Beatrice Lang
Electromagnets are used in AC electricity to power motors, generators, transformers, and inductors. They are made by winding wire, usually copper, into a coil around a magnetic core, often made of iron. The magnetic field is produced by an electric current, and the strength of the field can be adjusted by changing the amount of current. Unlike permanent magnets, electromagnets require a continuous supply of current to maintain their magnetic field. AC electromagnets have a constantly changing magnetic field, which causes energy losses in their magnetic cores, resulting in heat dissipation. This phenomenon is not observed in DC electromagnets, where the magnetic field is constant. AC electromagnets are advantageous in certain applications due to their ability to demagnetize objects and their use in transformers, which are essential for long-range electric power distribution.
| Characteristics | Values |
|---|---|
| Type of magnet used in AC electricity | Electromagnet |
| How it works | A current through a wire creates a magnetic field |
| Magnetic field | Constantly changing |
| Core material | Ferromagnetic or ferrimagnetic material (e.g. iron) |
| Applications | Transformers, inductors, AC motors and generators |
| Energy losses | Eddy currents and hysteresis losses |
| Voltage spikes | Caused by sudden changes in current |
| Diode usage | Prevents voltage spikes by providing a path for current to recirculate |
| Advantages over DC | Simpler design, greater reliability, lower manufacturing cost |
| Mutual induction | Two or more coils placed together induce voltage in each other |
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What You'll Learn
Transformers
The two windings are called the primary and secondary coils. The primary coil receives the input voltage and creates a magnetic field when energised by an alternating current (AC). The secondary coil is where the transformed voltage is induced, delivering the output to the load. The number of turns in each winding determines the transformer's voltage ratio. For example, if the secondary winding has twice as many turns as the primary, the output voltage will be double the input voltage.
The operation of a transformer is based on the principle of electromagnetic induction, discovered by Michael Faraday in 1831. When an alternating current (AC) flows through the primary coil of a transformer, it creates a constantly changing magnetic field around the coil. This magnetic field extends to the transformer's core, which channels the field to the secondary coil. As the magnetic field fluctuates, it induces a voltage in the secondary coil through electromagnetic induction.
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Inductors
In AC circuits, the change in current flow generates an induced electromotive force (EMF) that opposes the current flow. The effect of this current opposition is referred to as inductive reactance (symbol XL), which is measured in ohms. Inductive reactance is the opposition that an inductor offers to alternating current due to its phase-shifted storage and release of energy in its magnetic field. Reactance is symbolized by the capital letter “X” and is measured in ohms just like resistance (R). Inductive reactance increases with increasing frequency. In other words, the higher the frequency, the more it opposes the AC flow of electrons.
Inductive reactance depends on inductance and supply frequency and can be calculated using the formula: XL = 2πfL. Inductance is a property of an electrical component known as an inductor, which arises when current flows through it, generating a magnetic field. This magnetic field interacts with other parts of the circuit, leading to the phenomenon of inductive reactance, a form of opposition to the change in current. Inductive reactance contributes to the overall impedance of the circuit and has significant implications for signal filtering and frequency response.
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AC motors
The squirrel cage in an AC motor is a set of rotor bars connected to two rings, one at either end. When AC power is sent through the stator, it creates an electromagnetic field. The bars in the squirrel cage rotor are conductors, so they respond to the flipping of the stator's poles, creating its own magnetic field. This is how the rotor rotates and creates torque.
The key to an AC induction motor is that the rotor is always trying to catch up with the stator's magnetic field, which is always a little faster. This creates the torque needed to generate mechanical power. The speed of an AC motor can be altered by having additional sets of coils or poles that can be switched on and off to change the speed of magnetic field rotation. However, the frequency of the power supply can also be varied to control the motor speed.
Most AC motors use the squirrel-cage rotor, found in almost all domestic and light industrial applications. The squirrel-cage motor can be viewed as a transformer with a rotating secondary. When the rotor is not rotating in sync with the magnetic field, large rotor currents are induced, which magnetize the rotor and bring it into synchronization with the stator's field. As the mechanical load increases, so does the electrical load.
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Generators
The most common type of generator used today is the AC (alternating current) generator, which is used for daily power generation. AC generators produce a current that continuously alternates direction. Inside an AC generator, one or more rotors (wire coils) are put in motion by a mechanical force and rotate inside a magnetic field. The mechanical energy is then used to produce electricity.
The magnetic field in an AC generator is provided by permanent magnets or electromagnets. Permanent magnets are simple in that they require no system for the provision of field current, but they do not contain any means for controlling the output voltage. Electromagnets, on the other hand, allow for the adjustment of the current and voltage, thus controlling the strength of the magnetic field and the amount of current generated in the coil.
The process by which magnets generate electricity is called electromagnetic induction. When a magnet gets close to a wire, its force causes electrons to flow, producing electricity. If a magnet is moved in and out of a coil of wire, a current is generated in the coil. This current alternates direction as the magnet changes direction, producing an AC current. The faster the magnet moves and the stronger the magnet, the stronger the current.
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Voltage spikes
In an AC (alternating current) setup, electromagnets are used in transformers, inductors, and AC motors and generators. The magnetic field in this setup is constantly changing, which causes energy losses in the magnetic cores that are dissipated as heat. This phenomenon is not observed in DC setups, where the magnetic field is constant.
The impact of voltage spikes on electronic devices can be detrimental. Electronic devices are designed to handle a specific voltage range, and any sudden increase in voltage can lead to overheating and eventual damage. Voltage spikes can cause electrons to jump, harming integrated circuits and larger electrical components. To mitigate the effects of voltage spikes, devices like surge protectors or surge suppressors are used to limit excess voltage and prevent harm to sensitive electronics.
Additionally, in the context of electromagnets, voltage spikes can occur when the current through the magnet is suddenly changed. When the current is increased, energy from the circuit is stored in the magnetic field. However, when the current is abruptly turned off, the energy in the magnetic field is rapidly returned to the circuit, resulting in a large voltage spike. This spike can cause arcing across switch contacts, potentially damaging them. To address this issue, a capacitor or a diode (freewheeling diode or flyback diode) can be used to prevent voltage spikes by providing a path for the current to recirculate and dissipate safely.
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Frequently asked questions
An electromagnet is a type of magnet in which a magnetic field is produced by an electric current.
An AC electromagnet is used in AC electricity.
An AC electromagnet has a constantly changing magnetic field.
AC electromagnets are used to create electric generators, motors, and power distribution systems that are more efficient than DC. They are also used in transformers, inductors, and AC motors and generators.
The magnetic core of an AC electromagnet experiences energy losses, which are dissipated as heat. The ferromagnetic core will heat up due to eddy currents and hysteresis losses, reducing the magnet's performance.
