30.20
发电机：交流发电机
发电机通过在磁场中旋转线圈来感应电动势。 简单的交流发电机是一种交流发电机，它产生随时间呈正弦变化的电能。 一个简单的交流发电机由放置在均匀磁场内的导电环组成。 该环路通过电刷连接到与外部电路相连的开口环。
当线圈在磁场内旋转时，穿过线圈的磁通量呈正弦变化。 这种正弦变化的磁通量感应出也呈正弦变化的…
Electric generators convert mechanical energy into electrical energy. One of the examples is the simple alternator, which consists of a conducting loop rotating with constant angular velocity, placed inside a constant uniform magnetic field.
As the loop rotates, the magnetic flux through the loop changes. Here, the angle between the area vector and the magnetic field is a product of angular velocity and time. Faraday's law gives the induced emf because of the loop rotation.
The magnetic flux and the induced emf vary sinusoidally with time. When the magnetic field and the area vector are perpendicular, the magnetic flux is zero, but the induced emf is either maximum or minimum.
Similarly, magnetic flux is maximum and minimum when the magnetic field and area vector are parallel and antiparallel, resulting in zero induced emf.
The induced emf for an alternator can be maximized by enlarging the uniform magnetic field, area of the loop, and angular velocity.
Alternators can be used as a source of current in external circuits with the help of two slip rings and brushes.
View the full transcript and gain access to JoVE Core videos
Q1: How does an alternator convert mechanical energy into electrical energy?
An alternator converts mechanical energy into electrical energy through electromagnetic induction. A conducting loop rotates with constant angular velocity inside a uniform magnetic field. As the loop rotates, the magnetic flux through it changes, inducing an emf according to Faraday's law. This induced emf drives current through an external circuit via slip rings and brushes.
Q2: Why does the induced emf in an alternator vary sinusoidally with time?
The induced emf varies sinusoidally because the magnetic flux through the rotating loop changes sinusoidally. The angle between the area vector and magnetic field equals the product of angular velocity and time. As this angle changes continuously, the flux and induced emf follow a sinusoidal pattern, creating AC current that also varies sinusoidally.
Q3: What is the relationship between magnetic flux and induced emf at different rotation angles?
Magnetic flux and induced emf have an inverse relationship during rotation. When the magnetic field and area vector are perpendicular, magnetic flux is zero but induced emf reaches maximum or minimum. Conversely, when they are parallel or antiparallel, magnetic flux is maximum or minimum while induced emf is zero.
Q4: How can the induced emf in an alternator be maximized?
The induced emf can be maximized by increasing the uniform magnetic field strength, enlarging the loop area, increasing the angular velocity of rotation, or adding more loops to the coil. Each factor directly increases the rate of magnetic flux change, thereby amplifying the induced emf generated by the alternator.
Q5: What role do slip rings and brushes play in an alternator?
Slip rings and brushes connect the rotating conducting loop to the external circuit. The split rings rotate with the loop while brushes maintain electrical contact, allowing the sinusoidally varying induced current to flow continuously into external devices. This mechanism enables the alternator to supply AC power to external loads.
Q6: How do alternators in automobiles differ from simple laboratory alternators?
In automobile alternators, the design is reversed: the loop remains stationary while electromagnets rotate around it. This configuration is more practical for vehicle integration. Automotive alternators power ignition systems, lights, and entertainment systems, demonstrating induced electric fields applications in real-world technology.
Q7: What determines whether induced emf is at maximum or minimum during alternator rotation?
Induced emf reaches maximum or minimum when the rate of change of magnetic flux is zero. This occurs when the magnetic field and area vector are perpendicular, meaning the flux is momentarily constant. At these instants, the emf is either at its peak positive or negative value before reversing direction.