Prime mover selection

Since the invention of three-phase alternating current, I think electric motors are generally chosen as prime movers; after all, diesel engines and the like are rather troublesome.

What I mean is the criteria for choosing between synchronous motors and asynchronous motors

Selection between synchronous motors and asynchronous motors

I. Differences in design between synchronous motors and asynchronous motors: ① The main difference between synchronous and asynchronous motors lies in whether the rotor’s speed is consistent with the speed of the magnetic field rotating in the stator. If the rotor’s speed matches that of the stator, it is called a synchronous motor; if not, it is called an asynchronous motor. . . ②When the number of pole pairs is constant, there is a strict relationship between the motor’s speed and frequency; in motor terminology, this is referred to as synchronization. An asynchronous motor, also known as an induction motor, is primarily used as an electric motor, and its rotor speed during operation is always lower than that of a synchronous motor. ③By \"synchronization,\" it is meant that when current flows through the armature (stator) windings, a rotating magnetic field is generated in the air gap. The direction and speed of rotation of this magnetic field are identical to those of the rotor’s rotation, which is why it is called synchronization. In the case of an asynchronous motor, there is a relative speed between its rotating magnetic field and the rotor, which results in torque being generated. II. Why does synchronization occur, and why doesn’t it? The stator windings of synchronous motors and asynchronous motors are the same; the main difference lies in the structure of the rotor. The rotor of a synchronous motor has a DC excitation winding, so an external excitation power supply is required to supply current through slip rings ； The rotor of an asynchronous motor consists of short-circuited windings, which generate current through electromagnetic induction. In contrast, synchronous motors are more complex and costly. Both synchronous and asynchronous motors are AC power motors that rotate by being powered by a 50Hz AC power grid. In an asynchronous motor, alternating current is supplied to the stator, generating a rotating magnetic field; the rotor, in turn, generates its own magnetic field as a result of induction. The interaction between these two magnetic fields causes the rotor to rotate in conjunction with the rotating magnetic field of the stator. In this case, the rotor rotates more slowly than the stator’s rotating magnetic field; there is a slip, and since they are not synchronized, it is called an asynchronous machine. The stator of a synchronous motor is the same as that of an asynchronous motor, but its rotor has a constant magnetic field created by applying direct current, allowing the rotor to rotate in sync with the rotating magnetic field of the stator; this is what gives it the name synchronous motor. Simply put, in an asynchronous motor, no direct current excitation current is applied to the rotor, whereas in a synchronous motor, a direct current excitation current is applied to the rotor so that its rotation speed matches the speed of the magnetic field generated by the interaction between the stator and the rotor. III. Why is a direct-current excitation current applied to the rotor of a synchronous generator, rather than an alternating-current excitation current? Considering a power frequency of 50HZ, applying a DC excitation current to the rotor can induce a 50HZ potential in the stator windings. When an alternating excitation current is applied to the rotor, it generates two rotating magnetic fields: one in the forward direction and one in the reverse direction. The rotation speed of the forward rotating magnetic field adds to the rotation speed of the rotor, thereby inducing a 100HZ potential in the stator windings ； The rotation speed of the reverse rotating magnetic field cancels out that of the rotor, remaining relatively stationary with respect to the stator windings; no potential is generated. However, a DC component appears in the stator flux, which may lead to saturation. IV. How to choose between synchronous motors and asynchronous motors (based on functionality and applications)? ①Synchronous motors are mostly used in large generators. Asynchronous motors are almost exclusively used in electric vehicles. Synchronous motors can flexibly adjust the voltage and current phase on the input side through excitation, that is, the power factor ； The power factor of asynchronous motors is not adjustable, and it generally ranges between 0.75 and 0.85. Therefore, in large factories where asynchronous motors are used extensively, a synchronous motor can be added as a phase-shifting machine to regulate the power factor at the interface between the factory and the power grid. However, due to the high cost and heavy maintenance requirements of synchronous motors, capacitive power factor compensation is generally used nowadays. ②Synchronous motors have slightly higher efficiency than asynchronous motors; when selecting motors with a capacity of over 2000 KW, it is generally necessary to consider whether to use synchronous motors. However, since synchronous machines have excitation windings and slip rings, they require operators with high skill levels to control the excitation; moreover, their maintenance requirements are greater compared to the low-maintenance nature of asynchronous motors ； Therefore, for motors below 2500 KW nowadays, asynchronous motors are the preferred choice. At low power levels, the difference in efficiency becomes negligible. ③Asynchronous motors are simple, low-cost, and easy to install, use, and maintain, which is why they are widely used. The disadvantage is low efficiency, and a low power factor is harmful to the power grid. The high efficiency of synchronous motors makes them suitable for capacitive loads, as they can improve the power factor of the power grid. ④Synchronous machines require excitation voltage and current regulation, while asynchronous machines do not ； Synchronous machines can compensate the system for reactive power, while asynchronous machines require specialized equipment to provide power compensation. ⑤There are three main operating modes for synchronous motors, namely operating as generators, motors, and compensators. Operating as a generator is the primary mode of operation for synchronous motors, while operating as an electric motor is another important mode of operation for them. The power factor of synchronous motors can be adjusted; in applications where speed control is not required, using large synchronous motors can improve operational efficiency. In recent years, small synchronous motors have begun to be widely used in variable-frequency speed control systems. Synchronous motors can also be connected to the power grid as synchronous compensators. At this time, the motor has no mechanical load; it generates the required inductive or capacitive reactive power and supplies it to the power grid by adjusting the excitation current in the rotor, thereby improving the power factor of the grid or regulating its voltage. For general applications, an asynchronous motor is sufficient as a device drive.