Dynamic braking resistors and elevator efficiency

26th March 2020
Alex Lynn

As cities continue to grow, it seems like the only way is up. Increasing urbanisation in recent decades has led construction companies to build upwards, with the construction of over 500 high-rise towers in the pipeline for the city of London alone.

Designing sky scraping buildings requires a number of considerations, but one important question is how people are going to get from top to bottom quickly and smoothly.

Here, Simone Bruckner, Managing Director at Cressall Resistors, explains how dynamic braking resistors (DBRs) help take elevators to the next level.

The rise of vertical cities, such as Hong Kong, Shanghai and New York, are driven by efficiency. A United Nations (UN) report predicts that 2.5 billion more people will be living in cities by 2050, requiring more buildings for both housing and businesses. A service that is essential in any tall building is elevators. Not only responsible for transporting people to every floor of a building, it is also imperative that lifts and elevators carry out this duty safely to avoid serious injury.

If you were to climb the stairs from the ground to the 18th floor of your office building, you’d have to move the weight of your body against the downward force of gravity. During the ascent, the energy you expend is mostly converted into potential energy, so climbing the stairs results in an increase in your potential energy. You have more potential energy at the top of the building than at the bottom, even if it doesn’t feel any different.

An elevator is simply a device that increases or decreases a person’s potential energy without the need to move. However, managing this energy in an elevator is a little more complex. The elevator car is balanced by a heavy counterweight that makes it easier for the motor to raise and lower the car. This counterweight means that the motor needs to use much less force when moving the car up and down.

Elevators also have electromagnetic brakes that engage when the car stops. The electromagnets keep the brakes in the open position so that they automatically clamp shut if the elevator loses power. Elevators also have automatic braking systems near the top and the bottom of the elevator shaft so that if the elevator car moves too far in either direction, the brake brings it to a stop.

Safety is a lift manufacturer’s biggest concern. Dynamic braking resistors (DBRs) play a huge part in providing this, as without them the lift wouldn’t slow down in the time determined by the drive. It is therefore critical that the system works every time, without fail. While a DBR may seem like a standard piece of equipment for an elevator, there are a number of variables that must be taken into consideration to ensure the installed system is safe and functionable.

Before providing the right resistor for the specification, Cressall first checks the energy per stop, the duty cycle and the ohmic value. The first two are typically considered as one variable — the power of the resistor. The energy per stop is the sum of the kinetic, rotational and potential energies, minus any frictional losses and any electrical losses in the motor or inverter system.

With these calculations made, engineers can determine the required DBR peak and average power, which in turn helps to ascertain the right DBR for the job.

Another consideration is the amount of space available for the DBR. Often, the size and position of the cable knock-outs on the resistor’s enclosure can be problematic due to the intricate and varied size required for each elevator. At Cressall, we can provide custom and adapted fitting to many of our products, meaning that you can find a product to your exact specification.

With our cities on the rise, it is only inevitable that we’ll be relying on elevators to help us reach new heights. While speed and efficiency are essential to delivering this, safety will be the most important consideration when manufacturing elevators and lifts for high rise buildings. With braking systems in place, selecting the right resistor to tackle excess braking energy will help things run without a hitch.

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