When an asynchronous motor starts, the slip ratio S is close to 1. At this point, the power is high, but the power factor is low. When the motor operates at rated conditions, the slip ratio S is close to zero, meaning the slip is small, and the power is lower, but the power factor is higher. When using an inverter to start the motor, the output frequency is low, which helps keep the slip within the rated range. This ensures that the motor always operates at a high power factor.
Inverters control the motor's slip by adjusting the output frequency. If the ratio of the inverter's output frequency f to the output voltage U remains constant, the motor's magnetic flux Φ stays the same, keeping the excitation current (NIo) stable. However, if the flux Φ decreases, the excitation current also drops, which can reduce the power factor. Therefore, one of the main ways to improve the power factor is by controlling the motor's slip.
When the motor is under heavy load, the inverter can adjust the frequency. Compared to reducing the motor's flux Φ, adjusting the frequency helps reduce reactive current and improve the power factor. In simpler terms, "at low frequencies, the output voltage is lower, resulting in less reactive current." This means that when the motor is under heavy load, the inverter can decrease the voltage while maintaining the same frequency, which lowers the excitation current and reduces reactive power without changing the magnetic flux significantly.
By carefully managing the frequency-to-voltage ratio, the inverter can maintain optimal performance, ensuring the motor runs efficiently and with a high power factor. This approach not only enhances energy efficiency but also extends the motor's lifespan and reduces overall energy consumption.
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