in the induction motor allows for the creation of a rotating magnetic field, which in turn induces currents in the rotor and causes it to rotate.

When the prime-mover of an alternator supplying load to an infinite bus is suddenly shut down, the alternator will continue to run as a synchronous motor in the same direction. This is because the alternator is connected to an infinite bus, which provides a constant source of power. The alternator will continue to rotate due to the inertia of the rotor and will act as a synchronous motor, maintaining the same direction of rotation as before.

When the induction motor is in standstill, it means that the rotor is not moving. In this situation, the slip, which is the difference between the synchronous speed and the rotor speed, is zero. This is because there is no relative motion between the stator magnetic field and the rotor, resulting in no slip. Therefore, the correct answer is 1.

The operation of an induction motor is based on the principle of mutual induction. Mutual induction occurs when a changing current in one coil induces a voltage in a nearby coil. In an induction motor, an alternating current is passed through the stator winding, creating a rotating magnetic field. This rotating magnetic field induces a current in the rotor winding through mutual induction. The interaction between the stator and rotor magnetic fields causes the rotor to rotate, enabling the motor to operate.

The wattage rating for a ceiling motor will typically fall within the range of 50 to 150W. This range is considered appropriate for most ceiling motors, providing sufficient power for their operation without consuming excessive energy. Wattage ratings below 50W may be insufficient to effectively operate a ceiling motor, while ratings above 150W may be excessive and lead to unnecessary energy consumption. Therefore, the range of 50 to 150W is the most suitable option for a ceiling motor's wattage rating.

When a DC series motor is operated on AC supply, it will start and run but will have poor performance. This is because the AC supply causes the motor to experience excessive sparking, which can result in damage to the motor's windings. Additionally, the motor's efficiency and power factor will be negatively affected, leading to a decrease in overall performance. However, the motor will still be able to start and run, although not optimally.

The tappings of a transformer are provided at the middle of the HV side. This is done to ensure that the voltage can be adjusted as per the requirement. By placing the tappings at the middle of the HV side, the transformer can be easily switched between different voltage levels without affecting the overall performance of the transformer. This allows for greater flexibility and adaptability in various power transmission and distribution systems.

The power factor of a synchronous motor can be varied by varying the excitation. Excitation refers to the amount of current supplied to the motor's field winding, which creates a magnetic field. By adjusting the excitation, the strength of the magnetic field can be changed, which in turn affects the power factor. A higher excitation results in a leading power factor, while a lower excitation leads to a lagging power factor. Therefore, varying the excitation allows for control over the power factor of a synchronous motor.

Pole changing control is a method that can be conveniently used for speed control of a squirrel cage induction motor. This method involves changing the number of poles in the motor by altering the connections of the stator winding. By changing the number of poles, the motor's speed can be adjusted to meet the desired requirements. This method is commonly used in applications where variable speed control is needed, such as in fans, pumps, and conveyors. Rotor resistance control, cascade operation, and secondary foreign voltage control are not as commonly used or as convenient for speed control in squirrel cage induction motors.

A synchronous capacitor is an over-excited synchronous motor without mechanical load. Unlike a regular static capacitor bank, which is used to store and release electrical energy, a synchronous capacitor is a motor that operates in an over-excited state without any mechanical load. This means that it is designed to generate reactive power and help correct the power factor in an electrical system. By adjusting the excitation level, a synchronous capacitor can provide or absorb reactive power as needed, improving the efficiency and stability of the system.

Synchronous motors are not self-starting because the direction of instantaneous torque on the rotor reverses after each half cycle. This means that the rotor does not have a consistent torque applied to it in one direction, making it unable to start rotating on its own.

In parallel operation, load sharing by transformers is according to per unit impedance. This means that the transformers share the load based on their impedance values. Transformers with lower impedance values will take on a larger portion of the load, while transformers with higher impedance values will take on a smaller portion. This ensures that the load is distributed evenly among the transformers and prevents overloading of any single transformer.

A distribution transformer is selected on the basis of all day efficiency. This is because all day efficiency takes into account the total energy losses in the transformer over a 24-hour period, including both load and no-load losses. It provides a more accurate measure of the transformer's overall performance and energy efficiency compared to just considering voltage regulation or efficiency alone. By selecting a transformer with high all day efficiency, energy losses can be minimized, resulting in cost savings and improved overall system efficiency.

When starting a synchronous motor, the field winding should be short-circuited. This is because a synchronous motor relies on the interaction between the stator and rotor magnetic fields to operate. By short-circuiting the field winding, it prevents the motor from developing a magnetic field of its own and allows it to synchronize with the rotating magnetic field produced by the stator. This synchronization is necessary for the motor to start and operate efficiently. Therefore, short-circuiting the field winding is the correct option.

The relative speed between stator and rotor fluxes is zero because in a synchronous motor, the stator and rotor magnetic fields rotate at the same speed. This means that there is no relative motion between the two fields, resulting in a relative speed of zero.

In a 3-phase induction motor, iron loss refers to the energy dissipated in the stator and rotor cores as well as the teeth of the stator. This loss occurs due to the alternating magnetic field generated by the stator winding, which causes the magnetic domains in the iron core and teeth to constantly realign and generate heat. Therefore, the correct answer is stator core and stator teeth.

When a transformer is operating at a constant voltage, an increase in input frequency leads to a decrease in core loss. This is because core loss is primarily caused by hysteresis and eddy current losses, which are directly proportional to the frequency. Therefore, as the frequency increases, the core loss decreases.

The inductive reactance of a transformer depends on leakage flux. Leakage flux refers to the magnetic flux that does not link with both the primary and secondary windings of a transformer. This flux is responsible for creating a self-inductance in the windings, which in turn leads to inductive reactance. The amount of leakage flux is influenced by factors such as the design of the transformer, the spacing between the windings, and the insulation used. Therefore, the inductive reactance of a transformer is directly affected by the presence and magnitude of leakage flux.