Comparative Calculation of Energy Saving Between High-efficiency Permanent Magnet Motor and Asynchronous Motor

1. Efficiency and power factor

When the asynchronous motor is working, the rotor winding absorbs part of the electric energy for excitation from the power grid, which consumes the power grid. This part of the electric energy is eventually consumed as current in the rotor winding as heat. This loss accounts for about 20~30% of the total loss of the motor. It makes The efficiency of the motor decrease. The rotor excitation current is converted into an inductive current in the stator winding, causing the current entering the stator winding to lag behind the grid voltage by an angle, resulting in a reduction in the power factor of the motor.

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) of an asynchronous motor is <50%, its operating efficiency and operating power factor decrease significantly. , so it is generally required to operate within the economic zone, that is, the load rate is between 75% and 100%.

After the permanent magnet synchronous motor is embedded with permanent magnets 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 Motor efficiency is 4%~50%.

Since there is no induced current excitation in the permanent magnet motor rotor, the stator winding may be a purely resistive load, making the motor power factor infinitely close to 1. 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 of the permanent magnet synchronous motor is >20%, its operating efficiency and operating power factor change little, and the operating efficiency >80%.

Comparative Calculation of Energy Saving Between High-efficiency Permanent Magnet Motor and Asynchronous Motor插图

Figure 1 Efficiency, power factor, and load factor curves

2. Working temperature rise

When the asynchronous motor is working, current flows in the rotor winding, and this current is completely consumed in the form of heat energy, so a large amount of heat will be generated in the rotor winding, which will increase the temperature of the motor and affect the service life of the motor.

Due to the high efficiency of the permanent magnet motor, there is no resistance loss in the rotor winding, and there is little or almost no reactive current in the stator winding, resulting in a low-temperature rise of the motor. The extremely low-temperature rise also ensures the life of the permanent magnet and extends the life of the permanent magnet. The service life of the motor is also proved by its comprehensive application in elevator traction machines.

3. Impact on power grid operation

Due to the low power factor of the asynchronous motor, the motor absorbs a large amount of reactive current from the power grid, resulting in a large amount of reactive current in the power grid, power transmission and transformation equipment, and power generation equipment, which in turn reduces the quality factor of the power grid and aggravates the problems of the power grid and transmission equipment. A load of transformer equipment and power generation equipment, the reactive current in the power grid, power transmission and transformation equipment, and power generation equipment all consume part of the electric energy, causing the efficiency of the power grid to become low and affecting the effective use of electric energy. Also due to the low efficiency of asynchronous motors, in order to meet the output power requirements, it is necessary to absorb more electric energy from the power grid, which further increases the loss of electrical energy and increases the load on the power grid.

There is no induced current in the permanent magnet motor rotor, and the power factor of the motor is high, which improves the quality factor of the power grid and eliminates the need to install a compensator in the power grid. At the same time, due to the high efficiency of the permanent magnet motor, electric energy is also saved.

Comparative Calculation of Energy Saving Between High-efficiency Permanent Magnet Motor and Asynchronous Motor插图1

4. Energy saving calculation

When the efficiency of a motor under rated load increases from η1 to η2, the electric energy ws (kW·h) saved by running the motor for one year is as follows:

PN—-motor rated power (kW);

LF%–motor operating load rate;

Th—-Annual running time (h).

Taking the rated operating condition of the 22kW-4P motor as an example, the efficiency of the Y series asynchronous motor is 91.5%. After changing to a high-efficiency permanent magnet motor, when the efficiency increases to 94.7%, each unit saves electric energy per year: 4.09×103 kW.h.

Note: The above is calculated underrated working conditions. If the load rate changes and the speed range is wide, the energy-saving effect will be far greater than the rated point.

5. Summary

Compared with asynchronous motors, permanent magnet synchronous motors have obvious advantages. They have high efficiency, high power factor, good performance indicators, small size, lightweight, low-temperature rise, significant technical effects, and better improve the quality factor of the power grid, fully exerting the capacity of the existing power grid, saving investment in the power grid, and greatly solving the “big horse and small cart” phenomenon in electrical equipment.