Can a regular motor operate at variable frequency for a long time?

Personally, I think the stress on the non-driven side is much greater; it is for this reason that the motor attached to the pumps in my factory requires bearing replacement after a short time. It makes more sense to explain this using the principle of levers – the farther it is from the driving end, the greater its force will be.

Conventional asynchronous motors are designed for constant frequency and voltage, and thus cannot fully meet the requirements of variable-frequency speed control. The following are the effects of frequency converters on motors: 1. Efficiency and temperature rise of the motor. Regardless of the type of frequency converter, 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 taken into account. 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% to 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 shock voltage with a high gradient to the motor, putting the inter-turn insulation under severe stress. Furthermore, the rectangular chopped shock voltage generated by the PWM inverter, when added to the voltage at which the motor operates, poses 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 time 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. With variable frequency drives used for power supply, the motor can start at very low frequencies and voltages without inductive currents, and it can be braked quickly using the various braking methods provided by the variable frequency drive. 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 deteriorates the cooling conditions at low speeds, causes a sharp increase in temperature rise, and makes it difficult to achieve constant torque output. Based on our actual usage experience, if the frequency isn’t reduced too much, it’s possible to arrange to use it at a later time ； (We generally reduce the frequency to below 15 Hz.) ； The damage to the bearings may be caused by an excessively low frequency; in such cases, the cooling airflow is not sufficient to remove the heat generated by the motor when it operates at low frequencies. This leads to excessive current flowing in the motor shaft, and the grease inside the bearings melts as a result of the heat, flowing out of the bearings. Over time, this results in the bearings operating without sufficient grease, leading to their damage

That makes sense. When we discussed this issue with the motor manufacturer, they said they had no historical records of operation at low frequencies to prove that low frequencies were the cause of the short bearing lifespan. Our technical agreement with the motor factory specifies a frequency tuning range of 30~50HZ. We have asked the motor manufacturer and an international bearing brand to jointly prepare an analysis report, which we will share with everyone once it’s ready.