1. Electromagnetic design for ordinary asynchronous motor, the main performance parameters in the design of asynchronous motor are overload, starting performance, efficiency and power factor. As the critical transfer ratio is inversely proportional to the power frequency, it can start directly when the critical transfer rate is close to 1. Therefore, the overload capacity and starting performance are no longer. The key problem to be solved is how to improve the adaptability of the motor to the non sinusoidal power supply. The general way is as follows:
Reduce the stator and rotor resistance as much as possible. Reducing the resistance of the stator can reduce the copper consumption of the fundamental wave to compensate for the increase of copper consumption caused by the high harmonics.
In order to suppress the higher harmonics in the current, it is necessary to increase the inductance of the motor properly. However, the larger leakage effect of rotor slot is, the larger the skin effect is, and the higher harmonic copper consumption is also increased. Therefore, the size of motor leakage should take into account the rationality of impedance matching in the whole speed range.
The main magnetic circuit of the frequency conversion motor is generally designed as an unsaturated state. One is to consider the high order harmonics to deepen the magnetic circuit saturation; two is to improve the output voltage of the inverter in order to improve the output torque at low frequency.
2. When structural design is designed, the influence of non sinusoidal power supply on the insulation structure, vibration and noise cooling mode of the variable frequency motor is also considered. General attention is given to the following questions:
Insulation grade, generally F or higher, to strengthen the ground insulation and turn insulation strength, especially considering the ability of insulation to withstand shock voltage.
The vibration and noise of the motor should be taken into full consideration of the rigidity of the motor components and the whole, and the inherent frequency of the motor should be improved in order to avoid the resonance of every force wave.
Cooling mode: forced ventilation cooling is commonly used, namely the main motor cooling fan is driven by an independent motor.
To prevent shaft current measures, bearing insulation measures should be adopted for motors with capacity exceeding 160kW. The main reason is that it is easy to produce asymmetrical magnetic circuit, and also produce axial current. When the current produced by other high frequency components is combined together, the axial current will increase greatly, which leads to the damage of the bearing, so insulation measures are generally adopted.
For constant power variable frequency motor, when the speed exceeds 3000/min, special grease with high temperature resistance should be used to compensate the temperature rise of the bearing.
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