Characteristics of VFD
1 Electromagnetic Design
For a common asynchronous motor, major performance parameters considered in design include the overload capacity, start performance ,efficiency and power. As for a VFD, its breakdown slip is inversely proportional to the power frequency, and it can start directly when the breakdown slip is near 1, therefore, the overload capacity and start performance doesn’t need to consider, the key problem to be solved is how to improve the adaptability of the motor to non sinusoidal power supplies. Methods are as follows:
- Reduce the stator and rotor resistance as much as possible. Reduce the stator resistance can reduce the copper consumption of fundamental wave, thus making up for copper consumption caused by higher harmonics.
- In order to suppress the higher harmonics in the current, it is necessary to increase the inductance of the motor properly. However, the rotor slot leakage reactance and skin effect are large, copper consumption of higher harmonics thereof increases. So the motor’s leakage reactance shall consider the rationality of impedance matching in the whole speed regulation range.
- Main magnetic circuit of the VFD is generally designed to be unsaturated, that’s because the higher harmonics will deepen the saturation of magnetic circuit, on the other hand, output voltage of the inverter shall be properly raised to increase the output torque at a low frequency.
2 Structure Design
Structure design mainly considers the influence of non-sinusoidal power supply characteristics on insulation structure, vibration and noise, cooling mode of a VFD. Generally, attentions shall be paid to following problems:
- Insulation grade, which is general grade F or higher. Strengthen the ground insulation and turns insulation, particularly, consider the ability of insulation to withstand impulse voltage.
- As for vibration and noise, the rigidity of motor components and whole assembly shall be fully considered, try to improve their natural frequencies, thus avoiding resonance with secondary force waves.
- Cooling method: Generally, forced aeration-cooling is adopted, namely the radiator fan of main motor is driven by an independent motor.
- Measures to prevent shaft current. Bearing insulation measures shall be taken for motors with a capacity exceeding 160KW. Because such a motor is easy to generate unbalanced magnetic circuit and shaft current, when the current generated by other high-frequency components is combined together, the shaft current will greatly increased, thus damaging the shaft. So insulation measures are required.
- For a constant power VFD, when its rotate rate exceeds 3000/min, high temperature resistant special grease shall be adopted to make up for temperature rise of the bearing.
In summary, asynchronous motor and VFD have their own characteristics respectively, it mainly depends on your working conditions. On considering the cost, asynchronous motor is the superior choice than the VFD if possible.