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Synchronous And Induction Motor: Which Are The Main Differences Between Them?
Aug 16, 2017

The synchronous motor it’s a type of AC electric motor in which the rotation period is synchronized with the frequency of the supply voltage, typically three-phase.

This type of electric motor is composed by a rotor on which are located different magnetic poles of alternate polarity created by permanent magnets or by windings supplied by direct current, also said excitation current, and a stator on which are present the windings of the power supply circuit.

The rotation of the shaft is synchronized with the frequency of the supply current.

Until a few years ago, with the engine stopped, the application of the AC voltage generated a starting problem. Due to the inertia effect, the rotor had no time to follow the rotating magnetic field, remaining stationary. The engine was started using an asynchronous motor and subsequently, after having disconnected the latter, being connected to the supply voltage and plugged the mechanical user load. If the synchronous motor was slowed or accelerated beyond a certain limit, it triggered a series of oscillations that led to the stop of the engine and this could provoke strong overcurrents that could damage the engine. For this reason it was implemented an overcurrents protection, such as a magneto-thermal protection switch.

Nowadays, with the advent of modern inverters it is possible to vary the supply voltage in amplitude and frequency so that the generated field is always synchronous to the rotation of the rotor also during the starting phase. Thanks to this device there’s no more need for the asynchronous engine to start the motor.

The asynchronous motor, invented by Galileo Ferraris in 1887, on the other hand is a type of AC electric motor in which the rotation frequency is not equal to or a submultiple of the network frequency. The asynchronous motor is also said induction motor thanks to its operating principle.

The asynchronous motor consists of a fixed part (the stator) and a rotating part (rotor). Typically the stator contains the rotor. Both pieces are drilled to let through the leads that will be crossed by the current.

Lo statore ospita normalmente un avvolgimento trifase, i cui conduttori sono distribuiti in modo che una terna di correnti sinusoidali nel tempo produca una distribuzione spaziale di campo magnetico sinusoidale rotante.

The stator usually houses a three-phase winding, whose conductors are distributed in such a way that a set of three sine currents, in a given time, produces a sinusoidal rotating magnetic field spatial distribution.

Most induction motors today contain a rotational element (the rotor) dubbed a squirrel cage. The cylindrical squirrel cage consists of heavy copper, aluminum, or brass bars set into grooves and connected at both ends by conductive rings that electrically short the bars together. The rotation of the magnetic field take place at a fixed speed, relative to the supply frequency, called synchronism speed. In both the synchronous and induction motor, the arrangement of the electrical wires is made in order to take advantage of the alternating current that create a rotating magnetic field. In the synchronous motor the rotor is usually constituted by a permanent magnet. In the induction motor instead, on the rotor are present the windings which are short-circuited. Both in the synchronous and asynchronous motor, the stator currents generate a rotating magnetic field, which interacts with the magnetic field of the rotor, making it rotate.