Frequency converters do have an impact on motors . Ordinary motors are designed for constant frequency and voltage, and therefore cannot fully adapt to the requirements of variable frequency speed control. So how do frequency converters affect three-phase asynchronous motors?
1. The electric motor's adaptability to frequent starting and braking.
With frequency converter power supply, the motor can start at very low frequency and voltage without inrush current, and can be quickly braked using various braking methods provided by the frequency converter, creating conditions for frequent starting and braking. However, the motor's mechanical and electromagnetic systems are under cyclic alternating forces, leading to fatigue and accelerated aging of the mechanical and insulation structures.
2. Cooling at low speeds
The impedance of an asynchronous motor is not ideal. When the power supply frequency is low, the losses caused by high-order harmonics in the power supply are relatively large.
When the speed of a conventional asynchronous motor decreases, the cooling airflow decreases proportionally to the cube of the speed, which worsens the low-speed cooling of the motor, causes a sharp increase in temperature, and makes it difficult to achieve constant torque output.
3. Harmonic electromagnetic noise and vibration
The time harmonics contained in the variable frequency power supply interfere with the inherent spatial harmonics of the electromagnetic part of the motor, forming various electromagnetic excitation forces. When an asynchronous motor is powered by a frequency converter, the vibration and noise caused by electromagnetic, mechanical, and ventilation factors become more complex.
Because electric 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. When the frequency of the electromagnetic force wave coincides with or is close to the natural vibration frequency of the motor body, resonance will occur, thereby increasing noise.
4. Insulation strength of the electric motor
Currently, many small and medium-sized frequency converters use PWM control. The carrier frequency is about several thousand to tens of kilohertz, which means that the stator winding of the motor has to withstand a very high voltage rise rate, which is equivalent to applying a very steep surge voltage to the motor, putting the inter-turn insulation of the motor to a very severe test.
In addition, the rectangular chopper impulse voltage generated by the PWM inverter superimposed on the motor operating voltage will threaten the motor's insulation to ground, and the insulation to ground will age faster under repeated high voltage impacts.
5. Electric motor efficiency and temperature rise
Regardless of the type of frequency converter, they all generate harmonic voltages and currents to varying degrees during operation, causing the motor to operate under non-sinusoidal voltage and current conditions.
Taking the commonly used sinusoidal PWM inverter as an example, its low-order harmonics are basically zero, and the remaining high-order harmonic components, which are about twice the carrier frequency, are: 2u+1 (u is the modulation ratio). High-order harmonics will cause an increase in the stator copper loss, rotor copper (aluminum) loss, iron loss and additional losses of the motor, with the rotor copper (aluminum) loss being the most significant.
Because 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 the motor to generate extra heat, reduce efficiency, and decrease output power. For example, if a regular three-phase asynchronous motor is operated under non-sinusoidal power conditions from a frequency converter, its temperature rise will generally increase by 10%–20%.
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