Permanent magnet motor
1. Control problems
After making the permanent magnet motor, the magnetic field can be maintained without the external energy, but it is also difficult to adjust and control the magnetic field from the outside. The permanent magnetic generator can't adjust its output voltage and power factor from the outside. The permanent magnet DC motor can not use the method of changing excitation to adjust its speed.
2. The problem of irreversible demagnetization
If the design or improper use of permanent magnet motor in high (NdFeB permanent magnet) or low (ferrite permanent magnet) temperature, armature reaction in impact current, or in severe mechanical vibration may cause irreversible demagnetization, or loss of excitation, the motor performance is reduced, can't even use.
3. The cost problem
Ferrite permanent magnet motor, especially micro permanent magnet DC motor, has been widely applied because of its simple structure and low quality, and its total cost is generally lower than that of electric excitation motor. The cost of rare-earth permanent magnet motor is usually higher than that of electric excitation motor because of the high price of rare-earth permanent magnet, which requires compensation for its high performance and operation cost.
Therefore, the permanent magnet motor is suitable for small power occasions.
Compared with permanent magnet motor excitation motor control is simple, not afraid of vibration, easy assembly: as long as the excitation current can control the magnetic field strength, and the excitation current is much smaller than the armature current, so the excitation controller is simple, high reliability, low cost, usually used for high power applications, such as large generator and motor.