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Braking the mould in lift control

The Dynamic Brake Resistor (DBR) used in the control panel of a lift is a mission critical item – if not a glamorous one. Failure or inaccurate sizing would result in the lift not being able to stop in accordance with the rest of the control system. Although accidents of this kind are almost unheard of, it is essential for the panel builder to choose the right supplier. So, when Daventry-based Digital Lift Controls set about sourcing a partner, the decision to entrust the job to Cressall Resistors was not taken lightly.

Digital Lift Controls was established eight years ago by managing director Yan Phoenix and has since built up an enviable reputation among OEMs as the partner of choice for replacement lift control panels. The company both designs and manufactures control systems which are used in applications ranging from retail and healthcare to hotels and local authorities. End users of the company’s products, it ships nearly 700 panels a year, include Sainsbury’s, Boots and Marks & Spencer.

“We use Cressall’s Dynamic Braking Resistors in all of our panels,” explained Phoenix. “Although at first a DBR seems like a standard piece of equipment, there is quite a bit of work to be done for each application. We specify the operating parameters, its power and ohmic value, for each resistor according to its use and load. The factors we have to take into account include the speed and capacity of the lift and the regenerative energy produced by the motor. We use the energy per stop to determine the DBR’s peak power and the energy per stop plus the frequency to determine the DBR’s average power. We find that, typically, the slower the lift the more inefficient its operation is.”

When Digital Lift Controls receive an order, the firm first sends the details to the engineering team where the panel is designed and laid out. It is then kitted, fitted and wired before at least a 74 point safety check is conducted on the operation of the lift and the safety circuit is double checked. “Safety is absolutely our biggest concern every time,” Phoenix explained. “The DBRs are of course a big part of this. Without them the lift wouldn’t slow down in the time determined by the drive. It is absolutely critical that the system works every time and this reliability is what we provide, in conjunction with Cressall.”

After working with the customer on the specification, Digital Lift Controls passes on its requirements to Cressall where a DBR is designed and manufactured to meet them.

Before providing the right resistor for the speciation, Cressall first checks the energy per stop, the duty cycle and the ohmic value. The first two are normally considered as one variable – the power of the resistor. Cressall’s engineers then calculate the energy involved in each stop. This is the sum of the kinetic, rotational and potential energies, minus any frictional losses if these are significant, minus the electrical losses in the motor and/or inverter system. Because all the energy produced by the braking process is used in heating the resistor, Cressall needs to know the characteristics of the duty cycle before it can specify the right size for the DBR.

However, Phoenix claims that he doesn’t just work with Cressall for the company’s ability to specify accurately. Another reason is the company’s adaptability and willingness to be flexible in order to get the job done. “A good example of Cressall’s readiness to go the extra mile would be the size of the cable knock-outs on their enclosures,” he claims. “Hole sizes can be a real irritant for a panel builder but Cressall will punch them to the size we request. As a result, the DBRs come as a package that is easy to fit and integrate on to our enclosures. Because a resistor is a commodity product of sorts we are effectively buying an off the shelf item adapted to our specification.”

“It isn’t just the enclosures where they win out though. It might equally be that we need a different type of connector or something else as seemingly minor as that. However, it’s really useful to have a supplier able to help us meet these ‘small’ requirements every time. Our customers use us because we are flexible and we’ve made our equipment simple to use. I think Cressall reflect that philosophy as well,” continued Phoenix.

“Cressall’s range is quite wide, which in turn allows us to quote for a much wider range of applications. For instance, if the need is for a small but high power resistor for wall mounting or mounting in a very small enclosure we are now able to be extremely competitive,” Phoenix explained.

Digital Lift Controls claims that the replacement market for lift control panels is pretty buoyant, having been positively affected by a number of legislative changes over the last few years. For instance, the Disability Discrimination Act means that floor levelling in lifts now has to be much more accurate and as a result variable speed drives are now used much more often. It has also meant that the ‘car controls’ (the buttons you press to choose your floor) in the lift often need to be replaced. End users now demand more tactile buttons and a lot of companies have opted to lower the height of the car controls to make them easier to reach from a wheelchair. The cumulative effect has been more replacement lift panels and increased levels of business since the Act was introduced in October 2004.

Unusual applications have included wall climber lifts for the outside of buildings, which operate using a rack and pinion system. These lifts have their drive motor fitted on top of the car along with the brake and gearbox. The motor drives a pinion that, after engaging a toothed rack bolted to the lift mast, moves along the rack. This technology enables the lift car to climb up and down the mast at controlled speed. “These lifts produce much greater levels of energy, which the DBRs we use handle without any problem, thanks to the sizing work put in initially,” explained Phoenix.

“We definitively plan to continue buying from Cressall,” Phoenix concluded. “The customer service is good and they are always very responsive if we need something urgently. Occasionally, we will have to produce a panel at the last minute for a customer whose lift has broken down. This normally entails turning something around in 24 or 48 hours. Cressall will always make something in these conditions if they don’t have an item in stock. The result is that we never have to let our customers down. That kind of working relationship is the best you can ask for from a partner in your supply chain.”

Peter Duncan, deputy managing director of Cressall Resistors, said, “I’ve worked with Digital Lift Controls for a long time and during that period a bond of trust has developed between the two companies. As a result, it is a particularly satisfying application for us. I think the market that the company has found – replacing the mechanical controls in older lifts with digital controls – is one that looks sustainable for the foreseeable future.”

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