First, ordinary asynchronous motors are designed for constant frequency and constant voltage, and cannot fully meet the requirements of variable frequency speed regulation.
The impact of frequency converters on motors
1. Issues related to the efficiency and temperature rise of electric motors.
Regardless of the type of frequency converter, all generate harmonic voltages and currents to varying degrees during operation, causing the motor to operate under non-sinusoidal voltage and current conditions. According to available information, taking the commonly used sinusoidal PWM frequency converter as an example, its low-order harmonics are essentially zero, while the remaining high-order harmonic components, approximately twice the carrier frequency, are 2u+1 (u being the modulation ratio). These high-order harmonics increase stator copper losses, rotor copper (aluminum) losses, iron losses, and additional losses in the motor, with the most significant increase being in rotor copper (aluminum) losses. Because asynchronous motors rotate at a synchronous speed close to the fundamental frequency, the high-order harmonic voltages, cutting the rotor bars with a large slip, generate substantial rotor losses. In addition, the additional copper losses due to the skin effect must be considered. These losses all contribute to additional motor heating, reduced efficiency, and decreased output power. For example, operating a typical three-phase asynchronous motor under non-sinusoidal power conditions from a frequency converter output generally increases its temperature rise by 10%–20%.
2. Electric motor insulation strength issues
Currently, many small and medium-sized frequency converters use PWM control. Their carrier frequency is approximately several thousand to tens of kilohertz, which forces the motor stator windings to withstand a very high voltage rise rate, equivalent to applying a steep surge voltage to the motor, subjecting the inter-turn insulation to severe stress. Furthermore, the rectangular chopper surge voltage generated by the PWM frequency converter, superimposed on the motor's operating voltage, threatens the motor's insulation to ground, accelerating its aging under repeated high-voltage surges.
3. Harmonic electromagnetic noise and vibration
When a conventional asynchronous motor is powered by a frequency converter, the vibrations and noise caused by electromagnetic, mechanical, and ventilation factors become more complex. The time harmonics in the frequency converter interfere with the inherent spatial harmonics of the motor's electromagnetic components, creating various electromagnetic excitation forces. When the frequency of these electromagnetic force waves coincides with or is close to the natural vibration frequency of the motor body, resonance occurs, thus increasing noise. Because motors operate over a wide frequency range and have a large speed variation range, it is difficult for the frequencies of these electromagnetic force waves to avoid the natural vibration frequencies of the motor's components.
4. The motor's ability to adapt to frequent starting and braking.
Because the motor can be started at very low frequency and voltage without inrush current after being powered by a frequency converter, and can be quickly braked using various braking methods provided by the frequency converter, it creates conditions for frequent starting and braking. As a result, the mechanical and electromagnetic systems of the motor are under the action of cyclic alternating forces, which brings fatigue and accelerated aging problems to the mechanical and insulation structures.
5. Cooling issues at low speeds
First, the impedance of an asynchronous motor is not ideal. When the power supply frequency is low, the losses caused by high-order harmonics in the power supply are significant. Second, when the speed of a conventional asynchronous motor decreases, the cooling airflow decreases proportionally to the cube of the speed, resulting in poor low-speed cooling of the motor, a sharp increase in temperature, and difficulty in achieving constant torque output.
II. Characteristics of Variable Frequency Motors
1. Electromagnetic Design
For ordinary asynchronous motors, the main performance parameters considered during redesign are overload capacity, starting performance, efficiency, and power factor. However, for variable frequency motors, since the critical slip is inversely proportional to the power supply frequency, they can start directly when the critical slip is close to 1. Therefore, overload capacity and starting performance are no longer as important considerations. The key issue to address is how to improve the motor's adaptability to non-sinusoidal power supplies. The general approach is as follows:
1) Minimize stator and rotor resistance as much as possible. Reducing stator resistance lowers fundamental copper loss, thus compensating for increased copper loss caused by higher harmonics.
2) To suppress high-order harmonics in the current, the inductance of the motor needs to be appropriately increased. However, a larger rotor slot leakage reactance results in a larger skin effect and increased copper losses due to high-order harmonics. Therefore, the magnitude of the motor leakage reactance must take into account the rationality of impedance matching throughout the entire speed range.
3) The main magnetic circuit of a variable frequency motor is generally designed to be unsaturated. This is because higher harmonics will deepen the saturation of the magnetic circuit, and also because the output voltage of the frequency converter needs to be appropriately increased at low frequencies to improve the output torque.
2. Structural Design
When redesigning the structure, the main considerations are the impact of non-sinusoidal power supply characteristics on the insulation structure, vibration, noise, and cooling methods of the variable frequency motor. The following issues are generally taken into account:
1) Insulation class, generally F or higher, strengthen the insulation to ground and the insulation strength of the coils, and especially consider the insulation's ability to withstand impulse voltage.
2) Regarding the vibration and noise issues of the motor, the rigidity of the motor components and the whole should be fully considered, and efforts should be made to increase its natural frequency to avoid resonance with various force waves. 3) Cooling method: Generally, forced ventilation cooling is adopted, that is, the main motor cooling fan is driven by an independent motor.
4) Measures to prevent shaft current: Bearing insulation measures should be adopted for motors with a capacity exceeding 160KW. This is mainly because magnetic circuit asymmetry is prone to occur, which can also generate shaft current. When the current generated by other high-frequency components combines with it, the shaft current will increase greatly, leading to bearing damage. Therefore, insulation measures are generally required.
5) For constant power variable frequency motors, when the speed exceeds 3000 rpm, special high-temperature resistant grease should be used to compensate for the temperature rise of the bearings.
Variable frequency motors can operate continuously within the range of 0.1Hz to 130Hz, while ordinary motors can operate within the range of:
The two-electrode type operates continuously in the 20-65 Hz range.
The 4-pole version operates continuously in the 25-75Hz range.
The 6-pole version operates continuously in the 30-85Hz range.
The 8-pole version operates continuously in the 35-100Hz range.
