the reasons are as follows:
1.efficiency and temperature rise of motor
no matter what form of frequency converter, it will produce different degrees of high-order harmonic voltage and current in operation. high-order harmonic will cause the increase of motor stator copper loss, rotor copper loss (aluminum loss), iron loss and additional loss, most notably the loss of rotor. these losses will make the motor generate additional heat, reduce the efficiency and reduce the output power. if the ordinary motor is operated under the condition of frequency conversion, its temperature rise will generally increase by 10% - 20%.
2.cooling problem at low speed
firstly, the impedance of asynchronous motor is not ideal. when the power frequency is low, the loss caused by high-order harmonics in the power supply is large. secondly, when ordinary asynchronous motor operates at low frequency, it belongs to constant torque operation. although the voltage, frequency and power are lower than the rated operation value, the current of the motor will not change much, so the heating capacity of the motor will not be reduced, the cooling air volume of the motor is directly proportional to the third power of the speed. with the decrease of the speed, the cooling condition of the motor becomes worse, and the temperature rise of the motor will increase rapidly.
3.shaft voltage and shaft current of motor
bearing voltage refers to the capacitive coupling voltage in the radial clearance of the bearing. the bearing voltage is initially caused by the average potential of the stator winding and the ac voltage between the grounded stator core. when the motor is powered by sinusoidal voltage, the bearing voltage is actually equal to zero. when the frequency converter is used for power supply, due to the rectangular wave power supply, the neutral point potential is not zero, so the common mode voltage will be generated. if the motor is not insulated on the bearing seat, shaft circulating current, shaft grounding current and common mode current will be generated accordingly. great damage to bearing and load. the ordinary motor of our company does not need insulation treatment.
4.insulation of motor
since the motor is powered by the frequency converter, it generates fixed amplitude voltage rectangular pulses with different widths and frequencies (tens to thousands of khz), and the voltage rise time is between 50ns-400ns. in such a short rise time, when the frequency converter supplies power to the motor through the cable, repeated voltage sudden changes (surge voltage) will occur on the motor terminal, which is bound to reduce the service life of the insulation. for ordinary motors, electromagnetic wires, insulation and lead wiring cables have not been specially treated, and there are major hidden dangers under the power supply of frequency converter.
5.harmonic electromagnetic noise and vibration
the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complex when the ordinary asynchronous motor operates with variable frequency. each time harmonic contained in the variable frequency power supply interferes with the natural space harmonic of the electromagnetic part of the motor to form various electromagnetic excitation forces. when the frequency of the electromagnetic force wave is consistent with or close to the natural frequency of the motor, resonance will occur, which will increase the noise. if the working frequency range of the motor is wide and the speed changes greatly, the frequencies of various electromagnetic force waves are difficult to avoid the natural vibration of various parts of the motor.
therefore, if you need frequency conversion speed regulation, you'd better choose frequency conversion motor.