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Asynchronous Motors

Asynchronous motors or so-called induction motors represent most of the electric motors on the market today. These motors have a short-circuit rotor, which is formed by aluminum bars that are housed in the groove cores of the magnetic plate and at the ends. They are connected to each other by means of a ring (short-circuit ring).

The variable magnetic field created in the stator induces sinusoidal currents in the rotor cage bars, which in turn create a magnetic field in the rotor that opposes the stator field.

They also have a coiled rotor, this solution being less used and since poles with the same polarity repel each other, there is a force in the direction of rotation of the rotor.

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It rotates with a speed slightly less than the speed of synchronism and because of this characteristic, this type of motor can start directly from the network, without the help of any other motor or even of power devices.

An important aspect of this type of motor is the difference between the speed of synchronism and the rotor speed, known as sliding or slipping.

 

The Law of Faraday-Neumann-Lenz

The Faraday-Neumann-Lenz law, also known as Faraday’s law of induction or law of electromagnetic induction, is one of the basic principles of electromagnetism. It specifies how a magnetic field can interact with an electric circuit so that it can produce an electromotive force, that is, a phenomenon known as electromagnetic induction. It is considered as the basis of the operation of transformers, alternators, dynamos, inductors and other types of electric motors, generators and solenoids.

Michael Faraday can be attributed to the discovery of electromagnetic induction and the name of the law relating to this phenomenon, as proved by experiments by Faraday, although his explanation is limited to the concept of power lines.

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The first formulation of Faraday’s law was made by Franz Ernst Neumann in 1845, in which the electromotive force generated in a circuit by induction should be expressed by the negative of the magnetic flux derivative over time by the area delimited by that circuit. The negative sign refers to the direction of the circuit and the electric current, which can be expressed through the so-called Lenz Law, developed by Heinrich Lenz in the year 1834.

Variation of Voltage and Frequency

An electric motor needs to be able to perform its main function continuously, although it may not meet its performance specifications at rated voltage and frequency and may suffer some deviations. The temperature increases may be higher according to nominal voltage and frequency. An engine must be able to perform its main functions, and may present larger deviations, in terms of performance specifications at rated voltage and frequency.

Temperature increases may be higher than those at rated voltage and frequency, so prolonged operation is not recommended. The starting of a three-phase cage motor must be direct, via contactors. It must be considered that for a given motor, the torque and current curves need to be fixed, regardless of the load, for a constant voltage.

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If the motor starting current is high, some damaging consequences can occur, such as the high voltage drop in the mains supply system. Due to this factor, interference may be caused to equipment installed in the system. The protection system of cables and contactors will need to be oversized, generating a high cost.