Working principle of synchronous motors

The difference between a synchronous motor and an asynchronous motor lies in the speeds of the rotor and the stator. In a synchronous motor, the rotor is brought to match the speed of the stator; this is achieved by applying a voltage signal to the rotor at startup, which triggers the rectifier thyristors to conduct, thereby supplying voltage and current to the rotor and achieving synchronization. (The rotor of a synchronous motor must have coils).............

When a synchronous motor is in operation, three-phase symmetric currents flow through the stator’s three-phase windings, while a direct current flows through the rotor’s excitation winding. When three-phase alternating current is applied to the stator’s three-phase symmetrical windings, a rotating magnetic field is generated in the air gap. When a direct current is applied to the rotor excitation winding, a stationary magnetic field with constant polarity is generated. If the number of pole pairs in the rotor magnetic field is equal to that in the stator magnetic field, the rotor magnetic field rotates in synchronization with the rotating magnetic field of the stator due to the magnetic attraction exerted by the stator magnetic field; in other words, the rotor rotates at the same speed and in the same direction as the rotating magnetic field. This is the basic working principle of a synchronous motor. The speed of the stator’s rotating magnetic field and the rotor is known as the synchronous speed. Its size is determined solely by the power supply frequency and the number of pole pairs p in the stator and rotor, and it does not change with variations in load. The rotation direction of the stator’s rotating magnetic field or of the rotor is determined by the phase sequence of the three-phase currents flowing through the stator windings; changing this phase sequence will alter the rotation direction of the synchronous motor. The common starting methods for synchronous motors are auxiliary motor starting, asynchronous starting, and variable-frequency starting. At startup, the synchronous motor is first accelerated to a speed close to the synchronous speed, after which an excitation current is applied. The electromagnetic torque generated by the magnetic attraction between the stator and rotor fields of the synchronous motor is used to bring the rotor into synchronization. The starting process of a synchronous motor occurs in two stages: asynchronous starting and synchronization. When starting using these two starting methods, the rotor winding must not be short-circuited nor left open-circuited; instead, a resistor with a certain resistance value (usually 5 to 10 times the resistance of the rotor winding) should be connected in series before closing the circuit, in order to prevent startup failures or damage to the insulation of the rotor winding. The variable-frequency starting method enables smooth startup, and its use is becoming increasingly widespread. At startup, a direct current excitation current is first applied to the rotor windings, and the power supply frequency applied to the stator is gradually increased using an inverter, so that the rotor poles can establish a stable magnetic attraction with the rotating magnetic field right from the start of startup, thereby rotating synchronously and increasing in speed throughout the startup process until the rated speed is reached.

Synchronous motors are generally used for power generation, as well as in special applications such as precise control. Synchronous motors can operate in an over-excited state and are also applicable to reactive power compensation in power grids to improve the power factor of the grid. Asynchronous motors are generally inexpensive and used in applications with modest requirements, making them widely applicable.

Thank you. May I ask whether the exciter in front of the synchronous motor is synchronous or asynchronous? Is it used to generate direct current, or does it have some other purpose? Thank you very much

The exciter supplies power to the rotor’s excitation windings. In synchronous generators, they can be classified as separately excited and self-excited based on the method of supplying the excitation current. To change the excitation current of a generator, it is generally not done directly in its rotor circuit, as the current in that circuit is very high and makes direct regulation difficult. The usual approach is to adjust the excitation current of the exciter in order to regulate the rotor current of the generator. The excitation device of a synchronous motor serves three main functions: first, it enables the asynchronous start-up of the synchronous machine and helps it reach synchronous operation ； Second is the regulation and control of the excitation current after synchronization is achieved ； Third is the monitoring system failure, to ensure the safe operation of the synchronous machine.

Is driving synchronous or asynchronous easier?

Does \"driving\" refer to the motor starting? If so, then asynchronous machines are simple.

When a synchronous motor starts up, the DC current in the excitation cabinet is usually controlled manually or automatically. At what level should the value be controlled?

Generally, there are two ways to start a synchronous motor: one is to supply excitation first and then start synchronously； An asynchronous start, followed by polarized excitation. For the variable-frequency starting of synchronous motors, excitation is applied first followed by synchronous starting; however, incorrect determination of the rotor position often leads to failed motor startup. For the frequency conversion speed control renovation of synchronous motors, it is easy to use asynchronous starting along with polarized excitation. Therefore, the Maxf frequency conversion unit enables asynchronous soft starting of synchronous motors in order to achieve the rated starting torque; the motor is started at around 8 Hz and then subjected to polarized excitation. The specific magnitude of the excitation as well as the frequency at which it is applied can be adjusted according to different application scenarios. At this point, after undergoing slight damped oscillations in the angle between the rotor magnetic field and the stator magnetic field, the rotor poles of the motor are reliably attracted by the stator poles, and the synchronous motor enters synchronous operation. The inverter gradually accelerates to the desired frequency according to a pre-set acceleration rate. At this point, the angle between the armature voltage vector of the synchronous motor and the position of the rotor poles gradually increases to a constant value; under the attraction of the stator magnetic field, the rotor poles of the motor accelerate gradually to the desired speed, and thus the starting process of the synchronous motor is completed. For operating conditions that require restart under heavy load, in order to achieve a greater starting torque, the output voltage of the frequency conversion unit and the excitation current of the synchronous motor can be appropriately increased.

Principle of synchronous motors: Synchronous motors belong to the category of AC motors, and their stator windings are the same as those in asynchronous motors. Its rotor rotates at the same speed as the rotating magnetic field generated by the stator windings, which is why it is called a synchronous motor. For this reason, the current in a synchronous motor leads the voltage in phase; that is, a synchronous motor is a capacitive load. For this reason, synchronous motors are often used to improve the power factor of power supply systems. Synchronous motors generally come in two structural types: 1. The rotor is excited using direct current. The rotor of this motor is of the salient-pole type; the field coils mounted on the pole cores are connected in series to produce alternating opposite polarities, and two leads are connected to the two slip rings mounted on the shaft. The magnetic field coil is excited by a small DC generator or battery; in most synchronous motors, the DC generator is mounted on the motor shaft to supply the excitation current to the rotor pole coils. Since this synchronous motor cannot start automatically, a squirrel-cage winding is also installed on the rotor to enable the motor to start. The squirrel-cage windings are arranged around the rotor, and its structure is similar to that of an asynchronous motor. When a three-phase AC power supply is applied to the stator windings, a rotating magnetic field is generated within the motor. The squirrel-cage windings cut through these magnetic flux lines, resulting in induced currents that cause the motor to rotate. After the motor starts rotating, its speed gradually increases to a value slightly lower than that of the rotating magnetic field. At this point, the rotor’s magnetic field coils are excited by direct current, resulting in the formation of magnetic poles on the rotor. These magnetic poles attempt to keep up with the rotating magnetic poles on the stator, thereby increasing the speed of the motor’s rotor until it rotates in sync with the rotating magnetic field. 2. Synchronous motors whose rotors do not require excitation: Synchronous motors with unexcited rotors can be used with single-phase power supplies as well as with multi-phase power supplies. In this type of motor, one variant has a stator winding similar to that of a split-phase or multi-phase motor, along with a squirrel-cage rotor whose surface is cut into flat surfaces. It is therefore a salient-pole rotor, whose magnetic poles are made of a type of magnetized steel and can maintain their magnetism over time. The squirrel-cage winding is used to generate starting torque, and when the motor reaches a certain speed, the rotor poles synchronize with the frequency of the current in the stator coils. The polarity of the salient poles is induced by the stator; therefore, their number should be equal to the number of poles on the stator. When the motor reaches its intended speed, the squirrel-cage windings cease to function, and rotation is maintained by the rotor and magnetic poles following the stator’s magnetic poles in synchronization.