Currently, high-performance motors are rare earth permanent magnet motors, and NdFeB magnets are widely used in rare earth permanent magnet motors.
Advantages of Rare Earth Permanent Magnet Motors
High efficiency | The efficiency curve of the asynchronous motor is generally below 60% of the rated load and falls faster, and the efficiency is very low at light load. The rare earth permanent magnet motor efficiency curve is high and flat, with 20% ~ 120% rated load in the high-efficiency area. As a comparison, when an AC asynchronous motor is in work, the rotor winding absorbs part of the electrical energy from the battery and consumes electrical energy, this part of the electrical energy eventually turns into heat consumption in the rotor winding. The loss accounts for about 20 ~ 30% of the total loss of the motor, reducing the efficiency of the motor.
Image Source: Eepower & Magneticinnovations
High power factor | The measured power factor of a 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, with a high power factor, it does not need low-voltage reactive power compensation, and makes full use of the capacity of the power distribution system.
Small stator current | Because the rotor excitation current and reactive power are reduced, the stator current drops significantly, compared with the same capacity of the asynchronous motor. The stator’s current value can be reduced by 30%~50%. At the same time, due to the significant reduction of stator current, the temperature rise of the motor decreases, and the bearing grease and bearing life are also extended.
High Pull-out and Pull-in Torque | Rare earth permanent magnet synchronous motors have high stall torque and draw-in torque, which enable the motor to have high load capacity and can be smoothly drawn into synchronization.
Pull-out torque： The maximum torque that a stepper motor can produce without losing steps.
Pull-in torque： The minimum torque that a stepper motor needs to start moving from a standstill.
Image Source: Islproducts& Magneticinnovations
How to Choose High-quality NdFeB for Rare Earth Permanent Magnet Motors?
The magnetism of NdFeB is the strongest among all magnetic materials, and it has a large magnetic energy product (B⋅H)max in a small volume, which is the reason why electric car motors have been popularized on a large scale in recent years. However, the disadvantages of relatively low operating temperature and easy corrosion are also very obvious.
NdFeB Magnet Grade | There are only N, M, H, and SH grades of NdFeB magnets used in electric cars. Their magnetism performance is also from low to high. The temperature resistance of the N-grade NdFeB magnets is very low, only 80 degrees. They are only used in the electric car handle. M-grade NdFeB magnets could operate under 100 degrees and they are more widely used in electric car motors. H-grade NdFeB magnets’ temperature resistance reaches 120 degrees, which is suitable for high-power motors. As to the SH-grade NdFeB magnets, although their temperature resistance is 150 degrees, they are not used too much now because of the very high price.
|33 – 55
|10.5 – 10.8
|11.8 – 14.5
|33 – 53
|10.8 – 13.0
|11.8 – 14.2
|31 – 51
|10.6 – 13.0
|11.4 – 13.9
|31 – 49
|10.5 – 12.8
|11.4 – 13.7
|28 – 46
|9.5 – 12.5
|10.8 – 13.3
|26 – 43
|9.5 – 12.2
|10.5 – 12.5
|26 – 39
|9.5 – 11.6
|11.5 – 12.2
NdFeB Magnet Consistency | Is there no difference between NdFeB magnets of the same grade? No! The same grade of products also have differences, such as NdFeB magnet consistency. Usually, a motor needs more than 40 pieces of NdFeB magnets. When the magnet performance of mass-produced NdFeB magnets is stable in a certain range of values, the NdFeB magnets could form a uniform magnetic field, and the efficiency of the motor will be high. Otherwise, it will fall into the cycle of uneven magnetic field – power consumption – heating temperature rise – magnet steel demagnetization – more power consumption and then the increase of temperature.
Dysprosium | Another problem is the dysprosium content in the NdFeB magnet. Dysprosium has a very important role in NdFeB magnets. By adding a small but decisive amount. Dysprosium enhances the coercivity of NdFeB magnets, which in turn enhances the temperature resistance of NdFeB magnets. The so-called temperature resistance refers to the fact that the demagnetization of NdFeB magnets shall not exceed 3% within 2 hours at the test temperature in the operating environment of the hub motor, and this demagnetization is irreversible. The demagnetization coefficient of NdFeB magnets is different due to the difference in the amount of dysprosium in the same grade of material.
Since the price of dysprosium is very expensive, nearly 1-1.5 million dollars per ton, some magnet producers will reduce the amount of dysprosium added into the NdFeB magnets, which will cause the motor to consume more and more power after using for a while.
Image Source: Web
Users of rare earth permanent magnet motors should pay attention to the warranty period of the motor. Because brushless DC motors have been used in electric vehicles for nearly twenty years, the warranty period for electronic components has been stipulated accordingly: three years for the motor, one year for the controller battery, and three months for the rotor. If the material of the motor is qualified and the design is reasonable, the warranty period is not a problem.