1. Motor Efficiency and Temperature Rise
Regardless of the inverter type, harmonic voltages and currents are generated to some extent during operation, causing the motor to run under non-sinusoidal voltage and current conditions. By excluding irrelevant data, we can take the commonly used sinusoidal PWM inverter as an example. This type of inverter has very low harmonics, with the remaining high harmonics typically around twice the carrier frequency, expressed as 2u+1 (where u is the modulation ratio).
PWM, or Pulse Width Modulation, is a control technique that uses digital signals from a microprocessor to regulate analog circuits. It is widely applied in various fields such as measurement, communication, power control, and conversion due to its advantages like simplicity, flexibility, high efficiency, and fast dynamic response. PWM is commonly used in switching regulators and is categorized based on the control method. Alongside PWM, there are also PFM types and hybrid PWM/PFM systems. Many modern microcontrollers come equipped with built-in PWM controllers.
High-frequency harmonics can increase stator copper losses, rotor copper or aluminum losses, and iron losses, along with additional losses. The most significant impact is on rotor copper or aluminum consumption. Asynchronous motors are the most widely used and heavily demanded type of motor. They generate torque through the interaction between the air-gap rotating magnetic field and the induced current in the rotor winding, making them AC motors that convert electrical energy into mechanical energy. Since the rotor current is induced, they are also known as induction motors. Their speed is close to the synchronous speed corresponding to the fundamental frequency. When higher harmonic voltages pass through the rotor bars at high slip, large rotor losses occur. Additionally, skin effect contributes to extra copper losses. These losses lead to increased heat generation, reduced efficiency, and lower output power. Operating a standard three-phase asynchronous motor under the non-sinusoidal output of an inverter typically results in a temperature rise of 10% to 20%.
2. Motor Insulation Strength Issues
Many small and medium-sized inverters use PWM control, which operates at frequencies ranging from several thousand to tens of kilohertz. This causes the motor’s stator windings to experience a very high rate of voltage change, effectively applying a steep voltage to the motor. This puts the inter-turn insulation under harsh testing. Furthermore, the rectangular ripple surge voltage generated by the PWM inverter is superimposed on the motor’s operating voltage, posing a threat to the ground insulation. Repeated exposure to high voltage accelerates the aging of the ground insulation.
3. Harmonic Electromagnetic Noise and Vibration
When a standard asynchronous motor is powered by an inverter, the vibration and noise caused by electromagnetic, mechanical, and ventilation factors become more complex. A variable frequency power supply converts AC from the grid into DC and then back into AC, producing a pure sine wave. It allows for adjustable frequency and voltage within a certain range. Unlike traditional frequency converters used for speed control or ordinary AC voltage regulators, the ideal AC power supply is stable in frequency and voltage, has zero internal resistance, and produces a pure sine wave. Variable frequency power supplies closely resemble this ideal condition. Therefore, many developed countries now use variable frequency power supplies as standard power sources to provide optimal power environments for electrical devices and facilitate objective performance evaluations. There are two main types of variable frequency power supplies: linear amplification and SPWM switching. Harmonics from each stage can interfere with the motor’s inherent spatial harmonics, creating various electromagnetic forces. If the frequency of these forces matches or is close to the motor’s natural vibration frequency, resonance occurs, increasing noise levels. Given the wide operating frequency range and varying speeds, it's challenging for electromagnetic wave frequencies to avoid the motor’s inherent vibration frequencies.
4. Motor Ability for Frequent Start and Brake
Since the inverter provides power, the motor can start at low frequency and voltage without inrush current, and can be quickly braked using various braking modes offered by the inverter. This creates conditions for frequent starts and stops, subjecting the motor’s mechanical and electromagnetic systems to cyclic alternating forces, leading to fatigue and accelerated aging of both mechanical and insulating components.
5. Cooling Problem at Low Speed
First, the impedance of an asynchronous motor is not ideal. When the power source, which is a device that provides power to an electronic device, supplies the electrical energy required by all computer components, the loss caused by higher harmonics becomes greater when the frequency is lower than normal. Second, when a general asynchronous motor runs at low speed, the cooling air volume decreases proportionally to the cube of the rotational speed, worsening the motor’s cooling at low speeds and causing a sharp rise in temperature, making it difficult to maintain constant torque output.
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