Potential installation issues can lie in wait for those who expect a new drive to work straight off the bat. These problems, which range from electrical to environmental, may harm drives and lead to premature replacement.
“Although modern drives tolerate a fair amount of abuse, there are various installation issues that could compromise the initial investment without mitigating action,” states Jonathan Smith, power & control field business leader at Rockwell Automation.
According to Smith, these issues - in order of potential impact - include: connecting the power in reverse; failing to implement best-practice EMC prevention measures; failing to separate power cables from communication cables; insufficient cooling; compromised air quality; supply line challenges; and harmonics.
“Mistakenly connecting a standard three-phase drive in reverse will cause it to try and function, but into a mains supply situation, which a pretty good way of breaking it,” says Smith. “Another occasional issue that occurs typically during commissioning is failing to establish an effective earth connection from an EMC [electromagnetic compatibility] viewpoint. This earth type must have low impedance at high frequencies to help sink electromagnetic noise. Furthermore, using single-core cables for connections - which tend to drive up impedance at high frequencies due to the so-called ‘skin’ effect - should be avoided.”
UNDER THE SKIN
In the skin effect, high frequencies run down the exterior surface of copper wire rather than through the single core. A multi-strand cable is therefore preferable. In addition, the cable arrangement and earth should ideally be star-based to avoid the generation of high-frequency currents in the earth circuit itself.
“Another area of potential concern involves communication cabling,” adds Smith. “Today, most drives are connected on digital networks, so it’s desirable to keep power cabling away from signal cables. This is not always possible, which is why using communication cable that’s fit for purpose is paramount. It is easy to pick up a cheap Ethernet cable online, but will it feature metal-shelled RJ45 connectors? Will it have multi-screens? Will it be insulated to 600V?”
Also advising against bundling power and communications cables together in long lengths is Martien Heesterbeek, industrial automation business group product manager at drive technology firm Delta Electronics EMEA. “You really must keep them apart,” he says. “If it’s not possible, then ensure they only cross at 90°.”
He continues: “The issue of electromagnetic noise is application-dependent, but there are a number of best-practice approaches that can help minimise the problem. For instance, the cable between the motor and drive must be shielded, or housed in a metal ducting, with both ends connected to an earth termination that is divided in three.”
Plenty of environmental issues can also prove troublesome. For instance, installing a drive inside a panel without sufficient clearance around all sides can lead to cooling issues such as ‘hot spotting’ later down the line. Compromised air quality is cited as another potential problem. Although installing a standard filter will likely overcome any problems with dust, there could be other airborne hazards, such as corrosive gases and conductive dust.
“When you introduce our drives to a cabinet, you need space above and below because air flow is required through the inverter itself,” points out Heesterbeek. “The required clearance will typically be around 150 mm, depending on the drive’s power rating.”
This principle also applies in decentralised or distributed configurations whereby an IP55-rated drive is mounted directly to the wall, although clearance here is usually easier to find than in a control cabinet. Drives such as Delta's CFP2000 series are housed in an IP55-rated enclosure to provide effective protection against water, dust and other particles. A built-in EMC filter and DC choke also feature.
According to Heesterbeek, modern drives are pretty robust and will withstand even high levels of humidity and dust, including corrosive gases such as hydrogen sulphide. “Conductive dust is of course a potential problem, depending on how much you have,” he says. “Filters must be replaced in a timely fashion, and it might even be necessary to clean the drive on a regular basis, possibly even inside, which may not be cost-effective for small inverters.”
Adds Smith at Rockwell: “If small particles of conductive dust land on the drive, they can give a false conduction path or an arc-flash event, which is dangerous,” says Smith. “A pressurised plant room provides the optimum solution, and while this might sound extreme, it’s a minor investment compared with the potential downtime costs that might otherwise occur.”
A further issue picked out by Rockwell Automation involves situations where supply line challenges are encountered due to nearby equipment switching on and off. “Transients develop because of arcing contactors or the switching of highly reactive loads,” says Smith. “This is best resolved at source, but another option is to use input chokes/line reactors that suppress the transient through the provision of high-frequency isolation to the drive.”
On the subject of harmonics, over 90% of drives sold are six-pulse types, where a simple diode bridge feeds into a primarily capacitive load. “Waveform analysis breaks this non-linear [non-sinusoidal] load down into various harmonic components that can start to distort the voltage waveform and, in turn, affect equipment such as drives,” says Smith. “Good practice is to avoid putting all non-linear load on one transformer. We have software that models the impact of fitting drives.”
For sites which have a number of large drives, the power rating can quickly ramp up. To mitigate this issue, engineers should consider an active front-end drive such as the recently-introduced Rockwell Automation Powerflex 755T. “The Powerflex 755T is effectively an inverter at both ends – with the second inverter bridge connected to the supply,” explains Smith. “By switching this in a particular manner the current is force-commutated into a sinusoidal pattern, which overcomes the issue of line-side harmonics in a single swoop.”
In terms of harmonics, most of Delta’s larger drives – above 30kW – feature an integrated DC reactor to help minimise harmonic distortion on the mains supply. Using a drive such as the CFP2000, THDi (total harmonic distortion) is reduced to less than 48% thanks to its DC choke. There is also the opportunity to introduce filters that will reduce THDi to less than 5%, if required, while an active front-end drive is another solution that will provide effective suppression of harmonics.
BOX: HELP, REMOTE CONTROL NOT WORKING
Blaise Ford, managing director at Ilkeston-based Inverter Drive Systems, a specialist in installing and commissioning ABB drives, gets regular calls from customers saying their variable speed drive will not run in remote mode. This concern is particularly evident shortly after drive commissioning, when there is still a level of unfamiliarity with its functions.
“It’s fairly common for customers to find their drive unresponsive in remote control mode, but the solution is very simple,” says Ford. “Due to an accidental button press, the inverter has likely been set to ‘local’ control. This means the inverter is looking to the keypad for its start, direction and speed signals, not to remote sources, such as digital inputs, analogue inputs or fieldbus.”
Users simply need to locate the ‘Loc Rem’ button on the ABB drive and press it, which will return it to remote control.
“It’s the most frequent problem reported to us,” states Ford. “There can of course be many ‘proper’ reasons why a drive is not working – seized load, lost signals and so on -- but the most usual is that the inverter has been put into local control.”