What is the disadvantage of PMSM Fans?

We talked about advantages of PMSM HVLS fan few days ago, then what is the disadvantage?

1) Irreversible demagnetization problem. 

If under improper design or operation, or the PMSM runs at too high or too low temperature, performance of the motor will be degraded or even unusable when it works in the armature of surge current or severe mechanical vibration.

2) Cost problem. 

Price of lanthanon permanent magnets is relatively expensive, the cost of lanthanon PMSM is generally higher than electro-magnetic motors, which needs to be compensated by its high performance and operating cost savings.

3) Controlling system. 

A PMSM can maintain its magnetic field without external energy, but it also makes it extremely difficult to adjust and control its magnetic field from the outside. However, with the development of power electronic devices and control technologies such as MOSFET and IGBT, most of PMSM can be used for armature control without magnetic field control. The design requires the combination of three new technologies, permanent magnet materials, power electronics and microcomputer control, so that the permanent magnet synchronous motor operates in a brand new working conditions.