The difference between ordinary asynchronous motors and variable frequency motors: ordinary asynchronous motors are designed according to constant frequency and constant voltage, which cannot fully meet the requirements of variable frequency speed regulation.

　　The following is the influence of the inverter on the motor:

　　1. The efficiency and temperature rise of the motor

　　Regardless of the type of frequency converter, different degrees of harmonic voltage and current are generated during operation, making the motor run under non-sinusoidal voltage and current.According to the information, taking the commonly used sine wave PWM inverter as an example, its low-order harmonics are basically zero, and the remaining high-order harmonic components, which are about twice as large as the carrier frequency, are: 2u+1(u Is the modulation ratio).

　　High-order harmonics will cause the increase of stator copper loss, rotor copper (aluminum) loss, iron loss and additional loss, especially the rotor copper (aluminum) loss.Because the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, after the high-order harmonic voltage cuts the rotor bar with a large slip, a large rotor loss will be generated.In addition, the additional copper loss due to the skin effect needs to be considered.These losses will cause the motor to generate additional heat, reduce the efficiency, and reduce the output power. For example, if an ordinary three-phase asynchronous motor is operated under the condition of a non-sinusoidal power supply output by the frequency converter, its temperature rise will generally increase by 10% to 20%.

　　2. The insulation strength of the motor

　　At present, many small and medium-sized inverters adopt PWM control methods.Its carrier frequency is about several thousand to ten kilohertz, which makes the motor stator winding to withstand a very high rate of voltage rise, which is equivalent to imposing a steep impulse voltage on the motor, which makes the motor’s turn-to-turn insulation more resistant. The harsh test.In addition, the rectangular chopping impulse voltage generated by the PWM frequency converter is superimposed on the motor operating voltage, which poses a threat to the motor's ground insulation, and the ground insulation will accelerate its aging under repeated impacts of high voltage.

　　3. Harmonic electromagnetic noise and vibration

　　When an ordinary asynchronous motor is powered by a frequency converter, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated.The time harmonics contained in the variable frequency power supply interfere with the inherent spatial harmonics of the electromagnetic part of the motor to form various electromagnetic exciting forces.When the frequency of the electromagnetic force wave is consistent with or close to the natural vibration frequency of the motor body, resonance will occur, thereby increasing the noise.Due to the wide operating frequency range of the motor and the wide range of rotation speed, the frequency of various electromagnetic force waves is difficult to avoid the natural vibration frequency of each component of the motor.

　　4. The adaptability of the motor to frequent starting and braking

　　Since the inverter is used for power supply, the motor can be started at a very low frequency and voltage without inrush current, and various braking methods provided by the inverter can be used for rapid braking, in order to achieve frequent starting and braking The conditions are created, so that the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating force, which brings fatigue and accelerated aging problems to the mechanical structure and the insulating structure.

　　5. Cooling problem at low speed

　　First of all, the impedance of the asynchronous motor is not ideal. When the frequency of the power supply is lower, the loss caused by the higher harmonics in the power supply is relatively large.Secondly, when the rotation speed of an ordinary asynchronous motor decreases, the cooling air volume decreases in proportion to the third power of the rotation speed, resulting in the deterioration of the low-speed cooling condition of the motor and the sharp increase in temperature rise, making it difficult to achieve constant torque output.

　　6. Electromagnetic design

　　For ordinary asynchronous motors, the main performance parameters considered in the design are overload capacity, starting performance, efficiency and power factor.As for the frequency conversion motor, since the critical slip is inversely proportional to the power frequency, it can be started directly when the critical slip is close to 1. Therefore, the overload capacity and starting performance do not need to be considered too much, but the key issue to be solved is how to improve the motor pair The adaptability of non-sine wave power supply.The method is generally as follows:

　　1) Reduce the stator and rotor resistance as much as possible.Reducing the stator resistance can reduce the fundamental copper loss to compensate for the increase in copper loss caused by higher harmonics.

　　2) In order to suppress the higher harmonics in the current, the inductance of the motor needs to be appropriately increased.However, the rotor slot leakage reactance is large, the skin effect is also large, and the copper loss of high-order harmonics is also increased.Therefore, the size of the motor leakage reactance should take into account the rationality of impedance matching in the entire speed range.

　　3) The main magnetic circuit of the variable frequency motor is generally designed to be unsaturated. One is to consider that higher harmonics will deepen the saturation of the magnetic circuit, and the other is to appropriately increase the output voltage of the inverter in order to increase the output torque at low frequencies.

　　7. Structural design

　　In the structural design, the main consideration is also the influence of non-sinusoidal power supply characteristics on the insulation structure, vibration, and noise cooling methods of the variable frequency motor. Generally, the following issues should be paid attention to:

　　1) Insulation grade, generally F grade or higher, strengthen the insulation to ground and the insulation strength of the turns, and especially consider the ability of the insulation to withstand impulse voltage.

　　2) For the vibration and noise of the motor, it is necessary to fully consider the rigidity of the motor components and the whole, and try to increase its natural frequency to avoid resonance with various force waves.

　　3) Cooling method: Forced ventilation cooling is generally used, that is, the main motor cooling fan is driven by an independent motor.

　　4) Measures to prevent shaft currents, bearing insulation measures should be adopted for motors with a capacity exceeding 160KW.The main reason is that the asymmetry of the magnetic circuit is easy to occur, and shaft current is also generated. When the currents generated by other high frequency components work together, the shaft current will greatly increase, resulting in damage to the bearing, so generally insulation measures must be taken.

　　5) For constant power variable frequency motors, when the speed exceeds 3000/min, special high-temperature-resistant grease should be used to compensate for the temperature rise of the bearing.In addition: the variable frequency motor can run for a long time in the range of 0.1HZ～130HZ, the ordinary motor can run for a long time in the range of 2～20hz for 65 poles, the range of 4～25hz for 75 poles, and the range of 6～30hz for 85 poles. Operation, 8-pole is long-term operation in the range of 35-100hz.In view of the above situation, when the motor is in a working condition with a low speed and a high power, an ordinary three-phase asynchronous motor cannot be driven by a frequency converter.