Variable Speed Drives:
The Dark Side
While labeling variable speed drives (VSDs) as the Jekyll and Hyde of the industry would be ex- treme, it is accurate to say that they possess a, widely unknown, darker side. Here, Joshua Piccaver, electrical design engineer at power resistor manufacturer, Cressall, explains how electrical engineers can combatVSDs’ darker side.
According to ABB, the power and automation company,the addition of a VSD can reduce energy consumption by asmuch as 60%. This means that, if a 90-k W motor in continuous operation is combined with a VSD, financial savings canamass to over £ 9,000 per year.
A VSD can help achieve these savings by better catering tothe needs of a specific application—we could refer to this asthe device’s positive Dr. Jekyll side. Traditionally, inductionmotors run at fixed speeds and are suited to applications thatrequire a constant motor output speed, such as in pumps orfans. Yet, sometimes, varying motor output speeds are preferable to meet the changing requirements of the load, such as infans, pumps, and precision tools.
Also known as a frequency converter or adjustable speeddrive, a VSD is able to control the speed and torque of the motor to better match the process requirements of the machine itis driving. It is the slowing down, when necessary, that helpsrecoup energy and costs that would otherwise go to waste.
RIDING THE WAVE
Of course, the bottom line of any plant manager’s ambitionsis to reduce costs and improve operational efficiency, and a VSDhelps to achieve just that. While a manufacturer should not bedissuaded from purchasing VSDs for use with electrical equipment, they must pay attention to an “unwanted ingredient” thatthe device might add to the power mix.
When existing equipment has to share its power networkwith connected add-ons, harmonics can become a problem.These harmonics are voltage or current waveforms that havea different frequency to that of the network and may causedevices to behave erratically.
The undesirable Mr. Hyde aspect of a VSD is that it can createthese 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 conse-
quences 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. Reductionis one obvious remedy, which involves the use of AC linereactors, known as chokes. These chokes are fitted either inside or outside the drive, to reduce the harmonics to a levelwhere they no longer cause serious issues.
However, the use of a large choke can have major size andcost drawbacks, which makes the solution unsuitable forsome applications. An AC choke also has a voltage drop thatimpacts the system.
FILTER THEM OUT
Harmonics caused by VSDs can be reduced to acceptablelevels by using passive filter circuits that consist of inductors,capacitors, and resistors. The filter circuit allows the fundamental frequency to pass through while diverting any harmonic frequencies to the resistor bank. Here, the frequenciesare dissipated as heat and are removed from the system.
The introduction of a dampening resistor can also offer anumber of benefits to the system. They include better filteringcharacteristics for higher frequencies, reduced amplificationat parallel resonance frequency, as well as higher filter lossesat the fundamental frequency.
Cressall builds discharge resistors that meet the stringentoperating conditions of customers such as Siemens, Areva,and also the National Grid Company, both in the UK and itscounterparts overseas. Cressall’s design expertise in the fieldis 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 ahigh surface area, which gives it excellent heat dissipation andmakes it ideal for continuous filtering duties.
The active material, insulators, and mountings on expandedmesh resistor elements maximize the use of convection to avoidhot spots and local overheating. However, as the elements arethin, the expanded mesh can bow when exposed to high levelsof 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 improvingthe shape of the expanded mesh, the company has been ableto 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 labelVSDs as being solely a Mr. Hyde “electrical circuit villain”. After all, the additional levels of performance flexibility that thedevices give to motors are essential—as are the resulting costsavings. However, to stop VSDs from drifting to the dark side,unwanted levels of harmonics must be tackled to allow for optimal performance.
For more, visit newequiment.com/21144988
By Joshua Piccaver
VSDs are rightly hailed as effective energy savers and help industrial applications to reduce their poweroutputs, but their impact on power quality is less often discussed.
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