Ordinary asynchronous motors are designed for constant frequency and constant voltage, and cannot fully adapt to the requirements of variable frequency speed control. The following are the effects of frequency converters on motors:
1. Issues related to the efficiency and temperature rise of electric motors.
Regardless of the type of frequency converter, all generate harmonic voltages and currents to varying degrees during operation, causing the motor to operate under non-sinusoidal voltage and current conditions. According to available information, currently...
Taking the commonly used sinusoidal PWM type frequency converter 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 increase stator copper losses, rotor copper (aluminum) losses, iron losses, and additional losses in electric motors, with rotor copper (aluminum) losses being the most significant. Because asynchronous motors rotate at a synchronous speed close to the fundamental frequency, high-order harmonic voltages, with their large slip, cut the rotor bars, resulting in substantial rotor losses. In addition, the additional copper losses due to the skin effect must be considered. These losses cause the motor to generate extra heat, reduce efficiency, and decrease output power. For example, operating a typical three-phase asynchronous motor under non-sinusoidal power conditions from a frequency converter output generally increases its temperature rise by 10%-20%.
2. Electric motor insulation strength issues
Currently, many small and medium-sized frequency converters use PWM control. Their carrier frequency has a rise rate of several thousand to tens of kilohertz, equivalent to applying a very steep surge voltage to the motor. This causes the inter-turn insulation of the motor to endure a relatively severe voltage superimposed on the motor's operating voltage (4 to 6 times the voltage), posing a threat to the motor's insulation to ground. In addition to the high voltage repeated impacts on the stator windings, the rectangular chopping pulses generated by the PWM frequency converter accelerate the aging process.
3. Harmonic electromagnetic noise and vibration
When a conventional asynchronous motor is powered by a frequency converter, the vibrations and noise caused by electromagnetic, mechanical, and ventilation factors become more complex. The time harmonics in the frequency converter interfere with the inherent spatial harmonics of the motor's electromagnetic components, creating various electromagnetic excitation forces. When the frequency of these electromagnetic force waves coincides with or is close to the natural vibration frequency of the motor body, resonance occurs, thus increasing noise. Because motors operate over a wide frequency range and have a large speed variation range, it is difficult for the frequencies of these electromagnetic force waves to avoid the natural vibration frequencies of the motor's components.
4. The motor's ability to adapt to frequent starting and braking.
Because the motor can be started at very low frequency and voltage without inrush current after being powered by a frequency converter, and can be quickly braked using various braking methods provided by the frequency converter, it creates conditions for frequent starting and braking. As a result, the mechanical and electromagnetic systems of the motor are under the action of cyclic alternating forces, which brings fatigue and accelerated aging problems to the mechanical and insulation structures.
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