Permanent Magnet Synchronous Motor vs AC Induction Motor
Rare earth permanent magnet motor is a kind of permanent magnet motor that only appeared in the early 1970s. Due to the high magnetic energy product (BHmax) and high coercive force (especially high intrinsic coercive force Hcj) of rare earth permanent magnets, rare earth permanent magnet motors It has a series of advantages such as small size, light weight, high efficiency, and good characteristics. Combining with power electronics technology and microcomputer control technology, it has improved the performance of the motor and transmission system to a new level, and the side has also improved. The performance and level of technical equipment is an important development direction for the motor industry to adjust its industrial structure.
Rare earth permanent magnet motors include permanent magnet synchronous motors, permanent magnet generators, DC motors, brushless DC motors, AC permanent magnet servo motors, permanent magnet linear motors, special permanent magnet motors and related control systems, covering almost the entire motor industry .
Compared with traditional synchronous motors, rare earth permanent magnet motors replace the electric field of traditional motors with rare earth permanent magnet materials. It can not only achieve high performance unmatched by traditional electric excitation motors, but also can be made into special motors that can meet specific operating requirements, such as elevator traction motors, special motors for automobiles, etc.
Permanent Magnet Synchronous Motor
In principle, after the permanent magnet is embedded in the rotor of the permanent magnet synchronous motor, the inherent magnetic field of the magnetic steel is used as the excitation source to form a fixed magnetic pole, and a strong permanent magnetic field can be established without external energy. When the three-phase stator winding of the motor is connected to the three-phase alternating current, every 120° (the green, blue and red coils in the figure below) will create a magnetic field that attracts the permanent magnet of the rotor. Drive the rotor magnetic poles to rotate, and finally reach the rotor rotation speed equal to the stator magnetic field rotation speed. When working normally, the rotor and the stator magnetic field run synchronously, there is no induced current in the rotor, and there is no rotor resistance loss, and the motor efficiency can be increased by about 40% when starting.
Working principle of DC permanent magnet synchronous motor
As a comparison, when an AC asynchronous motor (induction motor) is working, the rotor winding has to absorb part of the electric energy from the battery for excitation, which consumes electric energy. This part of electric energy is finally consumed by current in the rotor winding. This loss accounts for about 10% of the total loss of the motor. 20~30%, it reduces the efficiency of the motor. And the weight is too big. The 11kW asynchronous motor weighs the most at 110kg, while the permanent magnet motor is only 48kg, which is equivalent to 43.6% of the weight of the AC asynchronous motor.
AC asynchronous motor
The advantages of rare earth permanent magnet motors are as follows:
(1) High efficiency: the efficiency curve of asynchronous motors generally drops rapidly when the rated load is below 60%, and the efficiency is very low at light loads. The efficiency curve of rare earth permanent magnet motor is high and flat, and it is in the high efficiency zone when the rated load is 20%~120%.
(2) High power factor: the measured value of the power factor of the rare earth permanent magnet synchronous motor is close to the limit value of 1.0, the power factor curve is as high and flat as the efficiency curve, the power factor is high, no low-voltage reactive power compensation is needed, and the capacity of the power distribution system is fully utilized
(3) The stator current is small: the rotor has no excitation current, the reactive power is reduced, and the stator current is significantly reduced. Compared with the asynchronous motor with the same capacity, the stator current value can be reduced by 30% to 50%. At the same time, because the stator current is greatly reduced, the temperature rise of the motor is reduced, and the bearing grease and bearing life are prolonged.
(4) High out-of-step torque and pull-in torque: Rare earth permanent magnet synchronous motors have high out-of-step torque and pull-in torque, so that the motor has a high load capacity and can be pulled into synchronization smoothly.
However, the disadvantages of this type of motor are also very clear, that is:
(1) High cost: Compared with the asynchronous motor of the same specification, the air gap between the stator and the rotor is smaller, and the processing precision of each component is high; the rotor structure is more complicated, and the price of rare earth magnetic steel materials is high; overall, the manufacturing cost of the motor is high.
(2) The impact is large when starting at full voltage: when starting at full voltage, the synchronous speed can be drawn in a very short time, the mechanical impact is large, the starting current is more than 10 times the rated current, and the impact on the power supply system is large, requiring the power supply system The capacity is large.
(3) Rare earth magnets are easy to demagnetize: when permanent magnet materials are subjected to vibration, high temperature and overload current, their magnetic permeability may decrease, or demagnetization may occur, reducing the performance of permanent magnet motors.
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