MARELLI MOTORS - THREE-PHASE AND SINGLE-PHASE ELECTRIC MOTORS

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La Marelli Motors S.p.A., a company based in Arzignana, in the province of Vicenza, has a long tradition dating back to 1891. With more than 100 years of experience Marelli Motori is recognized as a leading supplier in the Power Generation, Industrial, Petrochemical and Marine sectors offering a complete range of Motors and Generators in Low, Medium and High Voltage.

This section is devoted to electric motors, by which term is usually referred to as an electric machine in which the input power is of the electrical type and the output power is of the mechanical type, assuming the function of an actuator. This type of electric machine is based, similarly to the electric generator, on electromagnetic forces interacting between a system of currents and a magnetic field.

Several distinctions can be made based on other references: for example, the distinction between synchronous motors, in which the power frequency is a multiple of the rotational frequency, and asynchronous motors, in which the two frequencies are different; therefore, usually the categories into which electric motors are classified are asynchronous motor, synchronous motor, or DC motor.

The synchronous motor is a type of alternating current electric motor in which the stator, usually three-phase, generates a rotating magnetic field. There is a magnetic field in the rotor (generated by a permanent magnet or DC-powered winding) that is attracted to the rotating magnetic field of the stator, generating the driving torque.

Starting this type of motor is relatively complex because it has a pulse torque curve centered on the frequency of the stator supply current; this means that the rotor has drive torque only and exclusively if it is turning at the same frequency as the stator alternating current. Therefore, when the motor is stopped, the application of the alternating voltage is unable to produce motor starting because the rotor has zero torque. Therefore, the motor is initially brought to its final rotational speed by means of an asynchronous motor, then, after disconnecting the latter, the supply voltage is connected at the same rotational frequency as achieved and, subsequently, the mechanical user load is inserted. In addition to physically having 2 motors in parallel, this can also be accomplished with specially made synchronous motors (provided with an additional squirrel-cage rotor that provides asynchronous behavior), then switch to synchronous mode. In recent years, the use of power electronics has drastically simplified starting; in fact, it allows both the supply voltage (and thus current) and frequency to be adjusted. Thus starting from zero frequency and increasing it very gradually, a torque capable of accelerating the motor from standstill is continuously driven. Drives that enable this mode (inverters or cycloconverters) are made with semiconductor components such as the thyristor or Insulated Gate Bipolar Transistor (IGBT) transistor.

Because of the limited practicality of the synchronous motor, its use with direct supply from the mains is limited to fields of application where a particularly precise and stable rotational speed is required, for example, in the paper industry, where the perfect synchronism of several motors makes it possible to avoid sheet breakage. It is, on the other hand, widely used to drive variable-speed loads where powered by static converter (inverter), as is the case, for example, in most electric vehicles (almost all except those of Tesla Motors, which are asynchronous). There are also small synchronous motors with automatic start-up and single-phase power supply used in timing mechanisms such as household washing machine timers and at one time in some clocks, taking advantage of the good accuracy of the mains frequency.

Compared with an asynchronous motor, the synchronous motor is unable to adapt to significant changes in the resisting torque; in fact, if once at full speed the rotation is braked or accelerated beyond a certain limit, a series of oscillations are triggered that bring the motor to a standstill and can cause severe overcurrents such as to damage the motor; overcurrent protection must also be provided.

The asynchronous motor is a type of alternating-current electric motor in which the rotational frequency is not the same as or a submultiple of the mains frequency, that is, it is not "synchronous" with it; this is why it differs from synchronous motors. The asynchronous motor is also called an induction motor by virtue of its principle of operation.

When, due to an external force, the rotor has a speed greater than the rotating field of the stator the asynchronous motor can be used as an asynchronous generator with or without the use of capacitors depending on whether it is connected to the grid or not. It is used for small powers, in occurrences where ease of use is preferred over the synchronous motor (which requires the use of inverters) even at the expense of efficiency. However, efficiency is a function of maximum power and decreases as the number of poles increases, generally ranging from 0.67 for three-phase motors to 0.97 for larger motors

The motor consists of a fixed part called the stator and a moving part called the rotor. The stator consists of a stack of laminations having the shape of a circular crown. The grooves inside the stator plate pack accommodate the conductors (enameled copper wire) of the stator winding, which can be either three-phase or two-phase (depending on the type of alternating current supply). The rotor is located inside the stator and consists of a stack of laminations having the shape of a circular crown. It has an inner hole for the rotating shaft to pass through, and outer grooves (rotor slots) to accommodate the rotor winding. The latter can be of two types:

• wound rotor (also called ring rotor);
• squirrel-cage rotor (also called short-circuited).

A small space called an air gap is left between the stator and rotor to allow free rotation of the rotor. This thin thickness of air (which acts as a dielectric) is a few tenths of a millimeter or as small as mechanical tolerances allow. Stator windings are usually encased in resins, which also provide excellent protection from water and weathering.

The stator generally contains an even number of windings since, normally, there are two for each power phase. A three-phase, or three-phase, motor will therefore have at least six windings, that is, one pole pair for each phase, while a two-phase motor will typically have four windings. The two windings of each pole pair are connected in series and physically arranged opposite each other. In the case of the three-phase motor with six windings, the pole pairs have a phase shift of 120° physical and electrical; on the other hand, in the two-phase motor, the two pole pairs have a phase shift of 90° physical and electrical.

Another distinction that can be made is on the differentiation of three-phase and single-phase motors.

The three-phase motor is a type of electric motor whose operation is based on the application of Galileo Ferraris' rotating magnetic field principle to a three-phase set of input currents. In fact, to operate it requires the use of three-phase system of currents, displaced from each other in time and space by 120 electrical degrees. The neutral may or may not be accessible depending on the intended application.

Single-phase motor is a type of electric motor that is powered by single-phase alternating current