What is an AC synchronous motor? What are the characteristics of AC synchronous motors
AC synchronous motor is a constant-speed drive motor whose rotor speed maintains a constant proportional relationship with the power frequency. It is widely used in electronic instrumentation, modern office equipment, textile machinery, etc. Synchronous motors are AC motors, and the stator windings are the same as asynchronous motors. Its rotor rotation speed is the same as the speed of the rotating magnetic field generated by the stator winding, so it is called a synchronous motor. Because of this, the current of the synchronous motor is ahead of the voltage in phase, that is, the synchronous motor is a capacitive load. For this reason, in many cases, synchronous motors are used to improve the power factor of the power supply system. AC synchronous motor
Features
Synchronous motor for motor operation. Because the synchronous motor can run at a leading power factor by adjusting the excitation current, it is beneficial to improve the power factor of the power grid. Therefore, large-scale equipment, such as large-scale blowers, water pumps, ball mills, compressors, rolling mills, etc., are commonly driven by synchronous motors. This advantage is particularly prominent when a synchronous motor is used in a large-scale equipment with a low speed. In addition, the speed of the synchronous motor is completely determined by the power frequency. When the frequency is constant, the speed of the motor is also constant, and it does not change with the load. This feature is of great significance in certain transmission systems, especially multi-machine synchronous transmission systems and precision speed regulation systems. The operating stability of the synchronous motor is also relatively high. Synchronous motors generally operate under over-excitation, and their overload capacity is greater than that of corresponding asynchronous motors. The torque of an asynchronous motor is proportional to the square of the voltage, while the torque of a synchronous motor is determined by the product of the voltage and the internal electromotive force generated by the motor's excitation current, that is, only proportional to the first power of the voltage. When the grid voltage suddenly drops to about 80% of the rated value, the asynchronous motor torque often drops to about 64%, and stops running due to the immobile load; while the synchronous motor torque does not drop much, and it can be forced Excitation to ensure the stable operation of the motor.
Structure The structure of the synchronous motor is basically the same as that of the synchronous generator, and the rotor is also divided into salient poles and hidden poles. But most synchronous motors are of salient pole type. The installation form is also divided into horizontal and vertical. In order to solve the starting problem of a synchronous motor, a starting winding is generally installed on the rotor. It can also suppress oscillations during operation, so it is also called damper winding. In addition to the above-mentioned traditional structure, there is also a claw-pole rotor structure without sliding contact. Taking a 6-pole motor as an example, two sets of claw-shaped magnetic poles are installed opposite to each other on the rotating shaft. One group protrudes 3 pole bodies axially to the right on the claw plate; the other group is installed on the right side in the opposite direction, so that the claw disc protrudes 3 pole bodies to the left axially. The polarities of the two sets of magnetic poles are opposite. After assembly, the outer circumferential surface of the magnetic pole is no longer round tile surface like a common salient pole motor, but a wedge-shaped tile surface, that is, the pole arc at one end is longer than the other end. The whole rotor shape is shown in the figure. The field winding is installed on the outer edge of the yoke on both sides. The magnetic flux generated by it passes through the lateral main air gap gm between the N and S poles, the axial air gaps g1 and g2 between the rotor and the stator, and then closes through the end cover and the base, as shown by the dotted line in the figure. In order to prevent the magnetic flux from short-circuiting through the shaft, the shaft should be made of non-magnetic steel; or the shaft should be divided into 3 sections, the middle section is made of non-magnetic steel. The main advantage of this structure is that there is no winding in the rotating part, and no sliding contact between the collector ring and the brush, so the operation is reliable, the insulation structure is simple, and the maintenance is convenient. But its main magnetic circuit is long and has more air gaps, which increases the power required for excitation; the motor housing has strong magnetism, which will cause the bearing to heat up; and the rotating shaft must also be magnetically insulated. Therefore, this kind of motor has not been widely promoted, and is only used in some special occasions, and the general capacity does not exceed a few hundred kilowatts.
Starting a synchronous motor can only produce an average torque at the synchronous speed. If the stator is connected to the grid immediately and the rotor is excited by DC when starting, the stator rotating magnetic field immediately rotates at a synchronous speed, and the rotor magnetic field is temporarily stationary due to the inertia of the rotor, and the electromagnetic torque generated at this time will be positive or negative. Alternating and its average value is zero, so the motor cannot start by itself. To start a synchronous motor, other methods must be used. There are mainly the following two methods.
①Asynchronous starting method: Install a cage-type starting winding on the main magnetic pole shoe of the motor. When starting, first short-circuit the excitation winding through a resistor, and then connect the stator winding to the grid. Relying on the asynchronous electromagnetic torque of the starting winding to make the motor speed up to close to the synchronous speed, and then passing the excitation current into the excitation winding to establish the main pole magnetic field, the synchronous electromagnetic torque can be used to pull the motor rotor into the synchronous speed.
②Auxiliary motor starting method: Generally, an induction motor with the same pole number as the synchronous motor (capacity is about 10-15% of the main machine) is used as the auxiliary motor, and the main machine is driven to a speed close to the synchronous speed, and then the power is switched to the stator of the main machine, and the excitation current Pass in the excitation winding to pull the host into the synchronous speed.