Sizing brake resistors for overhead cranes and hoists
When an engineering firm has built a reputation for reliability, flexibility, quality and durable technology, as Chapel-en-le-Frith based Street Crane has, that reputation must be guarded at all costs. As a result, when Street Crane chose a supplier of Dynamic Brake Resistors (DBRs) it turned to UK firm TPR Resistors. The key was finding a partner who could both reflect its own brand values and provide advice in accurately sizing resistors for Street Crane’s wide range of applications. Since the two companies began working together, TPR has acquired Cressall Resistors, a step forward which has further improved these skills and qualities. As a result of the move, both firms are now known as Cressall Resistors.
The DBRs in any application have to be highly reliable. However, this is particularly true in a high intensity manufacturing environment where a breakdown could prove extremely expensive in lost production. In the sixty years since it was established, Street Crane has become a trusted volume manufacturer of overhead cranes and hoists for exactly these kinds of mission critical environments. The company’s products range from factory cranes and portal cranes to electric wire rope hoists, electric chain hoists, jib cranes and mechanical handling equipment. The cranes find applications in areas such as steel handling, machine loading, stock moving, magnet cranes, and moving concrete sections. They can be found anywhere from a paper or steel mill to a power station.
“Our cranes work in three axes of motion,” explained Colin Wild, Street Crane’s chief electrical engineer. “There is the hoist motion for lifting, the cross travel function, which is often referred to as the crab because of its sideways movement, and the travel motion for moving backwards and forwards. All of these use inverter dynamic braking and DBRs as part of the overall control system.”
Wild specifies Yaskawa inverters, supplied by OMRON in the UK, and trusts his choice of DBRs to perfectly complement the Japanese firm’s equipment. Indeed, an incorrectly specified DBR can damage associated equipment, such as the inverter drive, if it fails. Wild specifies resistors in several formats according to where on the crane the control system is going to sit. The first format is stand-alone for applications where the control of all axes of motion is built into a single enclosure. However, if the control of each axis is spread around the crane in a diverse control system, the DBRs for each axis are supplied separately in discrete enclosures.
When the DBR is specified, Street Crane provides power, duty cycle and resistance specifications and in return is supplied a complete product which is mounted in an appropriate enclosure.
When supplied with these specifications, Cressall uses its sophisticated modelling software to simulate the actual duty cycle of the crane and accurately size the DBR. This allows the firm to offer the smallest safe size of resistor suitable for the application. As a result, Street Crane is provided with the most cost effective solution, occupying the smallest possible footprint.
Just as the DBR itself can vary according to the application, so the enclosures take a wide variety of shapes. Indeed, Wild believes that one of the strengths of his supplier is the company’s willingness to adapt enclosure sizes to suit the application. “We need completely different sizes of enclosure depending on where on the crane we put the controls,” he explained. “Furthermore, in the crane industry, putting equipment into roof space can create limitations on our design. However, customers always want the biggest lift capacity to provide flexibility, so this is something we have to cope with.”