A permanent magnet synchronous motor consists of a stator and a rotor. The stator is the same as that of an asynchronous motor, consisting of three-phase windings and a stator core. The rotor is equipped with pre-magnetized permanent magnets, which can establish a magnetic field in the surrounding space without external energy, simplifying the motor structure and saving energy. This article starts with the characteristics of permanent magnet synchronous motors and, combined with practical applications, illustrates the comprehensive benefits of promoting permanent magnet synchronous motors.
Outstanding advantages of permanent magnet synchronous motors
(1) Because the rotor is made of permanent magnets, the magnetic flux density is high, no excitation current is required, and excitation loss is eliminated. Compared with asynchronous motors, the excitation current of the stator winding and the copper and iron losses of the rotor are reduced, and the reactive current is significantly reduced. Since the stator and rotor magnetomotive forces are synchronized, the rotor core has no fundamental iron loss. Therefore, the efficiency (related to active power) and power factor (related to reactive power) are both higher than those of asynchronous motors. Permanent magnet synchronous motors are generally designed to have a high power factor and efficiency even when operating under light load.
The above figure shows a comparison curve of the efficiency and power factor of a permanent magnet synchronous motor and an asynchronous motor of the same specifications. From the perspective of load changes, the rare-earth permanent magnet synchronous motor can maintain high power factor and high efficiency within a range of 25% to 120% of its rated power, while a typical three-phase asynchronous motor can usually only maintain this level within 60% to 100% of its rated power.
(2) Permanent magnet synchronous motors have relatively stiff mechanical characteristics and are able to withstand motor torque disturbances caused by load changes. The rotor core of a permanent magnet synchronous motor can be made into a hollow structure to reduce rotor inertia, and the start-up and braking times are much faster than those of asynchronous motors. The high torque/inertia ratio makes permanent magnet synchronous motors more suitable for operation under fast response conditions than asynchronous motors.
(3) The size of a permanent magnet synchronous motor is significantly smaller than that of an asynchronous motor, and its weight is also relatively lighter. Under the same heat dissipation conditions and insulation materials, a permanent magnet synchronous motor has more than twice the power density of a three-phase asynchronous motor.
(4) The rotor structure is greatly simplified, making it easier to maintain and improving the stability of operation.
permanent magnet synchronous motor rotor assembly structure
Three-phase asynchronous motors require a high power factor, necessitating a very small air gap between the stator and rotor. The uniformity of this air gap is also crucial for safe operation and noise reduction. Therefore, asynchronous motors have relatively strict requirements for the dimensional and positional tolerances of components and the concentricity of assembly, limiting the freedom in selecting bearing clearance. Larger frame asynchronous motors typically use oil-lubricated bearings, requiring regular lubrication within specified operating intervals. Oil leakage or delayed lubrication accelerates bearing failure. Bearing maintenance constitutes a significant portion of three-phase asynchronous motor repair. Furthermore, the presence of induced current in the rotor of a three-phase asynchronous motor has led to increased research attention on bearing electro-corrosion in recent years.
Permanent magnet synchronous motors do not have these problems. Due to their larger air gap, the issues caused by the smaller air gap in asynchronous motors are not as noticeable in permanent magnet synchronous motors. In addition, the bearings of permanent magnet synchronous motors are grease-lubricated bearings with dust covers, and the bearings are pre-lubricated with an appropriate amount of high-quality grease at the factory, requiring no maintenance for their entire lifespan.
Typical applications of permanent magnet synchronous motors
1. Application solutions for electro-hydraulic servo injection molding machines
Traditional injection molding machines use three-phase asynchronous motors to drive quantitative pumps, requiring the motors to run continuously during operation. Through energy-saving modifications to the servo oil pump drive system, the rapid response and high starting torque of the permanent magnet synchronous motor allow the pump to generate pressure only when the injection molding process is complete. During material feeding and mold closing, the main motor of the oil pump remains off, achieving intermittent zero power consumption. Comparative studies show energy savings of up to 85%.
Thanks to superior servo motor algorithm control, the noise level is significantly lower than that of ordinary injection molding machines. Under ideal conditions with a low-noise screw pump, the overall noise level of the injection molding machine is below 70 decibels, achieving quiet operation and improving the working environment.
Energy-saving applications in iron ore dry separators
The equipment was used at a mining site in Northwest China, a dusty environment. It utilizes 125 electric motors. The winch requires a significant starting torque, but during normal operation, the motors run at 30%–50% of their rated power under light load. The previously used three-phase asynchronous motors had an efficiency of only about 65% and a power factor of about 0.4–0.6 , resulting in serious energy waste. The selection of permanent magnet synchronous motors significantly improved energy efficiency. The energy-saving test statistics are as follows:
User concerns
1. Motor lifespan
The overall lifespan of the motor depends on the lifespan of its bearings. The motor housing has an IP54 protection rating, which can be increased to IP65 in special environments, meeting the requirements of most dusty and humid environments. Provided the motor shaft extension is well-coaxial and the radial load on the shaft is appropriate, the minimum lifespan of the motor bearings is over 20,000 hours. Secondly, the lifespan of the cooling fan is crucial. The fan uses a single-phase 220V shaded-pole motor, which has a longer lifespan than capacitor-run motors. When operating in dusty and humid environments for extended periods, it is necessary to regularly remove any sticky substances adhering to the fan to prevent it from burning out due to excessive load.
2. Failure and Protection of Permanent Magnet Materials
The importance of permanent magnet materials to permanent magnet motors is self-evident, accounting for nearly a quarter of the total material cost of the motor. Therefore, the four parties have established enterprise standards for permanent magnet materials, including corrosion resistance, consistency, high-temperature demagnetization testing, and linear demagnetization curve testing. Current permanent magnet materials can operate for extended periods within the allowable temperature rise limits of motor windings, with a demagnetization rate of less than 2%. Conventional permanent magnet materials require the surface coating to withstand salt spray testing for more than 24 hours. For environments with severe oxidation and corrosion, users should contact the manufacturer to select permanent magnet materials with higher protection technologies.
3 coils burned out
Traditional asynchronous motors have a coil burnout failure rate exceeding 8%, primarily due to issues with the motor's thermal protection and the quality of electrical materials. Our CM500 series permanent magnet synchronous motors incorporate thermally sensitive elements within their coils. When the coil temperature rises to the F-class insulation limit of 105K, the driver uses the temperature signal collected by the thermally sensitive element to implement overheat protection, ensuring safe motor operation. Simultaneously, the motor stator employs a double vacuum impregnation process, significantly improving the stator's pass rate.
From an economic perspective, permanent magnet synchronous motors are particularly suitable for applications requiring heavy starting and light operation. Promoting their use has significant economic and social benefits and is crucial for energy conservation and emission reduction. Permanent magnet synchronous motors also offer valuable advantages in reliability and stability. Selecting a high-efficiency permanent magnet synchronous motor is a one-time investment with long-term benefits.
