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Lesson Two: Theory about electromagnetism
2 Theory about electromagnetism
This chapter will explain how electromagnetism works and how it is used to make an electrical motor to rotate.2.1Magnets and magnetic fields
A magnet is an object or material that creates a magnetic field. Magnetic fields are an invisible force that attracts ferromagnetic metals such as iron, steel, nickel and cobalt. Magnets have two poles, a north pole and a south pole. The magnetic field travels from the North Pole to the South Pole.
The magnet also attracts or repels other magnets depending on which way the poles are facing. If the poles are of the opposite polarity, they will attract each other. If the poles are of the same polarity, they will repel each other.
This information is relevant for understanding the induction motor but even more useful in understanding how reluctance and permanent magnet motors work, which will be talked about after the induction motor.
There are two kinds of magnets, permanent magnets and electromagnets. Permanent magnets can be of two types.
They can be made from rare-earth magnet materials like neodymium-iron-boron or be of ferromagnetic type where a ferromagnetic object like iron have been magnetized to have a persisting magnetic field.
Electromagnets are temporary magnets created with the use of a current and a ferromagnetic core such as iron.
The biggest advantage of these electromagnet is that they can be controlled by a current. (HyperPhysics)
2.2Electromagnets
A wire that conducts a current creates a magnetic field that will wrap itself around the wire, this idea is used in creating electromagnets.
Use your right hand fingers to determine which way the magnetic field will rotate if you place your thumb in same direction as the current in the wire.
This rule can also be used to determine which is the South and North Pole in an electromagnet. This rule is called the right-hand rule. If the current switches direction in the wire, the magnetic field will also switch direction.
Electromagnets are created by wrapping a conducting wire around an iron core and send a current trough the wire. Now the iron core will act as a magnet.
Figure 6. Right-Hand rule
Figure 7. Electromagnet
If we use DC in the wire, the magnet has a stationary north and South Pole. If we use AC, the direction of the current will change with the sinusoidal wave so 50 times a second if connected to a 50Hz supply. This means that the direction of the magnetic field created also will change as often, and the pole polarity will shift 50 times a second as well. Electromagnets can be turned on and off by controlling the current.
An electromagnet does not need an iron core but it helps conducting the magnetic field and “boosts” the power of the electromagnet as a magnetic field flows very poorly in air.
The windings and the core in the stator acts as electromagnets which in turn creates magnetic fields. (HyperPhysics)2.3Electromagnetic induction and Lorentz force
Electromagnetic induction is the idea that a current will be induced in a closed electrical circuit when exposed to a changing external magnetic field.
It also works so that a closed electrical circuit that is conducting a current and is exposed to an external magnetic field will be affected by a force and start to move in the direction the fingers point in Figure 8. That force is called the Lorentz force and is a base principle on how an electrical motor works. (HyperPhysics)
Figure 8. Left Hand Rule
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