1. Efficiency and power factor
When an asynchronous motor is operating, the rotor windings absorb some electrical energy from the power grid for excitation, consuming grid energy. This energy is ultimately dissipated as heat generated by current in the rotor windings. This loss accounts for approximately 20-30% of the total motor losses, reducing the motor's efficiency. The rotor excitation current, when referred to the stator windings, is an inductive current, causing the current entering the stator windings to lag behind the grid voltage by a certain angle, resulting in a decrease in the motor's power factor.
In addition, it can be seen from the efficiency and power factor curves of permanent magnet synchronous motors and asynchronous motors (Figure 1) that when the load rate (=P2/Pn) < 50%, the operating efficiency and operating power factor of asynchronous motors drop significantly. Therefore, they are generally required to operate within the economic zone, that is, the load rate is between 75% and 100%.
After permanent magnets are embedded on the rotor, the permanent magnets establish the rotor magnetic field. During normal operation, the rotor and stator magnetic fields run synchronously. There is no induced current in the rotor and no rotor resistance loss. This alone can improve the motor efficiency by 4% to 50%.
Since there is no induced current excitation in the rotor of the water magneto motor, the stator winding may be a purely resistive load, causing the motor power factor to approach 1. From the efficiency and power factor curves of permanent magnet synchronous motors and asynchronous motors (Figure 1), it can be seen that when the load rate is >20%, the operating efficiency and operating power factor of the permanent magnet synchronous motor do not change much, and the operating efficiency is >80%.
Figure 1. Efficiency and power factor versus load rate curves
2. Operating temperature rise
When an asynchronous motor is working, current flows through the rotor windings, and this current is completely consumed as heat energy. Therefore, a large amount of heat will be generated in the rotor windings, causing the motor temperature to rise and affecting the motor's service life.
Because permanent magnet motors are highly efficient, there is no resistance loss in the rotor windings, and little or no reactive current in the stator windings, resulting in low motor temperature rise. This extremely low temperature rise also ensures the lifespan of the permanent magnets and extends the motor's service life, as evidenced by its widespread application in elevator traction machines. 3. Impact on power grid operation
Because asynchronous motors have a low power factor, they absorb a large amount of reactive current from the power grid. This results in a significant amount of reactive current in the power grid, transmission and distribution equipment, and power generation equipment, which in turn lowers the power grid's quality factor and increases the load on these systems. Simultaneously, the reactive current consumes electrical energy in all these systems, leading to lower power grid efficiency and hindering the effective utilization of electrical energy. Similarly, due to the low efficiency of asynchronous motors, meeting output power requirements necessitates absorbing more electrical energy from the grid, further increasing energy loss and exacerbating the power grid load.
Permanent magnet motors have no induced current excitation in the rotor, resulting in a high power factor that improves the grid's quality factor, eliminating the need for compensators. Furthermore, the high efficiency of permanent magnet motors also saves electrical energy.
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