Efficiency and power factor comparison
When an asynchronous motor is operating, the rotor windings absorb some electrical energy from the power grid for excitation, thus 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, directly leading to a reduction in motor 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, resulting in a decrease in the motor's power factor.
Furthermore, as can be seen from the efficiency and power factor curves of permanent magnet synchronous motors and asynchronous motors in electric vehicles shown in Figure 1, the operating efficiency and power factor of asynchronous motors drop significantly when the load rate (=P2/Pn) < 50%. Therefore, they are generally required to operate within the economic zone, i.e., the load rate is between 75% and 100%.
After permanent magnets are embedded in the rotor of a permanent magnet synchronous motor, the rotor magnetic field is established by the permanent magnets. During normal operation, the rotor and stator magnetic fields run synchronously. There is no induced current in the rotor, and there is no rotor resistance loss. This alone can improve the motor efficiency by 4% to 50%. At the same time, since there is no induced current excitation in the rotor of a permanent magnet synchronous motor, the stator winding may be a purely resistive load, making the motor power factor almost 1. As can be seen from the efficiency and power factor curves of permanent magnet synchronous motors and asynchronous motors shown in Figure 1, when the load rate is >20%, the operating efficiency and operating power factor of permanent magnet synchronous motors do not change much, and the operating efficiency is >80%.
Starting torque
When starting an asynchronous motor, it is required that the motor have a sufficiently large starting torque, but it is also desirable that the starting current not be too large, so as to avoid excessive voltage drop in the power grid, which could affect the normal operation of other motors and electrical equipment connected to the grid. In addition, excessive starting current will subject the motor itself to excessive electrical force, and frequent starting may also cause the windings to overheat. Therefore, the starting design of asynchronous motors often faces a dilemma.
Permanent magnet synchronous motors generally also use asynchronous starting. Since the rotor windings of a permanent magnet synchronous motor are not active during normal operation, when designing a permanent magnet motor, the rotor windings can be made to fully meet the requirements of high starting torque. For example, the starting torque multiple can be increased from 1.8 times that of an asynchronous motor to 2.5 times, or even more, which effectively solves the problem of "overpowered motors for small vehicles" in conventional power equipment.
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, which seriously affects the service life of the motor.
As for permanent magnet synchronous motors, due to their high efficiency, the absence of resistance loss in the rotor windings, and the low or almost non-existent reactive current in the stator windings, the motor temperature rises significantly, thus extending the motor's service life.
Impact on power grid operation
Because asynchronous motors have a low power factor, they need to 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, thus lowering the power grid's quality factor. This not only increases the load on the power grid, transmission and distribution equipment, and power generation equipment, but also consumes some electrical energy in all these systems, leading to lower power grid efficiency and affecting the effective utilization of electrical energy. Similarly, due to the low efficiency of asynchronous motors, to meet the required output power, they must absorb more electrical energy from the grid, further increasing energy loss and exacerbating the power grid load.
Permanent magnet synchronous motors (PMSMs) have no induced current excitation in their rotors, resulting in a high power factor. This not only improves the quality factor of the power grid, eliminating the need for reactive power compensation equipment, but also saves grid energy due to the high efficiency of PMSMs.
