Explanation of closed loop control of frequency converter with encoder

The frequency conversion control closed loop mainly refers to the speed closed loop.

Variable-frequency motors need speed feedback. During the speed change process of the motor starting, accelerating and decelerating, the motor's drive current needs to match the back electromotive force generated by the motor at the actual speed due to the “generator effect”. Impedance of the electromotive force is not matched, the driving force of the motor is not enough, and the speed cannot meet the output requirements, or the motor does not reach the output speed value due to the excessive load of the motor. The back electromotive force is weak in proportion to the speed. Motor coil or driver. Speed feedback The information fed back in time can calculate the actual speed and derive the matching of the back electromotive force and the drive current, thereby protecting the motor and the drive.

The speed closed-loop feedback of variable frequency motor has about three modes:

1. Hall sensor, there are mostly three Hall sensors on the rotating diameter of the motor, feedback the three-phase position change. Due to the limited information provided by the sensor to the motor for a week, the speed accuracy is low, and it is difficult to distinguish at low speeds.

2. The so-called sensorless technology-the use of coils to turn up, self-induced back electromotive force. However, the back electromotive force is weak during the process of starting to low speed. If the background impedance of the induction circuit is present, this weak induction will be "eaten", and the actual feedback obtained at low speed is very unstable.

3. Rotary encoder, with higher resolution (for example, 1024 pulses per revolution), can obtain higher speed accuracy, especially when starting to low speed.

According to the above description, it can be seen that the frequency converter (especially vector frequency conversion) with encoder is mainly effective at low speed start. The driving current can be calculated finely to prevent the current from being too small and the driving force is insufficient (no speed), or the motor stalls due to a locked rotor , The back electromotive force is not enough and the driving current is overcurrent, which is easy to burn the device or the motor.

The above situation is particularly important when restarting a lifting motor. To prevent the inverter from slipping to protect the motor from stalling, an encoder must be installed for the restarting and lifting inverter.

Pay attention to the content of the vector frequency conversion manual. Generally, there is feedback from the encoder, and the low speed can be very low.

In addition, some inverters are equipped with PG card position closed-loop mode. The encoder feedbacks to the inverter with position control function (PG card) for position closed-loop control, or the encoder signal is sent to the PLC, and the PLC commands the inverter to decelerate and Braking is used for position closed-loop control. At this time, I suggest using an absolute encoder.

Energy saving of variable frequency motors has always been a topic of discussion. Motors often start over-current design from starting to low speed to normal movement. At low speeds, because the back electromotive force is very low, external impedance is required to match, which actually consumes a lot of energy. Consume on the external impedance. The popularization and use of encoders can refine the drive current and reduce this part of loss. Someone has calculated that more than 40% of the world's electric energy is used in motors, and the energy consumption at start-up accounts for the largest proportion. If the motors can achieve high-efficiency and energy-saving start-ups, it is equivalent to more Fukushima nuclear power plants.

Therefore, the closed loop of the inverter encoder should be a trend.