Variable frequency control closed loop mainly refers to speed closed loop.
Variable frequency motors require speed feedback. During the speed changes of starting, accelerating, decelerating, and stopping, the motor's drive current needs to match the back electromotive force (EMF) generated by the "generator effect" at the actual speed. If the motor drive current and the back EMF impedance do not match, the motor drive force will be insufficient to reach the required speed, or the motor may not reach the output speed due to excessive load. The back EMF, being proportional to the speed, will be weak, causing a sudden increase in motor current and potentially burning out the motor coils or driver. Timely speed feedback allows for the calculation of the actual speed and the matching of the back EMF and drive current, thus protecting the motor and driver.
There are approximately three modes of closed-loop speed feedback for variable frequency motors:
1. Hall effect sensors: Most motors use three Hall effect sensors to provide feedback on three-phase position changes. However, because the sensors provide limited information for each revolution of the motor, their speed accuracy is low, making it difficult to distinguish speeds at low speeds.
2. The so-called sensorless technology utilizes the rotation of a coil to induce a back electromotive force. However, the back electromotive force is weak during the startup and low-speed operation. If the inherent impedance of the induction circuit is present, this weak induction is "eaten up," resulting in very unstable feedback at low speeds.
3. Rotary encoders , with higher resolution (e.g., 1024 pulses per revolution), can achieve higher speed accuracy, especially at low speeds during startup.
Based on the above description, it can be seen that the encoder of the frequency converter (especially the vector frequency converter) is mainly effective at low-speed start-up. It can calculate the drive current in detail to prevent insufficient driving force due to insufficient current (no speed) or insufficient back electromotive force due to stalled motor, which can easily burn out the components or motor.
The above situation is particularly important for start-up and lifting motors to prevent the inverter from slipping off the hook to protect the motor from stalling. Therefore, start-up and lifting inverters must be equipped with encoders.
Pay attention to the manual for vector frequency converters; they usually have encoder feedback, and can achieve very low speeds.
In addition, some frequency converters are equipped with a PG card position closed-loop mode. The encoder feeds back to the frequency converter (PG card) with position control function to perform position closed-loop control, or the encoder signal is sent to the PLC, and the PLC instructs the frequency converter to decelerate and brake to perform position closed-loop control. In this case, I suggest using an absolute encoder.
Energy saving in variable frequency motors has always been a topic of discussion. From startup to low speed to normal operation, motors often have overcurrent designs during startup, and at low speeds, due to the very low back electromotive force, external impedance is required for matching. In fact, this consumes a lot of energy in the external impedance. The widespread use of encoders can refine the drive current and reduce this loss. It has been calculated that more than 40% of the world's electricity is used for motors, and the energy consumption during startup accounts for the largest proportion. If motors could achieve efficient and energy-saving startup, it would be equivalent to building several more Fukushima nuclear power plants.
Therefore, closed-loop inverter encoders should be a trend.
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