I hope you are doing well. I am currently involved in a project to revamp a system that requires stopping a mixer at a specific position. This system utilizes Mitsubishi components (FX3U, FR E740 15kW), 1 electromagnetic brake, and 2 inductive sensors for position detection. The mixer runs at rated speed (through a gear box, motor 1450 RPM, shaft 50 RPM) for about 25 minutes before stopping at a designated position. Since I am new to working with large inertia applications like this, I am wondering if it is possible to stop the mixer at the command position. Additionally, I would appreciate any advice on the proper sequence to stop it safely and smoothly. Thank you for your assistance.
Using a sensor as a slowdown mechanism before reaching the stop position can be beneficial in controlling the speed of the VFD. For example, running the VFD at full speed until a certain time limit is reached, then slowing down the mixer when it reaches the slowdown position sensor. This method involves a fast rampdown time on the VFD to ensure a smooth stop at the sensor. This approach has been successfully implemented in various applications, such as indexing conveyors where fast transfer speeds are crucial. To control the speed, you can use an analogue setpoint or digital setpoints with PLC outputs for normal and jog speeds. Adjusting parameters during commissioning, such as VFD ramp up/down times, speed setpoints, and sensor distances, is essential to prevent overshooting and minimize stress on the gearbox and shaft. It is advised to avoid using magnetic brakes until the mixer has fully stopped, as this can potentially interfere with the VFD operation. Safety measures should be considered, such as implementing a brake if there is a risk of the mixer blades moving uncontrollably.
The FR-E740 is a Variable Frequency Drive (VFD) commonly used for controlling conveyor systems, powder blenders, and liquid mixers. Following Parky's recommendation, I implement a gradual stop process by first slowing the motor speed until the first switch is triggered, then switching to creep speed and stopping when the second switch is actuated. This method ensures a smooth and controlled stop for optimal equipment performance.
If you're looking to achieve a more precise and reliable stopping point, consider installing a Variable Frequency Drive (VFD) as suggested by Parky. A client of mine saw significant benefits after installing a VFD on a ball mill, such as reduced wear on the drivetrain and a faster empty/refill process. Their stopping strategy involved decelerating to a low speed and then utilizing the drive's DC injection braking, triggered by a prox switch, for the final stop. They only kept the DC on for approximately ten seconds to allow the contents to settle. Due to the high gear ratio between the motor and the mill, they were able to forgo using a holding brake. This investment quickly paid off for them.
Hello, I appreciate your advice. I have created a diagram of the machine which shows all the mechanical components that have been installed and are difficult to change. Currently, I am in the surveying stage and developing solutions at my office, so I do not have much data on the characteristics of the machine or motor. For basic operation, I am not required to adjust the speed setpoint. The machine is only required to start at the rated speed, run forward for 25 minutes, then reverse for another 25 minutes before stopping precisely at the discharge position. Following Parky's suggestion, I believe it is challenging to stop at the correct position solely by relying on slowing down due to varying loads with each batch. My idea is to slow down the machine to 5-10Hz after the 25 minutes of forward and reverse operation. During this time, if the machine reaches the sensor, I will activate the electromagnetic brake. This solution may seem simple, but I am unsure if it will put too much stress on the motor. It may be necessary to test this on-site, but I would appreciate any advice or solutions you can provide beforehand. Thank you.
While a slowdown sensor may not be necessary, it is important to operate the brakes on inverters differently to prevent tripping the VFD. Typically, brakes are connected to motor phases, causing the brake to release when power is applied - a practice not suitable for VFDs. To address this, wire the brake (energize to release) to a separate contactor controlled by the PLC. However, be cautious of potential issues when transitioning to slow speed, as the sensor may be detected before the motor has fully ramped down. In such cases, implementing a time delay is recommended to ensure the VFD has adequate time to ramp down before checking if the mixer is in the discharge position. In the realm of food/mixing systems, specific projects have included mixers utilizing opposing screw agitators. The mixing sequence involves a change of direction for the screws, followed by them rotating in the same direction on discharge (or opposed, depending on the screw orientation). Some examples include plants like Carl Scnell vessels used for pickles and spreads. When it comes to operating the mixer, two options are presented. Option 1 involves releasing the brake, running the inverter to the required mix speed, waiting a specified time, stopping the motors, reversing the motor, and repeating the process. Option 2 follows a similar process but includes a delay after reaching the slow speed to allow the motor to ramp down before stopping the VFD and applying the brake. It is crucial to ensure precise timing to avoid strain on the gearbox/mixer and prevent tripping the inverter. Implementing a delay to allow the motor to reach the slow speed before sensor detection and actions like stopping the motor or applying the brake are key to preventing tripping of the inverter. Remember, a slight delay can prevent potential issues and ensure smooth operation of the system.
Certainly, it is possible to stop the mixer at the commanded position with the right control strategy. One option would be to incorporate a PID (Proportional, Integral, Derivative) controller. The inductive sensors you mentioned can provide real-time feedback to the system, which can be used to calculate error values (i.e. the difference between the commanded position and the actual position). The PID controller can then use this error value to adjust the motor speed and/or brake activation for precise positioning. As for the safe stopping sequence; initially, you can slow down the mixer gradually instead of instant stopping. Once it reaches a manageable speed, then you can engage the electromagnetic brake. Of course, you'd need to tune the PID values carefully and perhaps incorporate some logic to prevent potential damage due to sudden stops.
It sounds like you're tackling a pretty interesting challenge! To stop the mixer at the commanded position, it’s definitely possible with the right approach. You might want to consider using a deceleration ramp in your control program to create a smooth stop and reduce stress on the motor and mechanical components. Make sure your inductive sensors are calibrated correctly to reliably detect the position, and I recommend implementing a fail-safe routine that engages the electromagnetic brake if the sensors don’t detect the position as expected. Always test the system at lower speeds first to ensure everything is working as planned before going full speed. Good luck with your project!
It sounds like an interesting project! To stop your mixer at the right position, you’ll definitely want to integrate precise control over the braking process. Since you're using an electromagnetic brake, consider implementing a deceleration ramp in your control logic to avoid sudden stops, which can be jarring for both the machine and your sensors. Also, using the inductive sensors as feedback is crucial—make sure they are set to trigger the brake well before the desired position to allow for smoother stopping. You might also want to monitor the motor’s current to avoid overheating, especially given the high inertia. Good luck, and feel free to share your progress!
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Answer: - To stop an AC motor at a specific position using Mitsubishi components, you can utilize inductive sensors for position detection along with an electromagnetic brake. Ensure proper programming and sequencing to achieve the desired stopping position.
Answer: - Yes, it is possible to stop a mixer at a designated position with large inertia by properly configuring and programming the Mitsubishi components in the system. Utilize the available components like the FX3U and FR E740 15kW in conjunction with electromagnetic brakes and sensors for accurate position control.
Answer: - To safely and smoothly stop a mixer at a specific position, ensure that the sequence includes gradually reducing the motor speed, engaging the electromagnetic brake, and using the inductive sensors for accurate position detection. Proper programming and testing are essential to ensure a controlled and precise stop.
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