The effects of frequency converters on ordinary asynchronous motors include:
1
The issues of efficiency and temperature rise in ordinary asynchronous motors. Regardless of the type of frequency converter, it generates harmonic voltage and harmonic current to varying degrees during operation, causing ordinary asynchronous motors to operate under non-sinusoidal voltage and current conditions.
Among them, high-order harmonics have the greatest impact on the operating efficiency and temperature rise of ordinary asynchronous motors. High-order harmonics will cause an increase in stator copper loss, rotor copper (aluminum) loss, iron loss and additional losses of ordinary asynchronous motors, with the rotor copper (aluminum) loss being the most significant.
Because ordinary asynchronous motors rotate at a synchronous speed close to the fundamental frequency, high-order harmonic voltages will generate significant rotor losses after cutting the rotor bars with a large slip.
In addition, the additional copper losses caused by the skin effect must also be considered. These losses will cause ordinary asynchronous motors to generate extra heat, reduce efficiency, and decrease output power. If an ordinary asynchronous motor is operated under non-sinusoidal power supply conditions from a frequency converter, its temperature rise will generally increase by 10% to 20%.
2
The insulation strength problem of ordinary asynchronous motors. Currently, most small and medium-sized frequency converters use PWM (Pulse Width Modulation) control. Its carrier frequency is about several thousand to tens of kilohertz, which means that the stator windings of ordinary asynchronous motors have to withstand a very high voltage rise rate, which is equivalent to applying a very steep impulse voltage to the ordinary asynchronous motor, putting the inter-turn insulation of the ordinary asynchronous motor to a very severe test.
In addition, the rectangular chopper impulse voltage generated by the PWM inverter superimposed on the operating voltage of the ordinary asynchronous motor will threaten the ground insulation of the ordinary asynchronous motor. The ground insulation will age faster under repeated high voltage impacts.
3
Harmonic electromagnetic noise and vibration. When a conventional asynchronous motor is powered by a frequency converter, the vibration and noise caused by electromagnetic, mechanical, and ventilation factors become more complex. The time harmonics contained in the frequency converter interfere with the inherent spatial harmonics of the electromagnetic part of the conventional asynchronous motor, forming various electromagnetic excitation forces.
When the frequency of electromagnetic force waves coincides with or is close to the natural vibration frequency of a conventional asynchronous motor, resonance will occur, thereby increasing noise. Because conventional asynchronous motors operate over a wide frequency range and have a large speed variation range, it is difficult for the frequencies of various electromagnetic force waves to avoid the natural vibration frequencies of the motor's components.
4
Ordinary asynchronous motors are not adaptable to frequent starting and braking. Because they are powered by frequency converters, ordinary asynchronous motors can start at very low frequencies and voltages without inrush current, and can be rapidly braked using various braking methods provided by the frequency converter. This creates conditions for frequent starting and braking. Consequently, the mechanical and electromagnetic systems of ordinary asynchronous motors are under the action of cyclic alternating forces, leading to fatigue and accelerated aging of the mechanical and insulation structures.
5
Cooling issues for ordinary asynchronous motors at low speeds. First, the impedance of ordinary asynchronous motors is not ideal, and when the power supply frequency is low, the losses caused by higher harmonics in the power supply are significant. Second, as the speed of an ordinary asynchronous motor decreases, the cooling airflow-to-speed ratio decreases, leading to poor low-speed cooling, a sharp increase in temperature, and difficulty in achieving constant torque output.
