Impact of Motor Frequency Inverters on Standard Asynchronous Motors

2024-08-10

Standard asynchronous motors are common power devices in industrial applications, and their performance can vary under different operating conditions. When paired with a motor frequency inverter, several new issues may arise. Below are the main impacts of using motor frequency inverters on standard asynchronous motors.

Challenges to Motor Insulation Strength from Motor Frequency Inverters

Modern motor frequency inverters typically use Pulse Width Modulation (PWM) control, with carrier frequencies reaching several kilohertz to tens of kilohertz. This high-frequency control requires the stator windings of standard asynchronous motors to withstand a higher voltage rise rate, thereby imposing greater demands on the motor's insulation system. The rectangular chopped impulse voltage generated by the motor frequency inverter accelerates the aging of the insulation materials in standard asynchronous motors, particularly affecting ground insulation, which is severely impacted by high-voltage surges. Prolonged operation under these conditions can significantly reduce the motor's insulation lifespan, increasing maintenance costs and the risk of failures.

Impact of Motor Frequency Inverters on Motor Efficiency and Temperature Rise

Motor frequency inverters generate a certain degree of harmonic voltage and harmonic current during operation, which can significantly affect the operating efficiency and temperature rise of standard asynchronous motors. Higher-order harmonics, in particular, can increase stator copper losses, rotor copper (or aluminum) losses, and iron losses, with rotor copper (or aluminum) losses being the most prominent. Since standard asynchronous motors operate at a synchronous speed close to the fundamental frequency, the harmonic voltage can cause substantial rotor losses, and the skin effect further increases additional copper losses. These additional losses lead to a 10% to 20% increase in motor temperature rise, reducing efficiency and thereby impacting the overall performance and lifespan of the equipment.

Cooling Issues at Low Speeds with Motor Frequency Inverters

Cooling at low speeds is a critical issue for standard asynchronous motors. Due to the lower supply frequency during motor frequency inverter operation, the motor's impedance performance deteriorates, and losses caused by higher-order harmonics increase, making low-speed cooling more challenging. As the speed decreases, the motor's cooling air volume decreases cubically, leading to a sharp increase in temperature rise. This not only affects the motor's ability to maintain constant torque output but also may result in overheating and premature failure of the equipment.

Adaptability of Motor Frequency Inverters to Motor Start-Up and Braking

When powered by a motor frequency inverter, standard asynchronous motors can start smoothly at low frequencies and voltages, while also enabling various rapid braking methods. This flexibility facilitates frequent start-up and braking operations. However, during frequent start-ups and braking, the motor's mechanical and electromagnetic systems are subjected to cyclic alternating forces, which can lead to fatigue and accelerated aging of the mechanical structure and insulation system.

Harmonic Noise and Vibration Issues

The application of motor frequency inverters can also cause electromagnetic noise and vibration problems. The interaction between the harmonics in the inverter's power supply and the inherent spatial harmonics of the motor can generate complex electromagnetic excitation forces. If the frequency of these electromagnetic force waves coincides with or is close to the motor's natural vibration frequency, resonance can occur, leading to increased noise. The wide range of operating frequencies and speeds of the motor makes such resonance phenomena difficult to avoid, further impacting the normal operation of the equipment and the working environment.