Variable frequency motors and industrial frequency motors are two completely different series of products, but it is a common practice for customers to use industrial frequency motors with variable frequency. Whether for motor manufacturers or supporting equipment manufacturers, this simple and easy method may be the most economical and quickest, but it has great risks: it may reduce the life of the motor or directly burn it out.

Since the use of industrial frequency motor frequency conversion is very common, there is undoubtedly a reasonable side to it. The speed of the asynchronous motor is positively correlated with the power frequency: that is, the higher the frequency, the higher the speed (not considering the influence of slip, they are proportional); the output torque and current are related to the load (= "dynamic load" + "static load or steady-state load"). If the influence of harmonic components is not very serious, the current increase at rated load is not large; the frequency conversion range of the motor during operation is not less than 30% of the rated frequency. There are often many cases that meet the above conditions. In actual operation, the speed is not so low that it affects ventilation and heat dissipation too much, and the current increase is not too large, so the motor can indeed work normally as expected by design.

The so-called great risk is due to the fact that most end users are not aware that when the frequency converter is powered, the inverter will output a large amount of high-frequency harmonics, and the motor may resonate at a certain speed. Therefore, it will be overloaded (the current increase is too large) at rated load, high-frequency harmonics will break through the insulation of the wire package or enameled wire, and there will be whistling or severe vibration during the speed regulation process. Sometimes it will even cause the inverter power module to explode. Ms. today will make a simple summary and conclusion on the particularity of the variable frequency motor, aiming to reduce the risk of the inverter power supply motor running and minimize the loss in case of an accident. Differences in performance parameter selection For ordinary asynchronous motors, the main performance parameters considered during design are overload capacity, starting performance, efficiency and power factor. For variable frequency motors, since the critical slip rate is inversely proportional to the power supply frequency, it can be started directly when the critical slip rate is close to 1. Therefore, the overload capacity and starting performance do not need to be considered too much, and the key issue to be solved is how to improve the motor's adaptability to non-sinusoidal power supply. Therefore, the main measures taken in the design link are:

● Reduce the stator and rotor resistance as much as possible. Because reducing the stator resistance can reduce the fundamental copper loss to compensate for the increase in copper loss caused by high-order harmonics.

● To suppress the high-order harmonics in the current, the inductance of the motor needs to be appropriately increased. However, the larger the rotor slot leakage reactance, the larger the skin effect, and the higher harmonic copper loss. Therefore, the size of the motor leakage reactance must take into account the rationality of the impedance matching within the entire speed regulation range.

● The main magnetic circuit is designed to be unsaturated, firstly because high-order harmonics will deepen the saturation of the magnetic circuit, and secondly because at low frequencies, the output voltage of the inverter should be appropriately increased in order to increase the output torque.

Structural design countermeasures When designing the structure, fully consider and evaluate the impact of non-sinusoidal power supply characteristics on the insulation structure, vibration, noise, cooling method, etc. of the variable frequency motor. The following methods can be used to ensure that the performance of the product meets the requirements:  ● Generally, the insulation level is F or higher, and the insulation to the ground and the insulation strength of the wire turns are strengthened, and the ability of the insulation to withstand impulse voltage should be considered in particular. Some motor manufacturers use thickened paint film electromagnetic wire, and standard motor manufacturers use special electromagnetic wire for variable frequency motors. From the performance index analysis of electromagnetic wire, the paint used in variable frequency electromagnetic wire is different from that of ordinary electromagnetic wire. The use of thickened wire will have a certain effect, but it cannot solve the fundamental problem. However, from the cost analysis, variable frequency electromagnetic wire is more expensive than ordinary electromagnetic wire.

● Fully consider the rigidity of the motor components and its overall structure to effectively avoid the vibration and noise problems of the motor, and avoid resonance with various force waves from the perspective of increasing the motor's natural frequency.

● Bearing insulation measures should be adopted for motors with a capacity of more than 132 kilowatts. This is mainly because variable frequency motors are prone to magnetic circuit asymmetry, which leads to shaft current when the motor is running. When the current generated by other high-frequency components is combined, the shaft current will increase greatly, resulting in bearing damage, so insulation measures are generally required.

● For constant power variable frequency motors, when the speed exceeds the synchronous speed of the 2P motor, a special high-temperature resistant grease should be used to ensure the lubrication effect of the bearing during operation. If necessary, special high-temperature resistant bearings should be selected.

Different motor manufacturers have different electromagnetic solutions for the same specifications, but the actual effect of motor operation will be somewhat different; the same product, due to the particularity of the operating conditions, will also lead to differences in the stability of products of the same specifications. Ms. suggested that motor manufacturers should try their best to fully understand the operating conditions of the motor and avoid actual risks of use through top-level design improvements of the product.

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