Trivia Questions On Electrical Machines

The power transformed from primary to secondary depends on the magnetic coupling between the primary and secondary windings. Magnetic coupling refers to the extent to which the magnetic field generated by the primary winding is transferred to the secondary winding. A stronger magnetic coupling allows for more efficient power transfer between the windings. Therefore, the power transformed is directly influenced by the level of magnetic coupling between the primary and secondary windings.

The inductive reactance of a transformer depends on the magnetomotive force. The magnetomotive force is responsible for creating a magnetic field in the transformer's core, which induces a voltage across the windings. This voltage is directly proportional to the rate of change of magnetic flux, and the reactance is the opposition to this change. Therefore, the inductive reactance is directly influenced by the magnetomotive force, as it determines the strength of the magnetic field and the resulting voltage induced in the transformer.

Secondary ampere turns counterbalance the primary ampere turns. Ampere turns refer to the product of the number of turns in a coil and the current flowing through it. In a transformer, the primary coil produces a magnetic field, and the secondary coil produces a corresponding magnetic field. The secondary ampere turns are designed to oppose and balance out the primary ampere turns, ensuring that the transformer operates efficiently and transfers power effectively. This balance helps regulate the voltage and current in the transformer, allowing for proper electrical transmission.

Smooth cylindrical rotors are usually designed for 2 or 4 poles because these configurations provide a balanced distribution of magnetic flux and minimize rotor losses. The number of poles determines the speed of the motor and its torque characteristics. 2 or 4 poles are commonly used in applications that require moderate to high speeds and torque, such as small motors, fans, and pumps. These configurations also offer better efficiency and smoother operation compared to rotors with a higher number of poles.

Distributed winding is preferred over concentrated winding because it improves the generated emf wave-form and adds rigidity and mechanical strength to the winding. This means that the output voltage waveform of the machine will be smoother and more stable, resulting in better performance. Additionally, the distributed winding design provides better support and stability to the winding, reducing the risk of mechanical failure.

An alternator with a higher value of SCR (short circuit ratio) will have better voltage regulation and a higher stability limit. This is because a higher SCR indicates a larger reactance in the alternator, which helps to regulate the voltage output. Additionally, a higher SCR also means that the alternator can handle larger fault currents without destabilizing the system, leading to a higher stability limit.

The flux in a transformer core remains constant irrespective of the load. This is because the transformer operates on the principle of electromagnetic induction, where the primary winding creates a magnetic field that induces a voltage in the secondary winding. The amount of flux generated in the core is determined by the primary voltage and the number of turns in the winding, and it remains constant as long as these factors are unchanged. The load on the secondary side does not affect the flux in the core, only the magnitude of the induced voltage.

The losses occurring in a power transformer can be categorized into copper losses and iron losses. Copper losses refer to the energy dissipated as heat in the transformer's copper windings due to the resistance of the wire. On the other hand, iron losses include hysteresis losses and eddy current losses, which occur in the transformer's iron core. Hysteresis losses result from the reversal of magnetization in the core material, while eddy current losses occur due to circulating currents induced in the core by the alternating magnetic field. Therefore, the correct answer is copper and iron.

Synchronous motors can be considered as doubly excited magnetic systems because they have two sources of magnetic field excitation. One source is the stator windings, which create a rotating magnetic field, and the other source is the rotor windings, which create a magnetic field that is synchronized with the stator field. This synchronization allows the motor to rotate at a constant speed, making it useful for applications where precise speed control is required. Solenoids and electromagnetic relays, on the other hand, typically have only one source of magnetic field excitation.

To produce a polyphase field, at least two windings are required. A polyphase field refers to a magnetic field that is produced by multiple currents or phases. These windings are typically found in electrical machines such as generators or motors. The presence of multiple windings allows for the generation of a rotating magnetic field, which is necessary for the operation of polyphase systems. Therefore, the correct answer is two.

A polyphase field refers to a type of electrical field that is characterized by having multiple phases or waveforms. In this case, the correct answer states that the polyphase field is constant in amplitude and rotating at synchronous speed. This means that the field has a consistent magnitude and is rotating at a speed that is synchronized with the frequency of the power supply.

A synchronous motor is a type of AC motor that operates at a constant speed determined by the frequency of the power supply. Unlike an induction motor, a synchronous motor requires a separate DC power source to create a magnetic field in the rotor. This is achieved using slip-rings, which are conductive rings that allow the flow of DC current into the rotor windings. In a 3-phase synchronous motor, two slip-rings are typically used - one for the positive DC supply and the other for the negative DC supply. Therefore, the correct answer is 2.

The emf induced in the primary lags behind the flux by 90 degrees. This means that the maximum emf occurs 90 degrees after the maximum flux. This is because the emf is induced in the primary coil due to the changing magnetic field created by the flux. As the flux changes, the emf is induced, but there is a time delay between the two.

The correct answer is "two coil-sides" because a coil typically consists of two sides or windings that are wound around a core. These coil-sides are usually connected to form a continuous circuit and are used in various electrical devices and systems. Each coil-side is considered a separate conductor, and they work together to create a magnetic field or induce voltage in the coil.

The voltage regulation of a well-designed transformer refers to its ability to maintain a relatively stable output voltage despite changes in the input voltage. A voltage regulation of 2% means that the transformer can maintain the output voltage within 2% of the rated voltage, even if there are fluctuations in the input voltage. This indicates a high level of efficiency and accuracy in the transformer's design and performance.

In transformer circuit mode, the core loss is represented as a shunt resistance. This means that the core loss is modeled as a resistance connected in parallel with the transformer circuit. The shunt resistance represents the energy losses that occur in the core due to eddy currents and hysteresis. By including the shunt resistance in the circuit model, the effects of core loss can be accurately accounted for in the analysis and design of the transformer.

The inductive reactance of a transformer depends on magnetomotive force. The magnetomotive force is responsible for creating the magnetic flux in the transformer's core, which in turn induces a voltage in the secondary winding. The inductive reactance is a measure of the opposition to the change in current flow caused by the magnetic field. Therefore, the strength of the magnetomotive force directly affects the inductive reactance of the transformer.

Large sized synchronous machines generate a significant amount of heat during operation, and it is crucial to have an efficient cooling medium to prevent overheating and ensure optimal performance. Hydrogen is commonly used as a cooling medium in large synchronous machines due to its excellent heat transfer properties. It has a high thermal conductivity and low density, allowing for efficient heat dissipation. Hydrogen also has a low viscosity, which reduces friction losses and improves overall machine efficiency. Additionally, hydrogen is non-toxic and non-corrosive, making it a safe and reliable choice for cooling large synchronous machines.

When a transformer is supplying a pure resistive load, the power factor on the primary side will be about 0.95 (lag). This means that the current flowing through the transformer will lag behind the voltage by a small angle. This is because in a resistive load, the current and voltage waveforms are in phase, but due to the inherent characteristics of the transformer, there will be a slight lag in the current waveform. Therefore, the power factor is close to 0.95 (lag) in this scenario.

The stator of modern alternators are wound for 60 degrees phase groups. This means that the coils in the stator are arranged in groups that are spaced 60 degrees apart from each other. This arrangement allows for a more efficient and balanced distribution of power in the alternator, resulting in smoother operation and less vibration. Additionally, the 60-degree phase groups help to reduce harmonic distortion and improve the overall performance of the alternator.