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Sizing brake resistors for critical applications

01 June 2009

In any application, dynamic brake resistors (DBRs) have to be 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 applications, and takes great care when specifying DBRs for its products

Street’s cranes work in three axes of motion. 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.

Chief electrical engineer, Colin Wild specifies Yaskawa inverters, supplied by Omron in the UK, and trusts his choice of DBRs to complement the Japanese firm’s equipment perfectly. Indeed, an incorrectly specified DBR can damage associated equipment, such as the inverter drive, if it fails.

Mr 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. Street provides power, duty cycle and resistance specifications to its supplier (in this case, Cressall) and, in return, is supplied a complete product appropriately mounted.

Once it has these specifications, Cressall uses its own modelling software to simulate the duty cycle of the crane and obtain an accurate sizing for the DBR. This procedure identifies the smallest safe size of resistor suitable for the application, saving the expense of over-sizing and ensuring that the DBR occupies the smallest possible footprint. One of Cressall’s strengths is its willingness to adapt enclosure sizes to suit the application, as Mr Wild explains.

“We need completely different sizes of enclosure depending on where on the crane we put the controls. Furthermore, in the crane industry, putting equipment into the roof space can impose limitations on our design. However, customers always want the biggest lift capacity to provide flexibility, so this is something we have to cope with.” Cressall director, Peter Duncan takes this need for flexibility very seriously.

“We pride ourselves on our ability to meet the customer’s requirements, even if those requirements are unusual,” he says. “For Street Crane, we often have to provide long thin enclosures to fit at the very top of the crane, near the roof of the site, allowing for a larger working space. We are always happy to do this.”

The need for a flexible DBR supplier is illustrated by another branch of Street Crane’s business. The firm also manufactures discrete components, which are often exported for assembly by overseas customers. While normally this poses few challenges for the DBR manufacturer, a component sold to a Canadian customer will have to take higher mains voltages into account.

“We opt for additional insulation in these cases,” explains Mr Duncan. “This isn’t always simple because the enclosures also have to be adequately ventilated but we always come up with a solution.” This facility to just hand over a problem and trust that it will come back solved is a key selling point as far as Mr Wild is concerned.

“As well as the flexibility and the good service, the products that have been provided have always represented an economical solution. They do well with delivery and even if they produce a complete custom design it never takes longer than a few weeks to supply. Furthermore, they always give us alternative options that can be delivered more quickly in case there is flexibility in our own selection. We think it’s essential that our suppliers reflect our own brand values in this way.”

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