What is V/F Control of Inverters?
V/F Control (Voltage/Frequency Control) of inverters is one of the most basic and widely used variable-frequency speed regulation methods. Its core principle is to synchronously adjust the output voltage (V) in a fixed proportion while changing the motor’s supply frequency (f) to regulate speed. This ensures the motor’s air-gap flux remains essentially constant, preventing magnetic circuit saturation or insufficient torque and enabling stable speed regulation.
I. Core Logic of V/F Control: Why “Synchronously Regulate V and F”?
To understand V/F control, you first need to grasp the motor’s core operating principle — the motor’s air-gap flux (Φ) is directly linked to voltage (V) and frequency (f). This relationship can be simplified using the formula for the induction electromotive force in the motor’s stator winding:
(Where E is the stator-induced electromotive force, approximately equal to the supply voltage V; N is the number of winding turns, an inherent parameter of the motor)
E ≈ 4.44 × f × N × Φ(Where E is the stator-induced electromotive force, approximately equal to the supply voltage V; N is the number of winding turns, an inherent parameter of the motor)
- When the frequency f decreases (motor decelerates): If the voltage V stays unchanged, the flux Φ will increase (per the formula), causing the motor’s magnetic circuit to saturate. This leads to a sharp rise in excitation current, which can overheat the motor, create noise, or even cause damage.
- When the frequency f increases (motor accelerates): If the voltage V remains unchanged, the flux Φ will decrease, resulting in a significant drop in the motor’s output torque. This leaves the motor unable to drive the load (a phenomenon commonly called “flux loss”).
Thus, the core goal of V/F control is to maintain a constant V/f ratio by “adjusting V in a fixed proportion with F”. This keeps the flux Φ essentially unchanged, allowing the motor to output stable torque at different speeds.
II. Key Parameter of V/F Control: The V/f Curve
The core of V/F control lies in the “proportional relationship between V and F,” which is defined by the V/f curve. Different curves suit different application scenarios, with common types listed below:
| Curve Type | Core Characteristics | Application Scenarios |
|---|---|---|
| Linear V/f Curve | V and F change linearly in a strictly fixed proportion (V/f = constant) | General constant-torque loads, such as conveyors, fans, and pumps |
| V/f Curve with Low-Frequency Compensation | In the low-frequency range (e.g., f < 5Hz), V increases at a higher rate than F. This compensates for the voltage drop across the stator resistance | Scenarios requiring high torque at low frequencies, such as elevator startup and machine tool brake release |
| Square-Law V/f Curve | V has a square relationship with F (V ∝ f²), so the increase in V is gentler at low frequencies | Fans, pumps, and other square-law loads (where load torque is proportional to the square of speed). This curve delivers energy-saving benefits |
Example: V/f Curve of a Standard Asynchronous Motor
Take a motor with a rated voltage of 380V and a rated frequency of 50Hz as an example:
- Rated point: When f = 50Hz, V = 380V, so the V/f ratio = 7.6 V/Hz;
- During speed regulation: If f drops to 25Hz (half speed), V must be reduced synchronously to 190V (25Hz × 7.6 V/Hz) to ensure constant flux.
III. Classification of V/F Control: Open-Loop vs. Closed-Loop
Based on whether motor speed feedback is required, V/F control falls into two categories, each suited to distinct application scenarios:
| Control Type | Principle | Advantages | Disadvantages | Application Scenarios |
|---|---|---|---|---|
| Open-Loop V/F Control | No speed feedback is used. The inverter only outputs the corresponding voltage based on the set frequency, relying on the motor’s inherent characteristics for speed regulation | Simple structure (no encoder needed), low cost, high reliability, and easy debugging | Low speed accuracy (slip occurs, so speed fluctuates with load changes) and slow dynamic response | Scenarios with low speed accuracy requirements: fans, pumps, conveyors, and standard machine tools |
| Closed-Loop V/F Control | Speed feedback (e.g., from an encoder or Hall sensor) is added. The system compares the “set speed” with the “actual speed” in real time and adjusts the output voltage/frequency accordingly | High speed accuracy (slip can be compensated), faster dynamic response, and strong load adaptability | Requires additional feedback devices, increases costs, and raises debugging complexity | Scenarios with moderate speed accuracy requirements: small cranes, injection molding machines, and printing machines |
IV. Advantages and Disadvantages of V/F Control
1. Advantages
- Simple Structure: No complex motor parameter identification is needed (unlike vector control, which requires a motor model), so inverter hardware costs are low;
- Strong Versatility: Works with all asynchronous motors. No debugging is required for specific motors, ensuring good compatibility;
- High Reliability: The control logic is simple, leading to a low failure rate and easy maintenance;
- Low Cost: No speed feedback device is required for open-loop systems, making it ideal for large-scale, low-cost applications.
2. Disadvantages
- Low Speed Accuracy: In open-loop mode, the system relies on motor slip. Load changes cause speed deviations (e.g., a fully loaded motor runs 2%–5% slower than an unloaded one);
- Poor Dynamic Response: There is a delay in voltage/frequency adjustment. When the load changes suddenly (e.g., sudden acceleration or deceleration), torque response is slow, and speed fluctuations are likely;
- Insufficient Low-Speed Torque: In the low-frequency range (f < 5Hz), the voltage drop across the stator resistance accounts for a larger proportion (V ≈ E + IR, where I is current and R is stator resistance). This reduces the actual flux, leading to lower low-speed torque (this issue can be mitigated with “low-frequency compensation” but not fully resolved).
V. Comparison: V/F Control vs. Other Control Methods
V/F control is the “basic version” of variable-frequency speed regulation. It must be distinguished from more advanced control methods (such as vector control and direct torque control) to avoid confusion:
| Control Method | Core Principle | Speed Accuracy | Dynamic Response | Low-Speed Torque | Application Scenarios |
|---|---|---|---|---|---|
| V/F Control | Maintains a constant V/f ratio to ensure constant flux | Low | Slow | Weak (needs compensation) | General constant-torque/square-law loads with low accuracy requirements |
| Vector Control | Decomposes motor current into “excitation current” and “torque current,” controlling each separately to simulate DC motor speed regulation | High | Fast | Strong | Scenarios requiring high accuracy and dynamic response: CNC machine tools and servo systems |
| Direct Torque Control (DTC) | Controls motor torque and flux directly without current decomposition | High | Faster | Strong | Scenarios requiring fast torque response: elevators, cranes, and electric vehicles |
VI. Typical Application Scenarios of V/F Control
Thanks to its “simplicity, low cost, and versatility,” V/F control is the most widely used variable-frequency speed regulation method in industry. Its key application scenarios include:
- Pump Loads: Centrifugal water pumps and oil pumps (square-law loads). The V/f curve matches the load’s characteristics, delivering significant energy savings;
- Fan Loads: Axial fans and centrifugal fans (similar to pumps). During speed regulation, power consumption decreases with the cube of speed, offering notable energy-saving benefits;
- Constant-Torque Loads: Conveyors, assembly lines, and standard machine tool spindles (low speed accuracy requirements, only stable operation is needed);
- Small General Equipment: Mixers, compressors, and packaging machines (no complex speed regulation is required; priority is given to low cost and reliability).
Summary
V/F control is the “entry-level” control strategy for inverters. Its core is synchronously adjusting voltage and frequency in proportion to maintain constant motor flux. Its greatest strengths are simplicity, versatility, and low cost — making it suitable for scenarios where speed accuracy and dynamic response are not critical. However, in high-precision or high-dynamic applications, it must be replaced by vector control or direct torque control. Understanding V/F control is the foundation for mastering variable-frequency speed regulation technology.