Compared to conventional three-phase asynchronous motors, permanent magnet synchronous motors offer the advantages of high starting torque, short startup time, and high overload capacity. This allows the installed capacity of the equipment’s drive motor to be reduced based on actual shaft power, saving energy while reducing fixed asset investment.
Permanent magnet synchronous motors are relatively easy to control, with speed determined solely by frequency. They operate smoothly and reliably, unaffected by load and voltage fluctuations. Their strictly synchronized speed provides excellent dynamic response, making them well-suited for variable-frequency control.
Permanent magnet synchronous motors also offer advantages in terms of low losses and temperature rise, and high power factor and efficiency. These factors, which align with the performance demands of motors, have driven their market dominance.
Why do permanent magnet motors have low losses and low temperature rise?
Because the magnetic field of a permanent magnet synchronous motor is generated by permanent magnets, this avoids the excitation losses (also known as copper losses) that would otherwise be caused by using excitation current to generate the magnetic field. When the motor is running, the rotor draws no current, significantly reducing the motor’s temperature rise. According to incomplete statistics, under the same load conditions, the temperature rise is approximately 20K lower.
Permanent Magnet Synchronous Motors Offer High Power Factor and Efficiency
Compared to asynchronous motors, permanent magnet synchronous motors offer significantly higher efficiency at light loads. They also boast a wide operating range, exceeding 90% within a load factor range of 25% to 120%. The rated efficiency of permanent magnet synchronous motors meets the current national standard for Class 1 energy efficiency, a significant advantage over asynchronous motors in terms of energy savings.
In actual operation, motors rarely operate at full power when driving loads. This is due to two factors: first, when selecting motors, designers typically determine motor power based on the load’s extreme operating conditions, which rarely occur. Furthermore, to prevent motor burnout under abnormal operating conditions, motor designs often include a power margin. Second, to ensure reliability, motor manufacturers typically include a certain power margin beyond the user’s specified power. Consequently, most motors in actual operation operate below 70% of their rated power, particularly when driving loads such as fans and pumps, where motors typically operate in the light-load range. For asynchronous motors, their light-load efficiency is very low, while permanent magnet synchronous motors can still maintain a relatively high efficiency in the light-load area.
Permanent magnet synchronous motors have a high power factor, independent of the number of poles. At full load, the power factor approaches unity. This results in lower motor currents than asynchronous motors, resulting in lower stator copper loss and higher efficiency. However, the power factor of asynchronous motors decreases as the number of poles increases. Furthermore, because of the high power factor of permanent magnet synchronous motors, the capacity of the power supply (transformer) supporting the motor can theoretically be reduced, which in turn reduces the specifications of the supporting switchgear and cables.
Disadvantages of Permanent Magnet Synchronous Motors
Permanent magnet synchronous motors also have their disadvantages. For example, their starting current is approximately nine times greater than that of asynchronous motors. They cannot be started using reduced voltage. Under reduced voltage power supply conditions, their asynchronous starting torque drops more than that of asynchronous motors, resulting in starting difficulties. Parameters for the self-starting characteristics and feedback current during system short-circuits of permanent magnet synchronous motors vary significantly among manufacturers. Furthermore, due to the difficulty in obtaining relevant data, the use of permanent magnet synchronous motors introduces uncertainties in the short-circuit performance of power systems and in starting calculation verification.
