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17 May, 2022

High-power braking resistors overcome traditional problems

16 September, 2019

REO has developed a range of braking resistors that, it claims, overcome problems traditionally associated with using high-power resistors in drives cabinets. The REOHM series 155 braking resistors come in a closed, IP66-rated format that prevents the entry of dust and airborne materials. They are connected to the drive by a screened cable, thus ensuring that emissions are not an issue.

Historically, high-power wire-wound braking resistors have needed large housings that affect how they are integrated into drives cabinets. The compact new resistors are claimed to overcome this. They can deliver up to 3.5kW of continuous power to drives with medium- and high-power frequency converters, and isolation voltages to 4.4kV.

Although traditional wire-wound resistors are relatively cheap to produce, they pose three key risks: the wires can uncoil; the housings are open to ingress; and the resistors can cause electromagnetic interference (EMI).

“The failure mode of a traditional wire-wound resistor tends to be catastrophic – especially with units with little or no enamelling – and often results in an uncoiling of the wire,” explains REO UK managing director, Steve Hughes. “Often, the resistor will still have power on at this point, creating a live DC conductor that can damage the drive and blow fuses. And this is among the best-case scenarios.

“The nature of them being inherently open – much like old-fashioned heater elements – means that they must be adequately housed and segregated from other sensitive parts of equipment, often on top of control cabinets,” Hughes adds. “But this can lead to issues, especially if there is likely to be conductive or explosive dust or material in the air, as in a paper mill. Likewise, units housed externally often require cabling that can unintentionally emit high-frequency noise from the drive, which can manifest as distorted currents in the control cabinet and nearby electrical systems.

REO’s IP66-protected braking resistors overcome problems of using high-power resistors in drives cabinets

Hughes contends that with the development of the new resistors, “these problems can become a thing of the past”. The enclosed IP66-rated design prevents dust and airborne materials from entering the resistors, while the screened cables eliminate the risk of EMI.

The resistors are rated up to IP66, protecting them against dust ingress and jets of water. The resistors have also passed salt-mist testing, making them suitable for offshore applications.

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