Waste grinder motor control cabinet with automatic reversing to free jammed waste, for a manufacturing plant

How the brief came about

This requirement came from a waste grinder already in use at the factory. The new control cabinet had to be installed on the grinder's existing motor and use the motor's electrical current as the signal indicating jammed waste. We started this work in 2019.

Requirements and constraints

The requirements for this work are set out in a requirements document (User Requirement). The document specifies the grinder's operating sequence, the system configuration and the system's software environment.

• Operating sequence: run forward to grind, detect jammed waste from the current, stop, reverse, then run forward to resume grinding
• Jam clearing is limited to 3 cycles. Once this limit is reached, the motor must stop and wait for the operator to press the Forward button on the cabinet.
• The front of the cabinet must have a Manual/Off/Auto mode selector switch, control buttons, status lights and an Emergency Stop button
• Remote system configuration through a Wireless Router
• System software environment: Python and a MySQL database on the Ubuntu operating system
• The cabinet must be installed on the motor of the waste grinder the factory is using

Why use the motor's electrical current to decide that waste is jammed?

We use the motor's electrical current because jammed waste overloads the motor and makes it draw a current that differs from normal, so the current value indicates a jam from the motor itself. The current sensor sends its readings to an Arduino ATmega328 board, and the cabinet uses this value as the condition for starting the reversing sequence.

We limit jam clearing to 3 cycles so that the cabinet does not keep reversing endlessly when a jam is beyond what reversing can clear. Once the cycles are complete, the operator must be the one who decides to keep running the machine.

1. 1

   Press Start to run the motor forward

   The operator presses the Start button on the front of the cabinet, and the motor turns in the forward direction (Forward) to grind the waste.

2. 2

   The current sensor detects jammed waste

   When waste jams and can no longer be ground, the motor current changes from normal, the front of the cabinet shows the Overload light, and the cabinet stops the motor.

3. 3

   Reverse to free the waste

   The cabinet commands the solid-state relay (SSR) and magnetic contactor to turn the motor in reverse (Reverse) to shift the jammed waste, then stops again before changing direction.

4. 4

   Resume grinding, or stop and wait for the operator

   The motor returns to running forward and resumes grinding. If it jams again, the cabinet repeats the same sequence. After 3 cycles, the motor stops and waits until the operator presses the Forward button on the cabinet.

5. 5

   Emergency stop and remote configuration

   The operator can press the emergency stop button at any time. System configuration is done through a remote connection via a Wireless Router.

What we designed and built

We carried out this work end to end, from system analysis through to installing the cabinet on the grinder's motor. The work is divided into hardware, software and documentation, as follows.

• Forward/Reverse motor control cabinet for a 30 hp motor, with cabinet drawings and electrical work
• Current sensing set: current sensor connected to an Arduino ATmega328 board
• Motor direction switching set: solid-state relay (SSR) and magnetic contactor
• Cabinet front: Start Forward, Stop and Run Reverse buttons, Manual/Off/Auto mode selector switch, status lights and emergency stop button
• Control board program, and a configuration screen accessed through a Wireless Router
• System testing and supporting documentation

What tests has this control cabinet passed?

This control cabinet passed program testing at our office in both Auto and Manual modes, and we installed the cabinet at the factory in 2020. The recorded results are the program test results for the 2 modes. No figures from continuous use on site were recorded, so this page does not report those figures.

What we learned

This work offers 3 principles for motor control cabinet jobs that must clear jams according to a sequence.

• The operating sequence and the number of times the cabinet may clear a jam by itself must be agreed in writing before programming. Once that number is reached, the cabinet must stop and wait for a person to decide, and must not continue trying by itself.
• The cabinet's installation location must be confirmed before manufacture and must be safe for maintenance. On this job, the installation area changed during the project, and the project records state that the installation area was not safe for maintenance.
• The scope of electrical and network (LAN) cabling on the factory side must be written clearly from the start of the work. On this job, the project records state that the LAN cabling scope was unclear.

How can you apply this approach to your grinder or shredder?

If your grinder or shredder jams often and the motor direction must be switched to free the material, you can use the same approach as this case: measure the motor current to detect the jam, then have the cabinet run a sequence of stopping, reversing and running forward for the agreed number of cycles. What needs adjusting depends on your motor, machine and installation area.

• Prepare the motor data from the nameplate, and record the symptoms that occur when the machine jams
• Agree in writing the operating sequence, the reversing time and the number of times the cabinet may clear a jam by itself before stopping to wait for a person
• Confirm the cabinet's installation location and the routes for electrical and LAN cabling before manufacture of the cabinet begins
• State clearly who prepares the area and runs the cabling on the factory's side