Name: how a dc generator makes current
Uploaded: 2023-09-18T17:11:20.000Z
Duration: 1 min 19 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Physics

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Motion and Stability- Forces and Interactions

PS.Phy.5.2

DC Generator

Ein Gleichstromgenerator wandelt mechanische Energie in elektrische Gleichstrom um. Der Grundaufbau des Gleichstromgenerators ähnelt dem der einfachen Lichtmaschine, nur dass die beiden Schleifringe durch einen einzigen Spaltring ersetzt werden.
Wenn sich die leitende Schleife in einem gleichmäßigen Magnetfeld dreht, ändert sich der magnetische Fluss durch die Schleife sinusförmig, was zu induzierter EMK führt.
Dabei fungiert der Spaltring als Kommutator. Der Kommutator schaltet die Kontakte für jede halbe Umdrehung der Schleife und sorgt so für die gleiche Polarität der induzierten EMK.
Ähnlich wie bei einer Lichtmaschine ist die induzierte EMK Null, wenn der magnetische Fluss entweder maximal oder minimal ist.
Aufgrund des Kommutators ist die induzierte EMK jedoch nie negativ und hat ein Maximum, wenn der magnetische Fluss Null ist.
Die Größe der durchschnittlichen induzierten EMK wird geschätzt, indem der sinusförmige Term aus dem einfachen Generator durch seinen Durchschnittswert ersetzt wird.
Die Größe der induzierten EMK ist proportional zur Winkelgeschwindigkeit für das gegebene Magnetfeld und die Fläche der Schleife.

<ol>
	<li>Young, H.D. und Freedman, R.A. (2012). Universitätsphysik mit moderner Physik. San Francisco, Kalifornien: Pearson. Abschnitt 29.4, Seiten 964-965.</li>
</ol>

gleichstromgenerator

Ein Wechselstromgenerator wandelt mechanische Energie in elektrische Energie um, die sinusförmig variiert und somit zu Wechselstrom führt. Ein Gleichstromgenerator hingegen wandelt mechanische Energie in elektrische Energie um, die als Gleichstromimpulse mit gleicher Polarität vorliegen. Der Aufbau eines Gleichstromgenerators ähnelt dem eines Wechselstromgenerators, mit der Ausnahme, dass das Paar von Schlupfringen durch einen einzigen geteilten Ring, auch Kommutator genannt, ersetzt wird. Der Kommutator fungiert wie ein periodischer Drehschalter und ändert die Kontakte zu den Bürsten für jede halbe Drehung der leitenden Schleife, so dass die induzierte Spannung immer die gleiche Polarität hat.

Das grundlegende Funktionsprinzip von Gleichstromgeneratoren basiert auf dem Gesetz der elektromagnetischen Induktion von Faraday. Die leitende Schleife wird im homogenen Magnetfeld gedreht, und diese Drehung verändert den periodisch durch die Schleife fließenden magnetischen Fluss. Wenn die leitende Schleife mit dem externen Stromkreis verbunden ist, induziert der sich ändernde Magnetfluss eine Spannung, die zu einem induzierten Strom führt. Die Verbindung zum externen Stromkreis erfolgt über einen Kommutator und ein Paar Bürsten.

Die Größe der induzierten Spannung ist maximal, wenn der magnetische Fluss durch die leitende Schleife null ist, und minimal, wenn der magnetische Fluss durch die leitende Schleife entweder maximal oder minimal ist. Der magnetische Fluss hat positive und negative Werte, aber die induzierte Spannung hat immer die gleiche Polarität und schwankt zwischen null und maximal. Der Durchschnitt der induzierten Spannung wird berechnet, indem der durch Sinusfunktion variierende Komponente der Durchschnitt genommen wird. Die Größe der induzierten Spannung kann erhöht werden, indem das angelegte homogene Magnetfeld, die Fläche der leitenden Schleife und die Winkelgeschwindigkeit der Rotation erhöht werden.

Gleichstromgenerator

Eine Lichtmaschine wandelt mechanische Energie in sinusförmig variierende elektrische Energie um, was zu Wechselstrom führt. Währenddessen wandelt ein Gleichstromgenerator ...

Education

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Unit 1

Einheit 1

Electromagnetic Induction

Elektromagnetische Induktion

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

how a dc generator makes current

A DC generator makes electrical energy from mechanical motion. It produces DC pulses with the same polarity, unlike an alternator, which gives sinusoidal AC current. The two machines have similar construction, but a DC generator uses a single split ring instead of a pair of slip rings.

That split ring is called a commutator. It works like a rotary switch that changes the contact with the brushes after each half turn of the conducting loop. This action keeps the induced emf at the same polarity, even as the loop keeps rotating.

The generator works by Faraday’s law of induction. A conducting loop turns inside a uniform magnetic field, so the magnetic flux through the loop changes over time. When the loop is connected to an external circuit, that changing flux induces an emf and then an induced current.

The commutator and brushes provide the link to the external circuit. The induced emf is largest when the magnetic flux through the loop is zero. It is smallest when the magnetic flux is at its maximum or minimum. The magnetic flux can change between positive and negative values, but the induced emf stays positive and varies between zero and a maximum value.

The average induced emf is found by averaging the sinusoidal part of the signal. Its size can be increased by making the uniform magnetic field stronger, increasing the area of the conducting loop, or increasing the angular velocity of rotation.

How a DC Generator Makes Current

induction

Magnetic Flux and Induced Current

Magnetic flux and induced current are linked when a magnetic field changes near a coil. Moving a bar magnet into or out of a coil induces an emf, which is an electromotive force. If the motion goes in the opposite direction, the emf has the opposite sign. Reversing the magnet poles also reverses the direction of the induced emf. The same effect appears when the coil moves instead of the magnet. Relative motion is what matters. A faster motion produces a larger emf. If the magnet and coil stay...

ampere-maxwell's law: problem-solving

Displacement Current in an RC Circuit

Displacement current in an RC circuit can be found by tracking the charging capacitor. A parallel-plate capacitor with capacitance C, plate area A, and plate separation d is connected to a resistor R and a battery of voltage V. When the current starts to flow at t = 0, the charge and voltage on the capacitor change with time. The first step is to use the RC-circuit result for the capacitor voltage at time t. With the z-axis pointing from the positive plate to the negative plate, the electric...

differential form of maxwell's equations

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dc generator

A DC generator makes electrical energy from mechanical motion. It produces DC pulses with the same polarity, unlike an alternator, which gives sinusoidal AC current. The two machines have similar construction, but a DC generator uses a single split ring instead of a pair of slip rings. That split ring is called a commutator. It works like a rotary switch that changes the contact with the brushes after each half turn of the conducting loop. This action keeps the induced emf at the same...

magnetic damping

Eddy Currents and Magnetic Braking

Eddy currents can create strong drag on moving objects. This drag is called magnetic damping. A metallic pendulum bob swinging between the poles of a strong magnet feels this force as it enters and leaves the field, so its motion dies out quickly. The effect is much smaller for a slotted metal plate. As the plate enters the magnetic field, a changing flux induces an emf, or electromotive force. The slots limit the size of the current loops, and nearby loops can carry currents in opposite...

superconductor

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faraday's law

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lenz's law

Magnet Motion and Induced Current

Magnet motion and induced current are linked by Lenz’s law. The law says that the induced emf, or electromotive force, drives current around a wire loop in a direction that opposes the change in magnetic flux that caused it. A moving bar magnet can create this effect in a coil. If the magnet moves toward the coil, the magnetic flux increases and an induced current appears. That current produces a magnetic field that pushes against the stronger field from the magnet. The same idea works in...

motional emf

Induced Current from Changing Magnetic Flux

Induced current from changing magnetic flux depends on how a magnetic field interacts with an area. Magnetic flux changes when the field strength changes, when the area it passes through changes, or when the field turns relative to the surface. When any of these factors vary, the magnetic flux also varies. A change in area can happen in two common ways. The flux through a rectangular loop increases as the loop moves into a magnetic field. The magnetic flux through a rotating coil also changes...

faraday disk dynamo

Faraday Disk Dynamo and DC Output

A Faraday disk dynamo is a DC generator that produces an emf constant in time. It uses a conducting disk that spins at constant angular velocity in a magnetic field. The magnetic field is perpendicular to the plane of the disk. As the disk rotates, the magnetic flux changes and an emf is induced. This causes opposite charges to build up at the rim and at the center of the disk. The direction of the induced emf depends on the direction of the magnetic field and the direction of rotation. If...

induced electric fields

Faraday’s Law and Induced EMF

Faraday’s law explains how a changing magnetic flux can create an induced emf in a circuit. When current is induced, work is being done on the conduction electrons in the wire. That work must come from a source, and the source is not a battery or a magnetic field. A battery does not have to be present in a circuit for current to be induced. Magnetic fields also do not do work on moving charges. Instead, the change in magnetic flux produces an electric field in the wire. This is called an...

induced electric fields: applications

Electric Fields from Changing Magnetism

An electric field can be induced by a changing magnetic field. This induced electric field is nonconservative, which means it does not do net work when a charge moves around a closed path. That behavior is different from an electrostatic field, which comes from a fixed charge distribution. An electrostatic field is conservative, so it does no net work over a closed path. Because of this difference, electric potential can be linked to the electrostatic field, but not to the induced field. When...

eddy currents

Eddy Currents in Recycling, Brakes, and Heat

Eddy currents affect how metals move, stop, and heat up. They form only in conductors, so they are useful for separating metals from nonmetals and for other practical devices. In recycling centers, trash can be sent down a ramp under a strong magnet. Metals in the trash slow down because eddy currents resist their motion. Nonmetals keep moving, so the materials separate. This method works for metals in general, not only ferromagnetic ones. Eddy currents also help in metal detectors. A...

displacement current

Maxwell’s Fix for Changing Currents

Maxwell’s correction to Ampère’s law explains how changing electric fields can create magnetic fields. The key idea is the displacement current, Id, which Maxwell added to the real conduction current, I. This extra term helps the law work when current is not steady. The displacement current is defined using the permittivity of free space, ε0, and the electric flux, ΦE. It is not a real flow of charge. Instead, it is an added term in Maxwell’s equations that acts like current in Ampère’s law. <br...

significance of displacement current

Maxwell’s Fix for Charging Capacitors

Maxwell’s fix for a charging capacitor is the displacement current in Ampère’s law. A displacement current is not a flow of charge like conduction current. It comes from a changing electric field. That is why it is defined using a time-varying electric field and has its own displacement current density. Maxwell added this term to the original Ampère’s law. The result is called generalized Ampère’s law. The added displacement current term lets the law work in regions where no charges are moving.

electromagnetic fields

Electric and Magnetic Fields in Motion

Electric and magnetic fields can come from different sources, but they can act in surprisingly similar ways. Static charges create electrostatic fields. These fields are conservative, which means a test charge does no net work after moving around a complete loop. Fields made by changing magnetic fields behave differently. They are not conservative, and they do net work on a charge. That work is proportional to how fast the magnetic flux changes. These fields also do not follow Gauss’s law for...

maxwell's equation of electromagnetism

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symmetry in maxwell's equations

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Electric and magnetic field symmetry is a key part of Maxwell's equations. After the fields are calculated with Maxwell's four equations, the Lorentz force equation gives the force those fields exert on a charged particle moving at a certain velocity. The Lorentz force law combines the effect of the electric field and the magnetic field on the moving charge. Maxwell's equations and the Lorentz force law together describe electricity and magnetism. The symmetry in Maxwell's framework is easier...

electric generator: alternator

How Alternators Produce AC Power

Alternators produce alternating current by rotating a conducting loop in a magnetic field. As the loop turns inside the field, the magnetic flux through the coil changes with time. That changing flux induces an emf, or electric voltage, in the circuit. A simple alternator uses a loop placed in a uniform magnetic field. The loop connects to split rings, and brushes carry the current to the external circuit. As the loop rotates, the magnetic flux changes sinusoidally, so the induced emf also...

back emf

Motor Current and Speed Changes

Motor current and speed changes are linked by back emf in an electric motor. A motor sends current through a wire loop in a magnetic field. The magnetic field pushes on the loop and creates torque, which turns a shaft and does mechanical work. As the coil turns, the magnetic flux through it changes. That changing flux induces an emf, or voltage, by Faraday’s law. The motor then acts like a generator whenever its coil rotates, whether the shaft is turned by an external drive or by the motor...

key terms and concepts

Learn how a DC generator uses a commutator, brushes, and magnetic flux changes to produce DC pulses from rotating motion and Faraday’s law.

How a DC Generator Makes Current | JoVE K12

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JoVE Core Physics introduces the theory and application of fundamental physics topics, including kinematics, dynamics, fluid mechanics, statics, waves, and acoustics, with concise and easy-to-understand animated videos. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

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Waves and Their Applications in Technologies for Information Transfer

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How a DC Generator Makes Current

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