VFD electric motor needs to withstand high-frequency magnetic field, so the insulation level should be higher than that of ordinary motors. In principle, ordinary motors cannot be driven by frequency converters, but in reality, to save costs, many ordinary motors are used instead of motor with integrated vfds in many occasions that require speed regulation. However, the speed regulation accuracy of ordinary motors is not high, and they are often used in the energy-saving transformation of fans and water pumps. When using ordinary motors instead of VFD electric motors, the carrier frequency of the frequency converter should be kept as low as possible to reduce the insulation damage caused by high frequency to the motor.
For ordinary asynchronous motors, the performance parameters mainly considered in the design of electromagnetic design are overload capacity, starting performance, efficiency, and power factor. However, for VFD electric motors, since the critical slip ratio is inversely proportional to the power supply frequency, it can be started directly when the critical slip ratio is close to 1. Therefore, no more consideration is needed for overload capacity and starting performance, and the key problem to be solved is how to improve the adaptability of the motor to non-sinusoidal power supply.
To suppress the high-order harmonic in the current, the motor inductance should be appropriately increased. However, the rotor slot leakage reactance is large, and its skin effect and copper loss of high-order harmonic are increased. Therefore, the size of the motor leakage reactance should consider the rationality of impedance matching in the entire speed range.
The main magnetic circuit of VFD electric motor is generally designed to be in a non-saturated state. Firstly, considering that high-order harmonics will deepen magnetic saturation, and secondly, considering that at low frequency, the output voltage of the inverter should be appropriately increased to improve the output torque.
In structural design from power control equipment company, the influence of non-sinusoidal power supply characteristics on the insulation structure, vibration, noise, and cooling method of VFD electric motor should be considered. Generally, the following issues are noted:
Insulation level: generally F level or higher, strengthen the insulation strength of grounding and winding insulation, especially consider the ability of insulation to withstand impulse voltage. Regarding the vibration and noise problems of motors, the rigidity of motor components and the overall rigidity should be fully considered to improve its natural frequency as much as possible, thereby avoiding resonance with various force waves.
Cooling method: generally adopt forced ventilation cooling, that is, the main motor cooling fan adopts independent motor drive. Measures to prevent shaft current: For motors with a capacity exceeding 160KW, bearing insulation measures should be taken. This is mainly because it is prone to produce magnetic circuit asymmetry and shaft current. When the current generated by other high-frequency components acts together, the shaft current will increase significantly, leading to bearing damage. Therefore, insulation measures should generally be taken. For constant power VFD electric motors, when the speed exceeds 3000/min, special high-temperature resistant grease should be used to compensate for the temperature rise of the bearing.