一. Characteristics of Permanent Magnet Synchronous Motors

  1. Voltage Regulation

  The automatic excitation system can be considered a negative feedback control system with voltage as the controlled variable. Reactive load current is the main reason for the decrease in generator terminal voltage. When the excitation current remains constant, the generator terminal voltage will decrease as the reactive current increases. However, to meet the user's requirements for power quality, the generator terminal voltage should remain essentially constant. 

  This is achieved by adjusting the generator's excitation current according to changes in reactive current.

  2. Reactive Power Regulation

  When the generator is running in parallel with the system, it can be considered to be operating with an infinitely large capacity power source. Changing the generator's excitation current also changes the induced electromotive force and stator current, and consequently, the generator's reactive current changes as well. 

  When the generator is running in parallel with an infinitely large capacity system, to change the generator's reactive power, the generator's excitation current must be adjusted. The change in generator excitation current in this case is not the usual "voltage regulation," but rather simply changes the reactive power delivered to the system.

  3. Reactive Load Distribution

  Generators operating in parallel distribute reactive current proportionally according to their respective rated capacities. Larger capacity generators should bear more reactive load, while smaller capacity generators should provide less reactive load. To achieve automatic distribution of reactive load, the generator's excitation current can be changed through an automatic high-voltage regulating excitation device to maintain its terminal voltage constant. 

  The slope of the generator's voltage regulation characteristic can also be adjusted to achieve a reasonable distribution of reactive load among the parallel-operating generators.

  二. Permanent Magnet Motor Drive Systems have the following advantages:

  1. Simple and Compact Structure

  Permanent magnet synchronous motors use permanent magnets to generate the air gap magnetic field, unlike commutator motors that use excitation coils to generate the air gap magnetic field, or induction motors that use the excitation component of the stator current to generate the air gap magnetic field. They have a simple structure, low losses, and high efficiency. 

  Permanent magnet synchronous motors can be divided into two types: surface-mounted permanent magnet synchronous motors (SPM) and interior permanent magnet synchronous motors (IPM). The domestically developed permanent magnet synchronous motor is an interior permanent magnet synchronous motor (IPM) with permanent magnets embedded inside the rotor core. 

  Because the permanent magnets are embedded within the rotor, the iron core provides excellent protection to the magnets, effectively preventing damage from centrifugal force, corrosion, and other potential damage during the manufacturing process. IPM motors have strong magnetic saliency, allowing for full utilization of reluctance torque through control, resulting in higher torque output. 

  Simultaneously, the saliency effect significantly increases the motor's field weakening control range, thereby greatly increasing the motor's speed range.

  2. High Efficiency and High Power Factor

  Permanent magnet synchronous motors combine the advantages of traditional asynchronous motors and electrically excited synchronous motors, and can achieve similar or even superior speed control characteristics compared to DC motors, resulting in overall performance improvement. Compared to asynchronous motors, permanent magnet synchronous motors do not require reactive excitation current, significantly improving the power factor, reducing stator current and stator copper losses. 

  Furthermore, there are no rotor copper losses during stable operation. The reduction in total losses reduces the capacity of the motor cooling system, thereby reducing corresponding additional losses. Consequently, their efficiency is 2-15 percentage points higher than asynchronous motors of the same specifications.

  3. Strong Dynamic Response and Overload Capacity

  Synchronous motors have a stronger ability to withstand torque disturbances than asynchronous motors and can respond more quickly. When the load torque of an asynchronous motor changes, the motor's slip rate must also change, meaning the motor speed changes accordingly. However, the inertia of the rotating parts of the system hinders the motor's rapid response. 

  When the load torque of a synchronous motor changes, as long as the motor's power angle changes appropriately, the speed remains at the original synchronous speed, and the inertia of the rotating parts does not affect the motor's rapid response to torque. The maximum torque of a permanent magnet synchronous motor can reach more than three times the rated torque, which is very beneficial for the stable operation of the motor system under conditions of large load torque variations.

  4. Small Size and Light Weight

  In recent years, with the continuous application of high-performance permanent magnet materials, the power density of permanent magnet synchronous motors has been greatly improved. 

  Compared with asynchronous motors of the same speed and capacity, permanent magnet synchronous motors have significantly reduced size and weight, allowing them to be used in many special applications.

  5. High reliability and low operating and maintenance costs

  Compared with DC motors and electrically excited synchronous motors, permanent magnet synchronous motors have no brushes, simplifying the structure and increasing reliability.

  The direct-drive system eliminates the gearbox, reducing failure points and improving system reliability. It also reduces operating and maintenance costs.

三. However, permanent magnet synchronous motors also have the following disadvantages:

  The motor cost is higher; in constant power mode, operation is more complex, and the control system cost is higher; the field weakening capability is poor, and the speed regulation range is limited; the power range is small, and due to the influence and limitations of magnetic material technology, the maximum power is only tens of kilowatts; at low speeds, the rated current is large, resulting in high losses and low efficiency; when subjected to vibration, high temperature, and overload current, the magnetic permeability of the permanent magnet material may decrease or demagnetization may occur, which will reduce the performance of the permanent magnet motor, and in severe cases, it may even damage the motor. Strict control must be exercised during use to prevent overload. 

  The magnetic field of the permanent magnet material is unchangeable; to increase the power of the motor, its size will be very large; it has poor corrosion resistance; and it is difficult to assemble.