1. What is the difference between high-voltage and low-voltage inverters?

In general, inverters can be divided into two categories: high-voltage inverters and low-voltage inverters.

In terms of application, the target motors differ. From a speed control perspective, both operate under the same principle. Structurally, however, there are significant differences, mainly due to the limited voltage tolerance of widely used IGBT modules. High-voltage inverters generate greater interference with driving circuits due to their high voltage, so optical fiber isolation is usually adopted.

2. Why does the output voltage of an inverter change in proportion to its frequency?

The torque of an induction motor is generated by the interaction between the magnetic flux and the current flowing through the rotor. At rated frequency, if the voltage remains constant while the frequency is reduced, the magnetic flux becomes excessive, causing saturation in the magnetic circuit, increased motor current, and potentially even burning out the motor. Therefore, the frequency and voltage must change proportionally—that is, as the frequency changes, the output voltage of the inverter is also controlled to maintain a constant magnetic flux and avoid magnetic saturation. This is known as VVVF (Variable Voltage Variable Frequency). The voltage here refers to the line or phase voltage RMS value of the motor.

3. When using a motor driven by a power frequency supply, a voltage drop causes an increase in current; for inverter-driven motors, will the current increase if both frequency and voltage decrease?

When the frequency decreases (low speed), if the same power is output, the current increases. However, under constant torque conditions, the current remains nearly unchanged.

4. How do starting current and starting torque behave when operating a motor with an inverter?

With inverter operation, as the motor accelerates, both frequency and voltage increase accordingly. Starting current is limited to below 150% of the rated current (depending on model, it ranges from 125% to 200%). With direct power frequency startup, the starting current is 6–7 times higher, resulting in mechanical and electrical shocks. Inverter-driven operation allows smooth starting (with longer start-up time). The starting current is about 1.2–1.5 times the rated current, and the starting torque is 70%–120% of the rated torque. For inverters with automatic torque boost features, the starting torque exceeds 100%, allowing full-load starting.

5. What does the V/f mode mean?

When the frequency decreases, the voltage also decreases proportionally—this issue was explained in Question 4. Maintaining a constant V/f ratio is the most basic control method for variable frequency speed regulation of induction motors. It controls both the frequency and voltage of the motor’s power supply simultaneously, keeping the V/f ratio constant to maintain a constant magnetic flux in the motor. Under rated operating conditions, the voltage drop across the stator resistance and leakage reactance is relatively small, so the terminal voltage of the motor is approximately equal to the induced EMF.

The main problem with constant V/f control is poor performance at low speeds. One reason is that at low speeds, the proportion of voltage drop across the stator resistance becomes more significant, making it no longer negligible. Thus, assuming the stator voltage is approximately equal to the induced EMF is inaccurate, and maintaining a constant V/f ratio no longer ensures a constant magnetic flux. A reduction in motor flux inevitably leads to a decrease in electromagnetic torque. Additionally, the dead time of the inverter’s power devices significantly affects low-speed performance. Dead time causes voltage drops and torque ripple, which can lead to oscillations in speed and current under certain conditions.

Constant V/f control is commonly used in general-purpose inverters. These inverters are mainly applied in fans, pumps, and applications where the speed range requirement is not high. The main advantage of V/f control is that it allows open-loop speed control of the motor.

6. When changing V and f proportionally, how does the motor torque change?

If the voltage is decreased exactly in proportion to the frequency, the AC impedance decreases while the resistance remains constant, leading to a tendency for torque to decrease at low speeds. Therefore, at low frequencies, the given V/f should be adjusted to slightly increase the output voltage to achieve sufficient starting torque. This compensation is called enhanced starting. Various methods can be used to implement this, including automatic adjustment, selecting a V/f mode, or adjusting a potentiometer.

7. What is meant by "open loop"?

When a speed sensor is installed on the motor and the actual speed is fed back to the control unit for regulation, it is called "closed loop". Operation without a speed sensor is referred to as "open loop". Most general-purpose inverters operate in open-loop mode.

8. Protective functions of high-voltage inverters:

Output overload, output overcurrent, grid overvoltage, grid undervoltage, grid power loss, DC bus overvoltage, DC bus undervoltage, transformer overheating, phase loss, control power failure, drive fault, power device overheating, cooling fan failure, external setpoint disconnection, ground fault, fiber optic fault, etc.

Changsha Sunye Electric Co., Ltd. (established in 2010) and Shenzhen Sunye Electric Co., Ltd. (established in 2002) are national high-tech enterprises integrating R&D, manufacturing, and sales of inverters, industry-specific integrated machines, servo drives, and new energy products.

Sunye Electric owns independent intellectual property rights. Its products have passed inspections and certifications by national authoritative institutions and have obtained multiple software copyrights and IP certificates. Each year, significant investment is made in researching new technologies and developing new products, resulting in numerous patents and software copyrights. The company possesses core platform technologies such as construction hoist drivers, high-performance vector inverters, servos, and permanent magnet synchronous motor controls. Medium- and low-voltage inverters and servo drives are widely used in industries such as lifting, stone processing, HVAC, machine tools, metal products, wire and cable, plastics, printing and packaging, textile and chemical fibers, building materials, metallurgy, coal mining, municipal services, and automotive manufacturing. Guided by the philosophy of “Innovation, Technology, Strength,” driven by the corporate spirit of “Unity, Progress, Pragmatism, Innovation,” and adhering to the business principle of “Sincerity and Excellence, Mutual Benefit and Coexistence,” the company aims to provide industry-leading products through comprehensive quality management, continuous improvement, and pursuit of zero defects.

Address: Sunye Electric Industrial Park, No. 669 Xinsheng Road, High-Tech Zone, Changsha City

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