Control and automation components from Eaton’s Electrical Sector formed the basis of the new Waste2O food digester produced by Mechline Developments
A programmable relay from Eaton’s easy range is at the heart of the Waste20 control system. It was chosen, in part, for the 8A switching capacity of its outputs. This is complemented by an Eaton M-MAX variable speed drive, with a single-phase input and a 3-phase output, and an Eaton ESR4 safety relay that is used to monitor the machine’s emergency stop circuits.
Closely resembling a top loading washing machine and only a little larger, Mechline’s Waste2O food digester helps solve the problem faced by all restaurants and food handling establishments of how to dispose of waste hygienically, inexpensively and with minimum environmental impact. Over the course of 24 hours, the digester converts up to 180kg of food waste into ‘grey’ water that can be easily and safely discharged into drain and sewer systems.
Waste food is put into the machine via a hatch, where it is mixed with a fluid containing bio-enzymatic bacteria. These bacteria, which are totally harmless, literally digest the waste, turning it into grey water with no solid content. During the digestion process, the mixture of waste food and digestion fluid requires intermittent agitation, and the mixture must also be kept at the correct temperature.
User needs
“Mechline wanted the operation of the machine to be fully automatic so that all the user has to do is tip in the food waste and press the start button,” said Philip Clarke, general manager of Electrical Manufactured Solutions (EMS), the designer and manufacturer of the Waste20 control system. “It was also important that the control system was small enough to fit within the body of the machine. To meet these requirements, we decided to base our design on an Eaton easy relay. These are very compact, straightforward to use and, because they are programmable, they allow changes to be made to the control system operation, either for fine tuning or for future upgrades, quickly and easily.”
The easy relay provides all of the interlocking and timing logic needed for the operation of the machine as well as controlling the water solenoid and dosing pump that regulates the flow of bio-enzymatic fluid into the digestion chamber.
The relay also controls the heating element that maintains the chamber at the correct temperature. Because it has outputs rated at 8A, the easy relay is able to switch the heating element directly, thereby eliminating the need to use an external interposing relay, which would have added to the cost, complexity and size of the control system.
Taking it to the MAX
To control the machine’s agitator motor, EMS chose a 0.25kW M-MAX inverter drive. This operates with a single-phase input, allowing the waste digester to be powered from an ordinary 13A mains socket, but provides a 3-phase output to the motor. This arrangement was chosen because 3-phase motors are, in general, less costly than single-phase types, and also because they are easier to reverse, which was an important requirement in this application.
The engineers at EMS were also able to make good use of the freely programmable outputs provided as standard in M-MAX drives. In this case, they programmed one of the outputs to respond to a motor current that is higher than normal, but less than the current needed for an overload trip.
This increased current indicates that too much waste food has been loaded, and the signal from the drive is used to stop the process and alert the operator. After the operator has removed the excess food, the machine restarts automatically when the access hatch is closed. This arrangement allows the machine to be restored to service quickly, without the investigation and checking that would be needed if the overload condition were allowed to reach the point where the drive tripped.
To provide the safety interlocking and emergency stop monitoring needed for the machine to meet the requirements for CE marking, the EMS engineers incorporated in their design an ESR5 safety relay. This is partnered with an RMQ-Titan emergency stop pushbutton, and position sensors to confirm that the access hatch is closed. Other Eaton control components used in the Waste2O food digester control system include FAZ miniature circuit breakers, and a T-series switch disconnector functioning as the main isolator.
To date, EMS has supplied Mechline with control systems for 50 Waste2O food digesters, and these are meeting all of Mechline’s requirements for ease of use and reliability.
