With AC motor speed control inverters;

What is V/F control?

When we talk about V / f control of an induction motor, it means changing the speed of the motor by changing the frequency, but at the same time keeping the voltage per unit frequency (the V / f ratio) constant. The correct V / Hz ratio equals the rated voltage of the motor divided by its rated frequency. V / Hz control is typically applied without feedback (i.e., open loop).

However, closed-loop V / Hz control with motor feedback is also possible. V / Hz control is simple and low-cost, but the cost and complexity increase in closed-loop applications. No control adjustment is required, but it can improve system performance.

For speed control inverter management, scalar methods optimize the motor flux and keep the magnetic field strength constant to provide continuous torque generation. Scalar methods, generally called V / Hz or V / f control, change the voltage (V) and frequency (f) supplied to the motor to ensure a fixed, stable ratio between them, so that the magnetic field remains constant regardless of motor speed.

With scalar-controlled speed adjustment, the speed accuracy is only within 2 to 3% of the nominal motor frequency, so these methods are not suitable for applications requiring precise speed control. Open-loop V / Hz control is unique in allowing a single drive to control multiple motors and is arguably the most commonly used motor speed control method.

This method is designed to meet application requirements where fast response and high-precision control are not necessary. In this mode, multiple motors can be connected to a single drive for motor control purposes. If motor parameters are unknown, the V / F control mode is recommended.

A standard speed controller (let's call it a Scalar Drive) implements a fixed V / Hz pattern to the motor under ideal conditions by generating a PWM signal motif. How the motor responds to this PWM pattern depends heavily on the load conditions. The scalar drive knows nothing about this, it just tells the motor what to do. For example, if it sends 27Hz to the motor and the motor rotates at 22Hz, the Scalar Drive doesn't know it. You can't do real torque control with a scalar drive, because there's no way to know the actual output torque of the motor (maybe a rough estimation from the current).

Speed controllers (commonly referred to as VFD worldwide) are adjustable speed drives that control the speed and torque of induction motors, typically by controlling the input voltage and frequency. Although various control techniques are available, most are used in V-F modes. To understand the reason, it is worth reviewing the vector control principle.

When it comes to electric motor control modes, the V / F control algorithm is one of the basic control modes commonly used in pump and fan systems. In this article, we will introduce the control principle, application, and more about the VF control method.

For induction motors, V/F control is definitely not the best speed control method; the vector control technique is the best method due to independent torque control. However, among scalar control techniques, this method means allowing the maximum torque of the motor, while other scalar control techniques permit undesirable slip.

The torque of an induction electric motor is the result of the interaction between the flux and rotor current. At rated frequency, if the voltage is set to a certain value and only the frequency is reduced, there will be high magnetic flux and magnetic circuit saturation (which can seriously burn the motor). Therefore, the frequency and voltage must be changed proportionally. When changing frequency, we must control the output voltage of the AC drive to keep the flux constant and avoid weak magnetic and magnetic saturation phenomena. This control method is commonly applied for fans and pumps.

Currently, most general applications use the V / F control method. VFDs with V / F control generally have a simple structure. However, these VFDs use open loop control, so it is difficult to achieve high control performance; also, at low frequencies, torque compensation is required to change low-frequency torque characteristics.

The voltage per hertz ratio must be kept constant, which means the voltage supplied to the motor will be low at low frequency—resulting in low torque / HP output. To fix this, a motor with a larger iron core than normal is required, allowing higher voltage input at low frequency (note the size difference between 50 Hz and 60 Hz motors due to the iron required to carry the magnetic flux).

In Summary: Dear members, no one has actually read the technical details explained above. In short, V/F control is the most basic motor speed control format, and probably around 90% of the drives in demand in our country operate in this form. If very low speeds (e.g., 5Hz or below) are not critical for high performance or speed losses, you can comfortably choose speed control devices with V-F control.

>> What is a speed controller?

>> What is Vector Control?

>> What is torque control?

>> What is a Crane Drive?

>> What is a braking resistor?

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