How Materials Become Magnetized
Ferromagnetic materials, such as iron, nickel, and cobalt, undergo magnetization when exposed to the magnetic field of a magnet. For instance, a permanent magnet can attract and magnetize objects like paper clips, nails, or iron filings. In this process, the object not only becomes attracted to the magnet but also acquires magnetism itself. However, this magnetism tends to diminish when the object is removed from the magnetic field.
The magnetic properties of the material impact both the magnetic flux density at the poles and the way flux density decreases with distance from the poles. Physical size also plays a role in flux density; for example, two disk magnets made from sintered Alnico may have similar densities near the pole, but the larger magnet exhibits higher flux density as you move away from the pole. This type of information is crucial for determining the effectiveness of a magnet in specific applications, especially considering the working distance.
Ferromagnetic materials possess minute magnetic domains formed within their atomic structure due to the orbital motion and spin of electrons. These domains can be visualized as very small bar magnets with north and south poles. In the absence of an external magnetic field, these domains are randomly oriented (Figure 1a). However, when the material is introduced into a magnetic field, the domains align themselves (Figure 1b), turning the object into a magnet.
The Hall Effect
The Hall effect is a phenomenon where a small voltage (a few volts) is generated on opposite sides of a thin current-carrying conductor or semiconductor known as the Hall element, which is placed in a magnetic field. This results in the appearance of a voltage called the Hall voltage across the Hall element. The Hall voltage is a consequence of the forces acting on electrons as they move through the magnetic field, leading to an accumulation of charge on one side of the Hall element. Although the effect was initially observed in conductors, it is more pronounced in semiconductors, which are commonly used in Hall-effect sensors.
In Hall-effect sensors, the magnetic field, electric current, and Hall voltage are all perpendicular to each other (as illustrated in Figure 2). This voltage can be amplified and utilized to detect the presence of a magnetic field, making it valuable in sensor applications.
Hall-effect sensors are favored for their small size, affordability, and lack of moving parts. They are non-contacting sensors, which contributes to their durability over numerous operations—a significant advantage over contacting sensors that may wear out. These sensors can detect the proximity of a magnet by sensing its magnetic field, making them suitable for position measurements or motion sensing. They are often employed in combination with other sensor elements for measuring current, temperature, or pressure.
The automotive industry extensively uses Hall-effect sensors for various purposes such as measuring throttle angle, determining crankshaft and camshaft positions, monitoring distributor position, tachometer readings, and gauging power seat and rear-view mirror positions. Additionally, these sensors find applications in measuring parameters for rotating devices like drills, fans, flow meter vanes, and disk speed detection.
Back EMF in DC Motors
When a DC motor is initiated, a magnetic field is established by the field windings. The armature current generates an additional magnetic field that interacts with the field winding’s magnetic field, initiating the motor’s rotation. As the armature windings start to spin within the magnetic field, generator action occurs. In other words, the rotating armature produces a voltage across it, which opposes the original applied voltage, following Lenz’s law. This self-generated voltage is termed “back electromotive force” or back EMF.
While the term “electromotive force” (EMF) was historically used for voltage, it is now less favored due to the recognition that voltage is not a “force” in the physics sense. However, “back EMF” is still employed to describe the self-generated voltage in motors. Back EMF, also known as counter EMF, plays a crucial role in reducing armature current when the motor is operating at a constant speed. It acts as a counteracting force to the applied voltage, limiting the flow of current in the armature windings as the motor reaches a stable rotational speed. Understanding and managing back EMF is essential in designing and controlling DC motors effectively.
Glossary of Electromagnetic Terms
Ampere-turn (At): The SI unit of magnetomotive force (mmf).
Electromagnetic field: A formation of a group of magnetic lines of force surrounding a conductor created by electrical current in the conductor.
Electromagnetic induction: The phenomenon or process by which a voltage is produced in a conductor when there is relative motion between the conductor and a magnetic or electromagnetic field.
Electromagnetism: The production of a magnetic field by current in a conductor.
Faraday’s law: A law stating that the voltage induced across a coil equals the number of turns in the coil times the rate of change of the magnetic flux.
Gauss: A CGS unit of flux density.
Hall effect: A change in current density across a conductor or semiconductor when current in the material is perpendicular to a magnetic field. The change in current density produces a small transverse voltage in the material, called the Hall voltage.
Hysteresis: A characteristic of a magnetic material whereby a change in magnetization lags the application of the magnetic field intensity.
Induced current (i ind): A current induced in a conductor as a result of a changing magnetic field.
Induced voltage (v ind): Voltage produced as a result of a changing magnetic field.
Lenz’s law: A physical law that states when the current through a coil changes, the polarity of the induced voltage created by the changing magnetic field is such that it always opposes the change in current that caused it. The current cannot change instantaneously.
Lines of force: Magnetic flux lines in a magnetic field radiating from the north pole to the south pole.
Magnetic field: A force field radiating from the north pole to the south pole of a magnet.
Magnetic flux: The lines of force between the north and south poles of a permanent magnet or an electromagnet.
Magnetic field intensity: The amount of mmf per unit length of magnetic material.
Magnetomotive force (mmf): The cause of a magnetic field, measured in ampere-turns.
Permeability: The measure of ease with which a magnetic field can be established in a material.
Relay: An electromagnetically controlled mechanical device in which electrical contacts are opened or closed by a magnetizing current.
Reluctance ( ): The opposition to the establishment of a magnetic field in a material.
Retentivity: The ability of a material, once magnetized, to maintain a magnetized state without the presence of a magnetizing force.
Solenoid: An electromagnetically controlled device in which the mechanical movement of a shaft or plunger is activated by a magnetizing current.
Speaker: An electromagnetic device that converts electrical signals to sound waves.
Tesla (T): The SI unit of flux density.
Weber (Wb): The SI unit of magnetic flux, which represents lines.