How to address the impact of inverters on motors

Thread Content

1. Issues related to motor efficiency and temperature rise: Regardless of the type of inverter, harmonic voltages and currents are generated to varying degrees during operation, causing the motor to run under non-sinusoidal voltage and current conditions. According to available information, taking the commonly used sine wave PWM-type frequency converters as an example, their lower harmonics are essentially zero; the higher harmonic components, which have a frequency about twice that of the carrier wave, are of the form 2u+1 (where u is the modulation ratio). 　　Higher harmonics cause an increase in the copper loss of the motor stator, the copper (aluminum) loss of the rotor, the iron loss, and additional losses, with the copper (aluminum) loss of the rotor being the most significant. Since an asynchronous motor rotates at a synchronous speed corresponding to the fundamental frequency, high-order harmonic voltages, when cutting across the rotor bars with a large slip, cause significant rotor losses. In addition, the additional copper loss caused by the skin effect must also be considered. All these losses cause the motor to generate additional heat, resulting in reduced efficiency and lower output power. For example, when a conventional three-phase asynchronous motor is operated under a non-sinusoidal power supply provided by an inverter, its temperature rise generally increases by 10%–20%. 　　2. Motor insulation strength issue 　At present, many small and medium-sized frequency converters use PWM control methods. His carrier frequency ranges from a few thousand to over ten thousand hertz, which causes the stator windings of the motor to experience very high voltage rise rates. This is equivalent to applying a sharp impulse voltage to the motor, putting the inter-turn insulation under severe stress. Furthermore, the rectangular chopped impulse voltages generated by PWM inverters, which are added to the voltage at which the motor operates, pose a threat to the motor’s insulation from ground; repeated exposure to such high voltages accelerates the aging of this insulation. 　　3. Harmonic electromagnetic noise and vibration When a conventional asynchronous motor is powered by an inverter, the vibrations and noises caused by electromagnetic, mechanical, ventilation, and other factors become more complex. The various temporal harmonics present in the variable-frequency power supply interact with the inherent spatial harmonics of the electromagnetic components of the motor, resulting in various electromagnetic excitation forces. When the frequency of the electromagnetic force wave matches or is close to the natural vibration frequency of the motor body, resonance occurs, thereby increasing noise. Due to the wide operating frequency range of electric motors and the large variation in rotational speed, it is difficult to avoid the natural vibration frequencies of the motor’s various components for various electromagnetic force waves. 　　4. The motor’s ability to handle frequent starting and braking. Since power is supplied via an inverter, the motor can start at very low frequencies and voltages without inductive currents, and it can be braked rapidly using the various braking methods provided by the inverter. This facilitates frequent starting and braking. As a result, the mechanical and electromagnetic systems of the motor are subjected to cyclic alternating forces, which leads to fatigue and accelerated aging of both the mechanical structure and the insulation. 　　5. Cooling issues at low speeds First of all, the impedance of asynchronous motors is not ideal; when the power supply frequency is low, the losses caused by higher harmonics in the power supply are significant. Secondly, as the speed of a conventional asynchronous motor decreases further, the volume of cooling air decreases in proportion to the cube of the speed, which worsens the cooling conditions at low speeds, causes a sharp increase in temperature rise, and makes it difficult to achieve constant torque output.