Motors are widely used. Before there was no frequency converter, we used the most power-frequency motors.

I. Ordinary asynchronous motors are designed according to constant frequency and constant voltage, and it is impossible to fully meet the requirements of frequency conversion and speed regulation. The following is the influence of the frequency converter on the motor:

1. The efficiency and temperature rise of the motor. Regardless of the type of frequency converter, different levels of harmonic voltage and current are generated during operation, so that the motor operates 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 that are about twice as large as the carrier frequency are 2u+1(u is the modulation ratio). Higher harmonics will cause an increase in motor stator copper loss, rotor copper (aluminum) loss, iron loss, and additional loss, the most significant is the rotor copper (aluminum) loss. Because the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, the high-order harmonic voltage cuts the rotor bar with a large slip, which will cause a large rotor loss. In addition, additional copper loss due to skin effects needs to be considered. These losses will cause the motor to generate extra heat, reduce the efficiency, and reduce the output power. If the ordinary three-phase asynchronous motor is operated under the condition of non-sinusoidal power output by the frequency converter, its temperature rise will generally increase by 10%-20%.

2. The insulation strength of the motor. At present, many small and medium-sized inverters use PWM control. Its carrier frequency is about several thousand to more than ten kilohertz, which makes the motor stator winding withstand a high voltage rise rate, which is equivalent to applying a steep impulse voltage to the motor, making the inter-turn insulation of the motor withstand more severe tests. In addition, the rectangular chopper impulse voltage generated by the PWM inverter is superimposed on the motor operating voltage, which will pose a threat to the motor’s ground insulation, and the ground insulation will accelerate aging under repeated high-voltage impacts.

3. Harmonic electromagnetic noise and vibration. When ordinary asynchronous motors are powered by frequency converters, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. The various time harmonics contained in the variable frequency power supply interfere with the inherent space harmonics of the electromagnetic part of the motor to form various electromagnetic excitation forces. When the frequency of the electromagnetic force wave is the same as or close to the natural vibration frequency of the motor body, a resonance phenomenon will occur, thereby increasing the noise. Due to the wide operating frequency range of the motor and the large range of speed variation, it is difficult for the frequency of various electromagnetic force waves 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 motor is powered by a frequency converter, the motor can be started at a very low frequency and voltage without inrush current and can be quickly braked by using various braking methods provided by the frequency converter, creating conditions for frequent starting and braking, so 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 insulation structure.

5. Cooling problem at low speed. First of all, the impedance of the asynchronous motor is not ideal. When the power frequency is low, the loss caused by high-order harmonics in the power supply is relatively large. Secondly, when the speed of ordinary asynchronous motors decreases, the cooling air volume decreases proportionally to the cube of the speed, resulting in the deterioration of the cooling condition of the motor at low speeds, a sharp increase in temperature rise, and it is difficult to achieve constant torque output.

II. The characteristics of frequency conversion motor

1. Electromagnetic design. For ordinary asynchronous motors, the main performance parameters considered in the redesign 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 supply 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 problem to be solved is how to improve the motor’s response to the power supply. Adaptability to non-sinusoidal power supplies. 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 high-order harmonics.

2) In order to suppress the higher harmonics in the current, it is necessary to increase the inductance of the motor appropriately. However, the leakage reactance of the rotor slot is large, the skin effect is also large, and the copper loss of high-order harmonics also increases. 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 in an unsaturated state. One is to consider that high-order harmonics will deepen the saturation of the magnetic circuit. The other is to consider that at low frequencies, the output voltage of the frequency converter should be appropriately increased in order to increase the output torque.

2. Structural design. When designing the structure, the influence of non-sinusoidal power characteristics on the insulation structure, vibration, and noise cooling methods of the variable frequency motor is mainly considered. Generally, the following issues should be paid attention to:

1) Insulation grade, generally F grade or higher, to strengthen the ground insulation and the insulation strength of the turns, especially the ability of the insulation to withstand the impact 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 each force wave.

3) Cooling method: Generally, forced ventilation is used for cooling, that is, the cooling fan of the main motor is driven by an independent motor.

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

5) For constant power variable frequency motors, when the speed exceeds 3000/min, special grease with high-temperature resistance should be used to compensate for the temperature rise of the bearing. The variable frequency motor can run for a long time in the range of 0.1HZ–130HZ, the ordinary motor can run in the range of 20–65hz for 2 poles, and the range of 25–75hz for 4 poles. The 6-pole is for long-term operation in the range of 30–85hz, and the 8-pole is for long-term operation in the range of 35–100hz.

Motor selection: 

First of all, the power of the motor should be selected according to the average power and maximum power required by the load when it is in motion, converted to the motor shaft side (there may be a reducer, pulley, and other reduction devices), and the overload capacity of the motor should also be considered. The motor manufacturer can provide the torque characteristic curve of the motor, and the characteristics of the motor will change at different temperatures. By the way: The order of selection is, of course, to select the motor first and then select the frequency converter according to the motor, because the ultimate goal of control is not the frequency converter or the motor, but the mechanical load.