Difference between AC asynchronous motor and permanent magnet synchronous motor

Update:14 Mar 2020

1. Different efficiency and power factors
When the asynchronous motor is working, the rotor winding needs to absorb part of the electric energy from the grid for excitation, which consumes the electric energy of the grid. This part of the electric energy is finally consumed by heating in the rotor winding with the current, which accounts for about 20-30% of the total loss of the motor, which reduces the efficiency of the motor.
When the excitation current of the rotor is converted to the stator winding, it is inductive current, which makes the current in the stator winding lag behind the grid voltage by an angle, resulting in the reduction of the power factor of the motor.
In addition, from the efficiency and power factor curve (Figure 1) of permanent magnet synchronous motor and asynchronous motor, it can be seen that when the load rate (= P2 / PN) < 50%, the operation efficiency and power factor of asynchronous motor are greatly reduced, so it is generally required to operate in the Economic Zone, that is, the load rate is between 75% - 100%.
After the permanent magnet is embedded in the rotor, the permanent magnet is used to establish the rotor magnetic field. In normal operation, the rotor and the stator magnetic field operate synchronously. There is no induced current in the rotor, and there is no rotor resistance loss. Only this one can improve the efficiency of the motor by 4% ~ 50%.
As there is no induced current excitation in the rotor of the water magnet motor, the stator winding may be pure resistive load, making the power factor of the motor almost 1. From the efficiency and power factor curve of the permanent magnet synchronous motor and the asynchronous motor (Figure 1), it can be seen that when the load rate of the permanent magnet synchronous motor is more than 20%, the operation efficiency and power factor of the permanent magnet synchronous motor will not change much, and the operation efficiency is more than 80%
2. Different starting torque
When the asynchronous motor starts, it is required that the motor has enough starting torque, but it is also hoped that the starting current should not be too large, so as to prevent the grid from generating too large voltage drop and affecting the normal operation of other motors and electrical equipment connected to the grid.
In addition, when the starting current is too large, the motor itself will be impacted by too much electric force. If the motor is started frequently, the winding may overheat. Therefore, the starting design of asynchronous motor is often faced with a dilemma.
The permanent magnet synchronous motor generally adopts asynchronous starting mode. Because the rotor winding does not work when the permanent magnet synchronous motor works normally, when designing the permanent magnet motor, the rotor winding can fully meet the requirements of high starting torque. For example, the starting torque multiple can be increased from 1.8 times of the asynchronous motor to 2.5 times, or even greater, which better solves the problem of "large horse drawn trolley" in the power equipment Elephant.

3. Different working temperature rise
As the asynchronous motor works, the rotor winding has current flow, and this current is completely consumed in the form of heat energy, so a lot of heat will be generated in the rotor winding, which will increase the Ju degree of the motor and affect the service life of the motor.
Because of the high efficiency of the permanent magnet motor, there is no resistance loss in the rotor winding, and there is little or no reactive current in the stator winding, which makes the temperature rise of the motor low and prolongs the service life of the motor.
4. Different impacts on grid operation
Due to the low power factor of asynchronous motor, the motor needs to absorb a large amount of reactive current from the grid, resulting in a large amount of reactive current in the grid, Han substation equipment and power generation equipment, thus reducing the quality factor of the grid and increasing the load of the grid, gun substation equipment and power generation equipment.
At the same time, the reactive current consumes part of the electric energy in the power grid, Han substation equipment and power generation equipment, resulting in the low efficiency of the power grid, affecting the effective use of electric energy.
Similarly, due to the low efficiency of asynchronous motor, in order to meet the demand of output power, it is bound to absorb more electric energy from the grid, further increase the loss of electric energy and increase the grid load.
There is no induction current excitation in the rotor of permanent magnet motor. The power factor of the motor is high, which improves the quality factor of the power grid, so that there is no need to install compensators in the power grid. At the same time, because of the high efficiency of the permanent magnet motor, the electric energy is also saved.

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