The Crucial Role of Compensation Techniques in Variable Frequency Drives

In the open-loop control of the frequency converter, compensation technology plays a vital role. This involves three key aspects: torque compensation, slip compensation and dead zone effect compensation.

Torque Compensation: Enhancing Low-Frequency Performance

During low-frequency operation, the voltage drop of the stator resistance is not negligible relative to the inverter output voltage. Therefore, torque compensation must be performed to ensure sufficient output voltage. Failure to compensate may result in insufficient output voltage, failure of the motor to start at low frequencies, or significant reduction in speed.

Slip compensation: solves problems when loads are heavier

Slip compensation is mainly aimed at the situation when the actual output speed of the motor is lower than the set speed when the load is heavy. These two compensation methods can be compensated by simple fixed values ​​in the implementation. An improved method is to use the current of the three-phase motor to calculate the compensation, but only based on the current amplitude. In fact, this method is scalar compensation. A more accurate compensation method is to vectorize the AC current of the three-phase motor and include the motor losses in the calculation, so that the compensation effect is better. However, this method is complex to calculate and depends on some parameters of the motor, so there are certain difficulties in the implementation process.

Dead-Time Compensation: Improving Output Current Waveform Smoothness

Dead zone compensation technology plays an important role in open-loop control, effectively improving the smoothness of the output current waveform, reducing harmonics, increasing the effective value of the output voltage, and reducing the oscillation of the motor current. Especially when a quiet environment needs to be maintained, dead zone compensation can increase the carrier frequency and prevent the motor from running at low frequencies.

Commonly used dead zone compensation techniques

1.Current zero-crossing direct compensation method: simple and easy to implement, but affected by noise in the current waveform, it is easily affected by load fluctuations and external interference, leading to zero-crossing misjudgment.

2.Current decomposition method based on stator magnetic field orientation: It avoids directly judging the current zero-crossing point. By decomposing the stator current in the rotating coordinate system, the relationship between the current vector angle and the dead zone voltage vector is obtained, and corresponding compensation is achieved. The effect is better combined with dead zone voltage pulse width compensation.

3.Dead zone voltage pulse width compensation method: By adjusting the voltage pulse width, the dead zone effect can be effectively compensated and system performance improved.

4.Dead zone prediction compensation method without current sensing: by predicting the current phase angle, and then compensating for the dead zone time. Although the current sensor can be eliminated to reduce costs, the accuracy and dynamic performance may be reduced because it does not adjust to external load changes.

In the control system of the frequency converter, the clever application of compensation technology makes the system more stable and efficient. Different types of compensation technologies cooperate with each other to ensure the normal operation of the motor, allowing the inverter to perform well under different working conditions.