DC Generator

A D.C. generator works on the principle of Faraday's law of electromagnetic induction. This law states that when a conductor is moved through a magnetic field or when there is a change in the magnetic field through a conductor, an electromotive force (EMF) is induced in the conductor. In a D.C. generator, a coil of wire is rotated within a magnetic field, causing the magnetic field to change through the coil. This change in magnetic field induces an EMF in the coil, which results in the generation of a direct current. Therefore, the correct answer is Faraday's law of electromagnetic induction.

When the speed of a shunt generator increases, the generated e.m.f. also increases. This is because the generated e.m.f. is directly proportional to the speed of the generator. In this case, the speed increases from 1000 r.p.m. to 1200 r.p.m., which is a 20% increase. Therefore, the generated e.m.f. will also increase by approximately 20%. Since the initial e.m.f. is 200 V, a 20% increase would result in an e.m.f. of 240 V.

The correct answer is Blv. According to Faraday's law of electromagnetic induction, the induced electromotive force (emf) is directly proportional to the magnetic flux density (B), the length of the conductor (l), and the velocity of the conductor (v). Therefore, the correct equation for the induced emf is Blv.

Ball bearings are commonly used to support rotor shafts because they provide smooth and efficient rotation. They consist of balls enclosed within a ring, which reduces friction and allows for high-speed operation. Ball bearings can handle both radial and axial loads, making them suitable for various applications. Additionally, they are durable and require minimal maintenance.

Mica is commonly used as an insulating material between the commutator segments. Mica is a mineral with excellent electrical insulation properties, high heat resistance, and good mechanical strength. It can withstand high temperatures and does not conduct electricity, making it an ideal choice for insulating the segments of a commutator. Mica is also resistant to chemicals and moisture, which further enhances its suitability for this application. Overall, mica provides the necessary insulation and protection to ensure the proper functioning of the commutator in electrical devices.

Mica is commonly used as an insulating material between the commutator segments because of its excellent electrical insulation properties. It has a high dielectric strength, which allows it to withstand high voltage without conducting electricity. Mica is also resistant to heat, chemicals, and moisture, making it a durable and reliable choice for insulating the commutator. Additionally, mica is a good thermal conductor, allowing it to dissipate heat effectively and prevent overheating of the commutator. Overall, mica is a suitable material for insulating the commutator, ensuring proper functioning and longevity of the electrical device.

Ball bearings are generally used to support the rotor shafts. Ball bearings consist of small metal balls that are held in a ring and allow for smooth rotation with minimal friction. They are commonly used in various applications where high radial and axial loads need to be supported. In the case of rotor shafts, ball bearings provide a reliable and efficient means of supporting the shafts and facilitating smooth rotation.

To achieve satisfactory commutation in D.C. machines, all of the mentioned factors are important. Brushes should be of proper grade and size to ensure good contact with the commutator. They should also smoothly run in the holders to avoid any friction or sparking. The commutator should be smooth and concentric, with proper undercutting to ensure effective switching of current between the armature coils. Therefore, all of the above factors are necessary for satisfactory commutation in D.C. machines.

Brushes are used in a DC motor to collect current from the commutator and transfer it to the external load. The commutator is a rotating device that switches the direction of the current in the armature coil, and the brushes make contact with the commutator segments to collect the current. The brushes are typically made of carbon or graphite, which have good electrical conductivity and can withstand the friction and heat generated during operation. By collecting the current from the commutator, the brushes ensure that the electrical power generated by the motor is transferred to the external load efficiently.

A motor is a device that uses electrical energy to produce mechanical energy. It does this by converting the electrical energy into rotational motion, which can be used to drive machinery or perform other mechanical tasks. This process is achieved through the interaction of magnetic fields and electric currents in the motor's components. Therefore, the correct answer is "Electrical energy into Mechanical energy."

In D.C. machines, mechanical losses primarily depend on the speed of the machine. This is because mechanical losses include friction and windage losses, which are caused by the movement of mechanical components such as bearings, brushes, and rotating parts. The faster the machine operates, the higher the friction and windage losses will be, resulting in more mechanical losses. Therefore, speed is the primary factor that determines the level of mechanical losses in D.C. machines.

In D.C. machines, mechanical losses primarily depend on the speed of the machine. This is because mechanical losses are caused by friction and windage, which increase as the speed of the machine increases. Therefore, the higher the speed of the D.C. machine, the greater the mechanical losses.

The correct answer is "all above". D.C. generators are connected to the busbars or disconnected from them only under the floating condition to avoid sudden loading of the prime mover, to avoid mechanical jerk to the shaft, and to avoid burning of switch contacts. By keeping the generators in a floating condition, these potential issues can be prevented, ensuring the smooth operation and longevity of the generator system.

The correct answer is "All of the above." This means that all the options mentioned - yoke, pole cores, and pole shoes - are parts of a field magnet. A field magnet typically consists of these components, which work together to generate a magnetic field. The yoke provides a closed magnetic circuit, while the pole cores and pole shoes help concentrate and direct the magnetic field in a desired manner. Therefore, all of these parts are essential for the functioning of a field magnet.

In a four-pole D.C. machine, the correct answer is "alternate poles are north and south." This means that the machine has two north poles and two south poles, with each pole alternating in polarity. This arrangement allows for the creation of a magnetic field that is essential for the operation of the machine. By having alternating north and south poles, the machine can generate the necessary electromagnetic forces required for its functioning.

The correct answer is rectangular because the word "brushes" implies that there are multiple brushes, and the shape of each brush is described as rectangular.

Electro magnets are used in large machines to create magnetic flux. Unlike permanent magnets, electro magnets are made by passing an electric current through a coil of wire, which generates a magnetic field. This magnetic field can be controlled and adjusted by varying the amount of current passing through the coil, making electro magnets suitable for use in large machines where magnetic flux needs to be created and manipulated. Permanent magnets, on the other hand, have a fixed magnetic field and cannot be easily adjusted or turned on/off like electro magnets. Temporary magnets are also not commonly used in large machines for creating magnetic flux.

For the production of dynamically induced emf, all three factors mentioned in the options are necessary. A magnetic field is required to create a flux, a conductor is needed to allow the flow of electrons, and the motion of the conductor with respect to the field is necessary to induce the emf. Therefore, all of the above options are essential for the production of dynamically induced emf.

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In a two-layer lap winding, each coil spans two poles. Since there are 16 coils, there must be 8 pole pairs. Therefore, the pole pitch, which is the distance between the centers of two adjacent poles, will be 8.

In a D.C. generator, the number of mechanical degrees and electrical degrees will be the same when the number of poles is 2. This means that for every mechanical degree the rotor moves, the electrical degrees also change by the same amount. This is because in a 2-pole generator, each pole represents 180 electrical degrees. Therefore, when there are 2 poles, the mechanical and electrical degrees will be in sync.

In the context of a D.C. generator, regulation refers to the ability of the generator to maintain a constant output voltage under varying load conditions. A 1% regulation means that the generator can maintain its output voltage within 1% of the rated voltage even when the load changes. This level of regulation is preferred because it indicates a high level of stability and accuracy in the generator's performance, ensuring consistent and reliable power supply.

The correct answer is "Any of the above". This means that the cause of rapid brush wear in D.C. generators can be any of the mentioned factors: severe sparking, rough commutator surface, or imperfect contact. Each of these factors can contribute to increased friction and wear on the brushes, leading to their rapid deterioration.

In lap winding, the number of brushes is always the same as the number of poles. This is because in lap winding, each coil is connected to two adjacent commutator segments, and each commutator segment is connected to a brush. Therefore, the number of brushes required is equal to the number of poles.

The brushes of D.C. machines are made of carbon. Carbon brushes are commonly used in D.C. machines because carbon is a good conductor of electricity and has low friction characteristics. It can withstand high temperatures and is durable, making it suitable for the constant sliding contact with the commutator in D.C. machines. Soft copper and hard copper are not typically used for brushes in D.C. machines.

The correct answer is "All of the above." This means that all of the given options - insulation failure between two commutator bars, insulation failure between two turns of a coil, and two or more turns of the same coil getting grounded - can cause a short-circuit in the armature winding.

Carbon is the most commonly used material for commutator brushes. This is because carbon has excellent electrical conductivity, low friction, and good wear resistance. It can effectively transfer electrical current between the commutator and the stationary parts of the motor. Carbon brushes also have self-lubricating properties, which reduce friction and heat generation during operation. Additionally, carbon brushes are relatively inexpensive and easy to replace, making them a popular choice in various electrical applications.

In D.C. generators, the brushes on the commutator remain in contact with conductors that lie under the interpolar region. The interpolar region is the area between the north and south poles of the generator. This region is important because it helps in reducing the sparking and improves the efficiency of the generator. By having the brushes in contact with the conductors in this region, the commutation process is improved, ensuring a smooth flow of current and reducing any potential issues with sparking or arcing.

Fleming's left hand rule is also called the 'motor rule'. This rule is used to determine the direction of the force experienced by a current-carrying conductor placed in a magnetic field. By using the left hand, with the thumb representing the direction of the force, the index finger representing the magnetic field, and the middle finger representing the current, the rule helps to determine the resulting motion or force on the conductor.

When two D.C. series generators are running in parallel, an equalizer bar is used so that two similar machines will pass approximately equal currents to the load. This is important to ensure that the load is shared equally between the generators and to prevent one generator from carrying a significantly higher load than the other. The equalizer bar allows for the adjustment of the field current in each generator, ensuring that they both contribute equally to the overall load.

The cause of rapid brush wear in D.C. generators can be any of the mentioned factors: severe sparking, rough commutator surface, or imperfect contact. Severe sparking occurs when there is excessive arcing between the brushes and the commutator, leading to increased wear. A rough commutator surface can cause uneven contact with the brushes, resulting in accelerated brush wear. Imperfect contact refers to a condition where the brushes do not make proper contact with the commutator, causing friction and wear. Therefore, any of these factors can contribute to rapid brush wear in D.C. generators.

A compound wound generator is a type of generator that has two field windings. One winding is connected in series with the armature, while the other is connected in parallel or shunt with the armature. This configuration allows the generator to have both the advantages of a series wound generator (high starting torque) and a shunt wound generator (good voltage regulation). Therefore, the correct answer is compound wound generator.

Iron losses in a D.C. machine are independent of variations in load. This means that the iron losses, which include hysteresis and eddy current losses, remain constant regardless of the amount of load on the machine. These losses are primarily caused by the magnetic properties of the iron core and the frequency of the magnetic field, rather than the load or the speed and voltage of the machine. Therefore, changes in load do not affect the iron losses in a D.C. machine.

In a D.C. generator, the magnetic field is produced by electromagnets. Unlike permanent magnets, electromagnets can be turned on and off, allowing for better control and adjustment of the magnetic field strength. This is important in a generator as it enables the conversion of mechanical energy into electrical energy. Therefore, the correct answer is electromagnets.

Copper brushes in D.C. machines are used where low voltage and high currents are involved. This is because copper is an excellent conductor of electricity and can handle high currents without overheating or causing excessive resistance. Additionally, copper brushes have good wear resistance and can maintain good contact with the commutator, ensuring efficient transfer of current. Therefore, copper brushes are suitable for applications where low voltage and high currents are present, such as in electric motors or generators.

This answer suggests that mica is harder than copper. Hardness is a property that measures a material's resistance to scratching or indentation. Mica is a mineral that is known for its hardness, typically ranging from 2.5 to 4 on the Mohs scale. Copper, on the other hand, is a metal that is relatively soft and has a hardness of around 2.5 on the Mohs scale. Therefore, mica is indeed harder than copper.

Copper brushes in D.C. machines are used where low voltage and high currents are involved. Copper is a good conductor of electricity and can handle high currents without overheating. By using copper brushes, the machine can efficiently transfer the high current from the power source to the rotating parts, ensuring smooth operation. Additionally, copper brushes have good wear resistance and can withstand the friction and heat generated during operation. Therefore, in applications where low voltage and high currents are present, copper brushes are the ideal choice.

Armature reaction of an unsaturated D.C. machine refers to the magnetic field produced by the armature current that interacts with the main magnetic field of the machine. In the case of crossmagnetising armature reaction, the armature current produces a magnetic field that is perpendicular to the main magnetic field, causing a distortion in the overall magnetic field. This can result in a reduction in the effective magnetic field strength, affecting the performance of the machine. Therefore, the correct answer is crossmagnetising.

Armature reaction refers to the magnetic effect produced by the armature current in a DC machine. In an unsaturated DC machine, the armature reaction is cross-magnetizing. This means that the magnetic field produced by the armature current interacts with the main field in such a way that it creates a magnetic field perpendicular to the main field. This cross-magnetizing effect can distort the main field and affect the performance of the machine.

In D.C. generators, lap winding is used for low voltage, high current. Lap winding is a type of armature winding where the armature coils are connected in parallel. This configuration allows for a larger number of turns per coil, resulting in a lower voltage output. However, since the coils are connected in parallel, the current flowing through each coil is additive, leading to a higher overall current output. Therefore, lap winding is suitable for applications that require a low voltage but high current output.

A D.C. welding generator typically uses a lap winding. Lap winding is a type of armature winding where the end of one coil overlaps with the start of the next coil. This arrangement allows for a better distribution of current and reduces the chances of short circuits. It also provides a higher voltage output and better control over the welding process. Therefore, a D.C. welding generator commonly utilizes a lap winding.

Iron losses in a D.C. machine are independent of variations in load. Iron losses, also known as core losses, occur in the iron core of the machine due to hysteresis and eddy current losses. These losses are constant and do not change with variations in load. The load only affects the copper losses in the machine, which are caused by the resistance of the windings and vary with the load. Therefore, variations in load do not impact the iron losses in a D.C. machine.

Core losses are also known as iron or magnetic losses. These losses occur in the core of electrical machines, such as transformers and motors, due to hysteresis and eddy current effects. Hysteresis losses are caused by the reversal of magnetization in the core material, while eddy current losses are caused by the circulating currents induced in the core. These losses result in energy dissipation in the form of heat and can significantly affect the efficiency and performance of the electrical machine. Therefore, core losses are an important consideration in the design and operation of electrical equipment.

The mechanical losses refer to the energy losses that occur due to friction and other mechanical factors in a system. These losses are typically a percentage of the full load losses, which are the losses that occur when the system is operating at full capacity. The correct answer, 10 to 20%, suggests that the mechanical losses are between 10% and 20% of the full load losses. This means that a significant portion of the energy losses in the system can be attributed to mechanical factors.

The wave winding will give the higher e.m.f. in a D.C. generator when the number of poles and the number of armature conductors is fixed. This is because in a wave winding, each coil spans across two poles, resulting in a higher number of parallel paths for the current to flow through. This increases the e.m.f. generated compared to a lap winding where each coil spans across only one pole. Therefore, the wave winding is more efficient in generating a higher e.m.f. in this scenario.

The correct answer is "Net torque – (Friction torque + Torque lost)". This is because the shaft torque is equal to the net torque acting on the shaft minus the sum of the friction torque and the torque lost. This equation takes into account all the forces and torques acting on the shaft and provides an accurate measure of the resulting torque.

All of the listed components (armature core, commutator, and cooling fan) are keyed to the shaft. This means that they are securely attached to the shaft using a key, which is a small piece of metal that fits into a groove on both the shaft and the component. This keying ensures that the components rotate along with the shaft, allowing for efficient and synchronized operation of the motor or machine.

Equalizer rings are required in case the armature is lap wound. In a lap wound armature, the armature coils are divided into two parallel paths, with each path consisting of multiple coils connected in series. The equalizer rings are used to ensure that the current flows evenly through all the coils in each parallel path. This helps in balancing the magnetic field and prevents any imbalance in the armature current, which can lead to overheating and damage to the armature winding. Therefore, the use of equalizer rings is necessary in lap wound armatures to maintain proper functioning and prevent any potential issues.

A self-excited generator is capable of building up without any residual magnetism in the poles because it uses a separate source of DC excitation to create a magnetic field. This means that even if there is no initial magnetism present in the poles, the generator can still generate the necessary magnetic field to induce voltage and start building up power. In contrast, the other types of generators mentioned (series, shunt, and compound) rely on the residual magnetism in the poles to initiate the generation process, so they would not be able to build up without any residual magnetism.

When applying Fleming's right-hand rule, the thumb represents the direction of the induced e.m.f. The forefinger points in the direction of the generated e.m.f. The middle finger represents the direction of the magnetic field or lines of flux. Therefore, if the forefinger points along the lines of flux, the thumb will point in the direction of the motion of the conductor. This is because the induced e.m.f. is generated due to the relative motion between the conductor and the magnetic field.