Electromagnetic and Structural Design of Variable Frequency Motor

Ⅰ. Electromagnetic design of VFD electric motor

For the electromagnetic design of ordinary asynchronous motors, the main performance parameters considered during the design are overload capacity, starting performance, efficiency and power factor. For VFD electric motors such as the explosion proof vfd, since the critical slip is inversely proportional to the power frequency, it can be started directly when the critical slip is close to 1. Therefore, overload capacity and starting performance no longer need to be considered too much, and the key problem to be solved is how to improve the adaptability of the motor to non-sinusoidal power sources. The method is generally as follows:

1. Reduce stator and rotor resistance as much as possible. The fundamental wave copper consumption can be reduced by reducing the stator resistance to compensate for the increase in copper loss caused by higher-order harmonics.

2. In order to suppress the high-order harmonics in the current, the inductance of the motor needs to be appropriately increased. However, the rotor slot leakage resistance is large, its skin effect is also large, and the copper loss of high-order harmonics also increases. Therefore, the size of the leakage reactance of the motor should take into account the rationality of impedance matching in the entire speed regulation range.

3. The main magnetic circuit of the vfd electronic motor is generally designed to be unsaturated. First, considering that higher-order harmonics will deepen the magnetic circuit saturation, and second, considering that at low frequencies, in order to increase the output torque, the output voltage of the inverter should be appropriately increased.

Ⅱ. Structural design of VFD electric motor

During the structural design of power control equipment company, mainly consider the influence of the non-sinusoidal power supply characteristics on the insulation structure, vibration, noise cooling method, etc. of the vfd electronic motor. Generally, attention is paid to the following issues:

1. Insulation grade, generally F grade or higher. Strengthen the ground insulation and line insulation strength, especially consider the insulation's ability to withstand impulse voltage.

2. For the vibration and noise issues of the vfd electronic motor, the rigidity of the motor components and the whole body should be fully considered. The natural frequency should be improved as much as possible to avoid resonance with each force wave.

3. Cooling method: Forced ventilation is generally used for cooling, that is, the main motor cooling fan is driven by an independent motor.

4. Measures to prevent shaft current. Bearing insulation measures should be adopted for motors with a capacity exceeding 160KW. The main reason is that it is easy to produce asymmetry of the magnetic circuit, and also produces shaft current. When the currents generated by other high-frequency components that work together, the shaft current will increase greatly, resulting in bearing damage. Therefore, generally insulation measures should be taken.

5. For vfd compatible motor with constant power, when the speed exceeds 3000/min, special grease with high temperature resistance should be used to compensate for the temperature rise of the bearing.