Beyond the factory floor

BEYOND THE FACTORY FLOOR

POWER RESISTORS AREN’T ONLY FOR INDUSTRIAL PLANTS

While many of us probably have a vague idea of a resistor’s function, you most likely think of them as part of an industrial plant or large-scale operation. In reality, you’re never too far away from this essential power component. Mark Barfield, engineering and R&D manager, explores the range of applications for industrial resistors.

In electronic circuits, resistors are used to reduce current flow, adjust signal levels, divide voltages and handle unnecessary influxes of power. High-power resistors that can dissipate large quantities of electrical power as heat have uses as part of motor controls, in power distribution systems or as test loads for generators.

To anybody that doesn’t possess an in-depth and technical knowledge of a resistor’s function, it may be difficult to understand how these applications are important to everyday functions.

UP AND AWAY

While a DBR may seem like a standard piece of elevator equipment, its design demands a number of variables in order to keep the lift safe and functioning. Key considerations include calculating the energy per stop, the duty cycle and the ohmic value. Once these factors have been determined, the resistor manufacturer can determine the required DBR peak and average power in order to produce the right DBR for the job.

Dynamic braking resistors (DBRs) are an essential component in elevator operations, where speed control is essential. Without them, the elevator mechanism wouldn’t slow down in the time determined by the drive, risking the lives of its passengers. When elevators and lifts descend, there is excess potential energy that usually drives the lift’s motor in reverse, making it operate like an alternator. But an alternator is responsible for charging and powering electrics, such as in an automotive charging system. This is far from what we want an elevator’s motor to do — we definitely don’t want the carriage to speed up during its descent — so this excess energy must be dissipated safely so that the elevator doesn’t descend too quickly and cause harm.

ALL ABOARD

Stopping a train also requires the dissipation of a vast amount of energy. Conventional disc brakes alone suffer a lot of wear, so dynamic braking is often used as an additional braking system to absorb the high amounts of energy generated by stopping electric trains.

Railway braking resistors operate in the same way as those on elevators. However, electrified railways also benefit from regenerative braking, where the power produced during braking is either immediately reused by other locomotives or is stored for later use. This method is particularly beneficial for intensively used underground rail services, as the generated power can be immediately fed back into the next approaching train.

Crowbar resistors, such as those supplied by Cressall, are another resistor type commonly found track side. These resistors are used in traction power supply circuits to deal with the effects of transient or longer lasting over-voltage conditions. A soft crowbar pulses to dissipate transient over-voltages, then if these persist or worsen the main breakers are opened and the system is short circuited using a hard crowbar to absorb the stored energy.

POWER PROTECTION

Power cuts are an inconvenience to anyone, at almost any time of day. But there are some buildings that cannot afford even a couple of minutes of blackout time. Take the care industry, for example. If a hospital was to plunge into darkness, surgery would be suspended, life-sustaining equipment would cut off and vulnerable patients would be placed at risk.

As a result, every hospital has a standby power supply plan in place in case of a power cut, so that the building never has to go a second without. A battery-powered uninterruptible power supply (UPS) can instantaneously take over if the regular power supply fails. In addition, most hospitals also have a diesel generator that kicks in when there isn’t a power supply from the grid. However, our fortune that power cuts are a rarity in the Western world can also be the generator’s downfall — it never has the opportunity to prove its power.

To make sure hospital generators are able to operate during power cuts, their efficiency must be tested using a fixed load bank. The load bank allows the building manager to verify the performance of emergency backup generators without interrupting ordinary power operations by regularly running on sets of at least 25 per cent of the generator’s rated power for 10-20 minutes. Running on load uses up expensive fuel, so the appropriate load for routine testing is the lowest one for the shortest time that will ensure the diesel and its ancillaries are brought up to their full working temperature.

Cressall’s load banks for fixed installations are designed as a stage bolt-on addition to the generator set, requiring a space of only 40–800 millimeters (mm) between the radiator and the acoustic splitters, making them an easy addition during any initial generator set up. Load banks are now easier than ever to operate thanks to features such as touch screen controllers and ethernet connectivity.

While power resistors may seem as though they belong in large, industrial operations, it’s never too difficult to identify where they are required in everyday life. Without this important piece of electrical equipment, many of our services that require power in order to function simply wouldn’t be safe and usable.

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Variable speed drives: the dark side

VARIABLE SPEED DRIVES: THE DARK SIDE

While labelling variable speed drives (VSDs) as the Jekyll and Hyde of industry would be extreme, it is accurate to say that they possess a, widely unknown, darker side. VSDs are rightly hailed as effective energy savers and help industrial applications to reduce their power outputs, but their impact on power quality is less often discussed. Here’s how electrical engineers can combat VSDs’ darker side.

According to ABB, the power and automation company, the addition of a VSD can reduce energy consumption by as much as 60 per cent. This means that, if a 90-kilowatt (kW) motor in continuous operation is combined with a VSD, financial savings can amass to over £9,000 per year.

A VSD can help achieve these savings by better catering for the needs of a specific application — we could refer to this as the device’s positive Dr. Jekyll side. Traditionally, induction motors run at fixed speeds and are suited to applications that require a constant motor output speed, such as in pumps or fans. Yet, sometimes, varying motor output speeds are preferable to meet the changing requirements of the load, such as in fans, pumps and precision tools.

Also known as a frequency converter or adjustable speed drive, a VSD is able to control the speed and torque of the motor to better match the process requirements of the machine it is driving. It is the slowing down, when necessary, that helps recoup energy and costs that would otherwise go to waste.

RIDING THE WAVE

Of course, the bottom line of any plant manager’s ambitions is to reduce costs and improve operational efficiency, and a VSD helps to achieve just that. While a manufacturer should not be dissuaded from purchasing VSDs for use with electrical equipment, they must pay attention to an “unwanted ingredient” that the device might add to the power mix.

When existing equipment has to share its power network with connected add-ons, harmonics can become a problem. These harmonics are voltage or current waveforms that have a different frequency to that of the network, and may cause devices to behave erratically.

The undesirable Mr. Hyde aspect of a VSD is that it can create these harmonic currents due to the conversion of an incoming alternative current (AC) waveform to a direct current (DC) source, in order to create modulated pulses that control the AC motor. This back and forth, from AC to DC, results in current waveforms that are greater than the network frequency can handle.

As a result of the unwanted currents, cables may overheat which damages their insulation. Other unwanted consequences include that motors can be at risk of overheating and becoming noisy; circuit breakers may trip; meters can give false readings; or equipment might fail altogether.

CUT THE CURRENTS

To prevent these unwanted effects from occurring, manufacturers can implement a number of techniques. Reduction is one obvious remedy, which involves the use of AC line reactors, known as chokes. These chokes are fitted either inside or outside the drive, to reduce the harmonics to a level where they no longer cause serious issues.

However, the use of a large choke can have major size and cost drawbacks, which makes the solution unsuitable for some applications. An AC choke also has a voltage drop that impacts the system.

FILTER THEM OUT

Harmonics caused by VSDs can be reduced to acceptable levels by using passive filter circuits that consist of inductors, capacitators and resistors. The filter circuit allows the fundamental frequency to pass through while diverting any harmonic frequencies to the resistor bank. Here, the frequencies are dissipated as heat and are removed from the system.

The introduction of a dampening resistor can also offer a number of benefits to the system. They include better filtering characteristics for higher frequencies, reduced amplification at parallel resonance frequency, as well as higher filter losses at the fundamental frequency.

Cressall builds discharge resistors that meet the stringent operating conditions of customers such as Siemens, Areva and also the National Grid Company, both in the UK and its counterparts overseas. Cressall’s design expertise in the field is well-known, as a result.

Based on Cressall’s experiences within the industry, perhaps the most commonly used material in the design of harmonic filter resistors is expanded mesh. This material has a high surface area, which gives it excellent heat dissipation and makes it ideal for continuous filtering duties.

The active material, insulators and mountings on expanded mesh resistor elements maximise the use of convection to avoid hot spots and local overheating. However, as the elements are thin, expanded mesh can bow when exposed to high levels of heat, and this uncontrollable bowing can cause sparks.

To remedy this, Cressall has developed a technique that allows bowing to take place in the same direction. By improving the shape of expanded mesh, the company has been able to prevent this fault from occurring so that dampening resistors made from expanded mesh can filter VSD harmonics, without the risk of sparking.

Given their many advantages, it wouldn’t be right to label VSDs as being solely a Mr. Hyde “electrical circuit villain”. After all, the additional levels of performance flexibility that the devices give to motors are essential — as are the resulting cost savings. However, to stop VSDs from drifting to the dark side, unwanted levels of harmonics must be tackled to allow for optimal performance.

To learn more about Cressall’s harmonic filtering technologies, click here

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