Jiaozuo brake

The key to debugging frequency converters for lifting machinery lies in modulating the dynamic logic. The quality of braking logic debugging directly affects the safe operation of the crane.

After using a frequency converter, the control of the brake should be achieved through the frequency converter.

Debugging of braking logic includes debugging of releasing the brake process and debugging of closing the brake process.

Release brake current: Before giving the release brake command, the frequency converter should first establish the release brake current.

This current is generally set at 70% to 120% of the rated current of the motor. Generally speaking, frequency converters with fast current dynamic response can set this current lower; For frequency converters with slow dynamic response to current, this current must be set higher. The current of the lifting mechanism and the translation and rotation mechanisms of the outdoor crane needs to be set higher; The current of the translation and rotation mechanisms of indoor cranes can be set lower.

This current value is set low, and when the brake is released, the motor torque is insufficient. The lifting mechanism will experience hook sliding phenomenon (when fully loaded), and the rotating mechanism of the translation mechanism will move in opposite directions (due to wind or slope); This current value is set too high, and when the brake is not opened, the locked rotor current is high, which is unfavorable for the motor.

It should be noted that this current also has directionality: for lifting mechanisms, the direction of this current is always positive (lifting direction), and for translation and rotation mechanisms, the direction of this current should be the same as the direction of motion.

Release brake frequency:

Before giving the brake release command, the frequency converter should first establish the brake release frequency.

When the frequency converter closed-loop current vector control is used, the motor can output zero speed full torque, and the frequency of the brake release can be set to zero theoretically, but in reality, it is often set between 0.1Hz and 0.3Hz.

When the frequency converter is used for current open-loop vector control, the motor does not have zero speed torque, and the frequency at which the motor can output full torque depends on the quality of the frequency converter and the coordination between the frequency converter and the motor. For most brands of frequency converters, this frequency is generally 0.5Hz or twice the motor slip frequency (whichever is greater). In fact, the frequency of the brake release is often set between 1Hz and 2.5Hz.

When the frequency converter adopts V/f control, the frequency at which the motor can output full torque depends on the motor. For different motors, this frequency can be between 2Hz and 20Hz.

The lower the frequency setting, the better the performance of the crane and the more convenient the debugging; However, when the setting is too low, the open loop lifting mechanism is prone to hook sliding (large load), and the open loop translation and rotation mechanisms are prone to abnormal movement (wind or slope). We not only need to consider the full load performance during debugging, but also the full load performance after long-term operation when the system performance changes

Release brake time:

After receiving the command to release the brake, it takes a certain amount of time to release. This time can be between 0.2s and 1.1s depending on the specifications and types of brakes.

Therefore, after the frequency converter gives the command to release the brake, slope acceleration cannot be carried out immediately. It is necessary to continue to maintain the output of the release brake current and release brake frequency unchanged during the release brake time.

The setting of the brake release time is too short, and the frequency converter starts to increase speed before the brake is released. There will be vibration immediately after the brake is released, and there may be slipping hooks during the lowering operation of the open-loop lifting mechanism; The setting of the brake release time is too long, the brake has been released and the frequency converter has not yet accelerated. The operation of the mechanism has stopped, and there may be a reverse pull phenomenon for the lowering operation of the open-loop lifting mechanism.

QQ Image 20221005161921

Closing brake time: After the brake receives the command to close the brake, it takes a certain amount of time to tighten. This time can be between 0.2s and 1.1s depending on the specifications and types of brakes.

Therefore, after the frequency converter gives the command to close the brake, the ramp deceleration cannot continue, and the frequency of the closing brake needs to be kept constant during the closing brake time.

If the closing brake time is set too short, the output torque of the motor before the brake is tightened is insufficient or closed, and the lifting mechanism will experience hook slipping, and the translation or rotation mechanism will experience abnormal movement; The setting of the closing brake time is too long, and the brake has already tightened and the frequency converter still has output, which is unfavorable for the motor.

After the time for closing the brake reaches, the frequency converter can immediately shut down the output without the need for further slope deceleration.

The names of the five parameters, namely current, frequency, time, frequency, and time, can be different, but the parameters must have complete functions. Otherwise, this frequency converter is not suitable for lifting machinery.