Characteristics of frequency conversion motor
1. For electromagnetic design for ordinary asynchronous motor, the main performance parameters in redesign are overload capacity, starting performance, efficiency and power factor. The variable frequency motor, because the critical slip is inversely proportional to the frequency of the power supply, can be started directly, in the critical slip ratio close to 1 so the overload capacity and starting performance do not need too much to consider, and the key problem to be solved is how to improve the ability to adapt to the non sinusoidal power supply motor.
The general way is as follows:
1) reduce the stator and rotor resistance as much as possible. Reducing the copper consumption of the base wave can be reduced by reducing the stator resistance, so as to make up for the increase of copper consumption caused by the high order harmonics.
2) in order to suppress the high order harmonic in the current, it is necessary to increase the inductance of the motor properly. But the reactance of the rotor slot is larger, and the high harmonic copper consumption is also increased. Therefore, the size of the motor leakage reactance should take into account the rationality of the impedance matching in the whole speed range.
3) the main magnetic circuit of the variable frequency motor is generally designed to be unsaturated. First, considering the higher harmonic, the saturation of the magnetic circuit will be deepened. The two is to increase the output voltage of the converter properly at low frequency when improving the output torque.
2, when the structural design is re designed, it also considers the influence of non sinusoidal power supply on the insulation structure, vibration, noise and cooling mode of the variable frequency motor.
General attention is given to the following questions:
1) insulation grade, generally F grade or higher, to strengthen the insulation and line turn insulation strength, especially to consider the ability of insulation resistance to the impact voltage.
2) for the vibration and noise problems of motor, we must give full consideration to the rigidity of motor components and the whole body, and try to improve their natural frequencies, so as to avoid resonance with the secondary force waves.
3) cooling mode: the general use of forced ventilation cooling, that is, the main motor cooling fan adopts an independent motor drive.
4) to prevent shaft current measures, bearing insulation measures should be adopted for capacity exceeding 160KW motor. It is mainly caused by the asymmetry of the magnetic circuit and the shaft current. When the current generated by the other high-frequency components is combined with each other, the shaft current will increase greatly, resulting in the bearing damage. Therefore, insulation measures should be adopted.
5) for the constant power frequency conversion motor, when the speed is more than 3000/min, the special grease with high temperature should be used in order to compensate the temperature rise of the bearing.