Advantages and disadvantages of variable-frequency pumps and fixed-frequency pumps

What are the differences between variable-frequency motors and fixed-frequency motors? 1. Ordinary asynchronous motors are designed for constant frequency and constant voltage, and they cannot fully meet the requirements of variable-frequency speed control. The following are the effects of frequency converters on motors: 1. Issues related to the motor’s efficiency and temperature rise. 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 in the motor stator, the copper (aluminum) loss in the rotor, the iron loss, and additional losses, with the copper (aluminum) loss in 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 means that the stator windings of the motor have to withstand very high voltage rise rates. This is equivalent to applying a shock voltage with a high gradient to the motor, placing severe stress on the inter-turn insulation of the motor. Furthermore, the rectangular chopped shock voltage generated by the PWM inverter, which is 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. Since power is supplied via an inverter, the motor can start at very low frequencies and voltages without inrush 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 the mechanical and insulation components. 　　 5. Cooling issues at low speeds: Firstly, the impedance of asynchronous motors is not ideal; when the power supply frequency is low, losses caused by high-order 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. II. Characteristics of variable-frequency motors 　　 1. Electromagnetic design 　For conventional asynchronous motors, the key performance parameters considered during design are overload capacity, starting performance, efficiency, and power factor. In contrast, since the critical slip rate of an induction motor is inversely proportional to the power supply frequency, it can start directly when the critical slip rate is close to 1. Therefore, overload capacity and starting performance do not need to be given excessive attention; the key issue to address is how to improve the motor’s ability to adapt to non-sinusoidal power supplies. The general approach is as follows: 1) Minimize the stator and rotor resistances as much as possible. 　　Reducing the stator resistance can lower the fundamental-wave copper loss, thereby compensating for the increase in copper loss caused by higher harmonics. 2) To suppress the higher harmonics in the current, it is necessary to appropriately increase the inductance of the motor. However, due to the large leakage reactance in the rotor slots, the skin effect is also significant, resulting in increased copper loss at higher harmonics. Therefore, the magnitude of the motor leakage reactance must take into account the rationality of impedance matching across the entire speed control range. 　　 3) The main magnetic circuit of variable-frequency motors is generally designed to be in a non-saturated state; this is done first to prevent high-order harmonics from exacerbating magnetic circuit saturation, and secondly to increase the output voltage of the inverter appropriately at low frequencies in order to boost the output torque. 　　 2. Structural Design During restructuring, the main considerations are the effects of the characteristics of non-sinusoidal power supplies on aspects such as the insulation structure, vibration, noise, and cooling methods of variable-frequency motors. The following points should generally be taken into account: 1) Insulation class, which is usually F grade or higher; it is necessary to enhance the insulation strength against ground as well as between windings, with particular attention paid to the insulation’s ability to withstand shock voltages. 　　 2) Regarding the vibration and noise issues of the motor, it is necessary to fully consider the stiffness of the motor’s components as well as the overall structure, and strive to increase its natural frequency in order to avoid resonance with various force waves. 　　 3) Cooling method: Forced ventilation cooling is generally used, that is, the cooling fan of the main motor is driven by an independent motor. 　　 4) Measures to prevent shaft current: Bearing insulation should be employed for motors with a capacity exceeding 160 KW. Primarily, it leads to magnetic circuit asymmetry, as well as the generation of shaft current. When the currents generated by other high-frequency components act together, the shaft current increases significantly, which can result in bearing damage; therefore, insulation measures are generally necessary. 　　 5) For constant-power variable-frequency motors, when the speed exceeds 3000/min, a special high-temperature-resistant lubricant should be used to compensate for the increase in bearing temperature. Variable-frequency motors can operate at 0. It can operate continuously within the range of 1HZ–130HZ. Ordinary motors can operate continuously within the following ranges: 2-pole motors can operate at 20–65HZ, 4-pole motors at 25–75HZ, 6-pole motors at 30–85HZ, and 8-pole motors at 35–100HZ