Basic Knowledge of High-Voltage Inverters

High-voltage inverters are essential for controlling the speed and torque of asynchronous motors by adjusting the frequency and voltage supplied to the motor. One of the most basic methods for variable frequency speed regulation is the V/f mode, where the voltage (V) decreases proportionally as the frequency (f) decreases. This method keeps the V/f ratio constant, ensuring that the magnetic flux of the motor remains steady, which is crucial for optimal motor performance. Under rated conditions, the voltage ｄｒｏｐ due to stator resistance and leakage reactance is minimal, and the terminal voltage of the motor and the induced potential are nearly equal. However, at low speeds, this constant V/f ratio control becomes less effective because the voltage ｄｒｏｐ in the stator resistance becomes more significant. This change reduces the magnetic flux in the motor, which in turn diminishes the electromagnetic torque. Additionally, the inverter’s power devices may introduce dead time, leading to voltage drops, torque pulsations, and even speed or current oscillations, further complicating low-speed operation.

Despite these challenges, V/f ratio constant control is widely used in general-purpose inverters, particularly in applications such as fans and water pumps, where the speed control range is not very large. The advantage of this method is that it allows for open-loop speed control, which simplifies operation and reduces costs, as it does not require a feedback system.

Torque Behavior with V/f Changes

When the frequency decreases and the voltage decreases proportionally in a V/f mode, the torque generated by the motor also tends to decrease. This occurs because, as the frequency drops, the AC impedance of the motor decreases, but the resistance remains unchanged. To counteract this reduction in torque at low speeds, the output voltage can be increased to maintain a certain level of starting torque. This compensation method is known as enhanced starting, and it can be achieved through various techniques, such as automatic compensation or manual adjustments using a potentiometer. The goal is to ensure that the motor can still provide adequate torque when starting or operating at low speeds, even though the proportional decrease in voltage and frequency reduces its overall torque capacity.

Open-Loop vs. Closed-Loop Control

In an open-loop control system, the motor operates without any feedback regarding its actual speed. This is the most common configuration for general-purpose inverters, where the system operates without the need for a speed sensor. While open-loop control is simple and cost-effective, it lacks the precision of closed-loop control, which uses a speed sensor to provide real-time feedback. In closed-loop systems, the inverter can adjust the motor's operation based on actual speed data, leading to more accurate control of motor performance. Closed-loop control is often preferred in applications where precise speed and torque regulation are critical.

Protection Functions of High-Voltage Inverters

High-voltage inverters are equipped with a variety of protection functions to ensure safe and reliable operation. These protection features help safeguard both the inverter and the connected motor. Some of the key protections include safeguards against output overload and overcurrent, which prevent the system from overloading and causing damage. Inverters also feature grid overvoltage and undervoltage protection, which ensure the system operates within safe voltage limits. Additional protections include DC bus overvoltage and undervoltage safeguards, which help maintain stable operation of the inverter's power supply. The inverter also has mechanisms to detect grid power failures, phase loss, and transformer overheating, which help prevent damage in case of electrical issues. Moreover, cooling fan failure alerts and power device overheating protections are in place to monitor and maintain proper operating temperatures. Other safety features include protection against control power failure, grounding faults, and optical fiber failure, ensuring that the inverter can continue to function properly and protect equipment from damage.