April 2019

38 n CONVEYORS AND MATERIAL HANDLING April 2019 www.drivesncontrols.com Protecting and controlling conveyors to keep deliveries on track C onveyor systems have been a key part of warehousing and logistics operations for decades. Traditionally, conveyor control systems have consisted of one or more large control panels, some local control panels, with the cabling for control, power and networking being wired directly back to the main panels. The main control panel would host the PLC, main controls, power distribution and circuit protection for the DC motors that drive the conveyor rollers. The net result was extensive cable runs distributing large DC currents over long distances. This was the archetypal way that conveyors were built to handle parcels. Today’s logistic centres face the challenge created by the high demand for products bought online and delivered on ever-shorter timescales. This means we need more and faster conveyor systems, often contained within the existing infrastructures of logistics centres. Typically, parcel conveyor lines use DC motor roller conveyors to move and sort the parcels efficiently. The modern approach to controlling the conveyors is to provide local DC power supplies. The challenge, however, is space. Automation engineers need a way to provide high DC current and protection close to the DC rollers. The new control architecture consists of a main control cabinet that houses the PLC and some controls. Local power distribution, control and circuit protection for the rollers’DC motors are located in a small panels below the conveyors. The choice of components for these local panels is critical due to the limited space available. Each DC motor roller typically needs 2A at 24V DC, so the more rollers that can be controlled locally, the better. This is dictated by the size of the DC power supply available. Power supplies with 40A outputs at 24V DC are common, followed by MCBs (miniature circuit breakers) and/or motor-rated MCBs and contactors to protect and control the DC motor rollers. This creates potential heat issues, with the temperatures inside panels reaching more than 60°C. This can reduce the lifespan of the components. A power supply with an efficiency of 90% or higher can help to reduce the power dissipated within the panel. Typically 40A, 24V DC power supplies are more than 120mmwide, while older supplies can be more than 270mmwide, taking up a lot of space in a 600mm control panel. But power supplies are now available that provide 40A, 24V DC in packages as narrow as 81mm. The aim is to maximise the number of DC motor rollers that can be controlled. These motors are inductive DC loads which, on start- up, draw large currents. If the labelled current on older supplies was exceeded, they would attempt to protect themselves by going into “crowbar”or“hiccup”mode, causing the supply to shut down. The latest power supplies have “power boost”functions that typically allow them to deliver 50% extra capacity for a specified time – for example, a 40A supply can deliver 60A for 5 seconds. This potentially allows more DC motor rollers to be used, depending on the inrush and continuous currents, cable lengths, and other factors. DC motor rollers are usually protected and controlled by breakers and contactors. The breakers are typically D type rated to cope with the inrush current of the motors, to prevent nuisance tripping. They typically need 10 to 15 times the rated current to protect the circuit, so if a 2A MCB is fitted, then a minimum fault current of 20A is required to trip the breaker. At full load, and even with the latest power supplies, the supply may shut down before the breaker trips, which is not safe. In a worst case, an overload could draw an additional fault current of 1A through the circuit. This could be on the furthest conveyor roller from the control panel, with a damaged cable creating a high impedance that would go undetected by the MCB and the power supply with potentially catastrophic results. To counter these issues, electronic fuses have been designed to support the new generation of power supplies and to give the correct level of protection. DC-rated electronic fuses covering 12, 24 and 48V DC systems, with selectable current ranges (typically 1– 10A), can detect fault currents of 1mA. Time characteristic curves are available that simulate the curves for MCBs to cover different load types – for example, type D for DC motor rollers. They can also provide status outputs and remote operation, with some models generating real-time data that can be made available on industrial networks. Electronic fuses are smaller than MCBs and can be switched remotely. More space can be created in the panel because a contactor may no longer be needed as the electronic fuse can be used to control the DC motor roller. n Conveyors systems that use DC motor rollers should ideally include protection circuitry close to the rollers. Phil Murby, marketing manager for control products at Lutze, describes a decentralised technology that provides such protection and offers other benefits. Modern power supplies for conveyor systems are smaller than previous generations and include functions such as“power boost”.