Electrical Machines: Transformers, Circuits, and Magnetic Fields
Electrical Machines
An electrical machine is a device that converts electrical energy into other forms of energy, or changes the power but with a different presentation. This energy goes through a period of storage in a magnetic field. They are divided into generators, motors, and transformers.
Induction
Induction is the phenomenon that causes the production of an electromotive force (EMF) in a medium or body exposed to a variable magnetic field, or in a movable medium relative to a static magnetic field.
Inductors
Inductors refer to the phenomenon whereby an electric current that varies over time in an electrical circuit produces another EMF or induced voltage variation opposite to the voltage inductor in the same circuit.
Magnetic Circuits
Magnetic circuits are made from pieces of thick material cut from thin sheet metal, coils, or strips of magnetic steel.
Nucleus
The nucleus is the magnetic circuit that transfers energy from one circuit to another, and its main function is to conduct the magnetic flux.
Windings
Windings are the circuits and load. The main function of the primary winding is to create a magnetic field with very small energy loss. The secondary winding should take advantage of the magnetic flux to produce an electromotive force.
Transformers
A transformer works only with AC because it needs the magnetic field to vary over the coil to induce electrical current. Types of transformers include power and measurement.
In a star transformer, line voltage and phase voltage are different, but the intensity of the line and phase are the same. In a delta transformer, it is reversed.
Autotransformer
An autotransformer has the same features as a transformer, with the difference that it has a winding on top of the other, isolated in each of the coils. The advantages are fewer losses, less weight, and material savings. The disadvantages are that it cannot be used in large transformers and cannot be used as a separator for networks.
Transformer Testing
Tests are done on a transformer to determine the losses that occur. The types of losses that occur are losses in the magnetic circuit (iron losses) and copper losses in the electrical circuit.
- Vacuum Test: A vacuum test provides the losses in the magnetic circuit by leaving the secondary circuit open and feeding the primary to its rated voltage.
- Short-Circuit Test: A short-circuit test is performed by shorting the transformer secondary with an ammeter and an adjustable voltmeter. This test reveals losses in the copper (Vdc), impedance, and resistance of the transformer, as well as the maximum Icc.
To measure the short-circuit, use an ammeter connected in series with the secondary output.
The apparatus used to measure insulation resistance is the Megger.
Fundamental Concepts
- Particles are formed by the neutral atom, proton, and electron.
- Metals are good conductors due to the ease with which electrons move through the last valence shell.
- Coulomb’s Law describes the value of the force to attract or repel two electrically charged particles located at a fixed distance.
- Three good insulators are plastic, wood, and rubber because they have high electrical resistance.
- Magnetic field strength is the magnetomotive force required to magnetize a unit length of material.
- The unit of magnetic field strength is oersteds.
- Magnetic flux is the total number of lines of force in a magnetic field.
- Magnetic permeability is the degree of facility that a medium provides to be traversed by the lines of force forming a magnetic field.
- Polarity depends on the direction of movement of the conductor in the magnetic field.
- Eddy current is an electrical phenomenon that occurs when a conductor passes through a varying magnetic field. To avoid this, stacked steel plates are used.