What are the main aspects of excellent performance of permanent magnet motors? Are there no disadvantages?
Compared with ordinary three-phase asynchronous motors, permanent magnet synchronous motors have the advantages of high starting torque, short starting time, and high overload capacity.
They can reduce the installed capacity of equipment driving motors according to the actual shaft power, saving energy and reducing investment in fixed assets at the same time.
Relatively speaking, permanent magnet synchronous motors are easy to control, with the speed determined only by frequency.
They operate smoothly and reliably without being affected by load or voltage fluctuations.
Due to the strict synchronization of the permanent magnet synchronous motor's speed, it has good dynamic response performance and is more suitable for variable frequency control.
The advantages of permanent magnet synchronous motors lie in their low losses and temperature rise, as well as high power factor and efficiency.
This is exactly what people pursue in terms of motor performance, which determines the market application status of permanent magnet motors.
Why do permanent magnet motors have low losses and low temperature rise?
Due to the fact that the magnetic field of the permanent magnet synchronous motor is generated by the permanent magnet, it avoids excitation losses caused by generating a magnetic field through excitation current, which is what we refer to as copper losses.
When the motor is running, there is no current flowing in the rotor, significantly reducing motor temperature rise.
According to incomplete statistics, under similar load conditions, the temperature rise can be reduced by about 20K.
High power factor and high efficiency of permanent magnet synchronous motor.
Compared to asynchronous motors, permanent magnet synchronous motors have much higher efficiency values under light loads.
They have a wide range of efficient operation and their efficiency is greater than 90% within the load range of 25% to 120%.
The rated efficiency of permanent magnet synchronous motors can meet the level 1 energy efficiency requirements of current national standards, which is their biggest advantage compared to asynchronous motors in terms of energy saving.
In actual operation, motors rarely operate at full power when driving loads.
There are several reasons for this: on one hand, during motor selection, designers generally determine the motor power based on the extreme operating conditions of the load. However, these extreme conditions occur very rarely. Additionally, in order to prevent motor burnout under abnormal conditions, designers also provide additional power margin for the motor during design.
On the other hand, motor manufacturers usually leave a certain amount of power margin beyond what users require in order to ensure reliability. As a result, most operating motors work below 70% of their rated power output in practice, especially when driving fan or pump loads where they typically operate in light load regions.
For asynchronous motors, their efficiency is very low under light loads; however, permanent magnet synchronous motors can still maintain high efficiency even under light load conditions.
Permanent magnet synchronous motors have a high power factor, which is independent of the motor rating. When operating at full load, the power factor of the motor is close to 1. Compared to asynchronous motors, this results in smaller motor currents and lower stator copper losses, leading to higher efficiency.
On the other hand, as the rating of asynchronous motors increases, their power factors decrease.
Additionally, due to the high power factor of permanent magnet synchronous motors, theoretically it is possible to reduce the capacity of the accompanying power supply (transformer), as well as decrease specifications for associated switchgear and cables.
Disadvantages of permanent magnet synchronous motors
Permanent magnet synchronous motors also have their disadvantages, such as: the starting current of permanent magnet synchronous motors is about 9 times that of asynchronous motors; permanent magnet synchronous motors cannot be started by voltage reduction.
Because under the condition of voltage reduction power supply, its asynchronous starting torque decreases more than that of asynchronous motors, which will cause difficulties in starting.
There are significant differences in the self-starting characteristics and feedback current during system short-circuit among different equipment manufacturers for permanent magnet synchronous motors, and due to the difficulty in obtaining relevant data, the application of permanent magnet synchronous motors will bring some uncertain factors to the short-circuit level and start-up calculation verification of power systems.
