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How selecting the right dynamic braking resistor can keep things running smoothly

15 January 2019

Historically, slowing down a train required brakemen to climb on top of the carriages to manually apply braking systems. This sometimes resulted in the wheels and carriages catching on fire from the heat the breaking system generated. With engineering advancements, the responsibility of slowing the moving locomotive was transferred to mechanical systems, including dynamic braking resistors (DBRs). Here, Mark Templeman, sales engineer at power resistor specialist, Cressall, discusses the considerations when selecting a DBR.

DBRs perform two vital roles for electric motors and are used across a variety of industries, from rail and marine transport to lift systems. They produce braking torque and absorb the energy generated through rheostatic or regenerative processes. Rheostatic braking dissipates the breaking energy in the motor as heat in a resistor, whereas regenerative braking feeds back the electric power into the system.

On to the next level

DBRs are key in ensuring lift safety as without them, the lift mechanism wouldn’t slow down in the time determined by the drive. While a DBR may seem like a standard piece of elevator equipment, a number of variables must be considered to ensure the system is safe and functionable.

These variables include the speed and capacity of the lift, energy per stop, ohmic value and the regenerative energy produced by the motor. Calculations based around the energy per stop and the energy per stop plus the frequency can determine the required DBR peak and average power, which in turn can help to ascertain the right DBR for the job.

Across the seas

Electrical drive systems are commonly used on marine and offshore applications like ships, oil rigs and crane barges for a range of power applications including propellers, driving winches cranes, conveyors and cable laying. By using electric drives and DBRs, users can benefit from increased control, reliability, mechanical simplicity and significant weight savings.

However, for many marine and offshore applications, the range of power required for DBRs can vary from a few kW to hundreds of MW. Being aware of the power requirements for the application in question is important, as if the power passing through the DBR exceeds its rating, both the resistor and connected machinery could be damaged.

Cooling methods can also impact selection. While water, air and oil cooling systems are often present on ships and offshore structures, they can impose unacceptable loads on the system should the built-in system be exceeded or broken.

In addition to its bespoke DBR offering, Cressall has produced a series of water-cooled resistors specifically for marine applications that are ideal for low and medium voltage applications like winches and cranes, propulsion drives and dummy loads.

Consistent considerations

Aside from lift and marine applications, DBRs can be used across industries including electric vehicles, wind turbines and elevator doors. When considering a DBR for any application, consider these key factors:

• Voltage.

If the DBR’s voltage is too low you can encounter arching, jumping or tripping that will all result in inconsistent performance and potential system failure.

• Size and weight

The amount of space available can dictate the type of DBR you use. If you have a lot of space, opting for a naturally ventilated resistor removes the need for additional cooling.

• Regulations

Industry regulations can also affect the type of DBR you opt for. For instance, ATEX explosion proof regulations come into play for petroleum plants or gas fields, which can require more expensive, sealed enclosures.

Knowing your exact needs can help reduce overall costs. While having a DBR that is too small or not powerful enough for its application can have clear consequences, using a DBR that is too big can also lead to problems.
Speaking to a specialist resistor supplier, like Cressall, will not only provide you with exactly the right tool, it can also help you save money, costly repairs or disaster down the line.

Thankfully, these days there’s no need to employ staff to slow down trains, make the lift stop on the right floor, or keep marine vessel propellers turning. Simply choose the right DBR for your needs and you’ll be plain sailing, stepping off on the next floor or stopping at the station without a worry.

For more information, please contact:

Martin Nicholls
Cressall Resistors
Evington Valley Road
Tel:  +44 (0)1162733633
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