Overcoming long cable run challenges

LONG CABLE runs are common in industrial enviroments. Systems are typically spread over wide areas, with motors often located far from VSDs, requiring connections back to centralised hubs or control centres.

Longer cable runs can act like antennas, making them more likely to pick up stray electromagnetic signals than shorter runs. Additionally, the increased cable length can introduce capacitance and resistance — storing and slowing down the flow of electricity and amplifying the effect of interference on the signal. These unwanted reflective waveforms impact system efficiency, increase power consumption and can lead to overheating.

Another consequence of these issues is unnecessary shutdowns in equipment like VSDs. This is otherwise known as a nuisance trip, where the system cuts out unexpectedly.

Cable run management

What can engineers do to avoid these issues? An answer is improving the management of longer cable runs. One method is to use shielded cables to reduce interference, while carefully routing them away from sources of noise. Another option is to keep cable lengths optimal, while ensuring they are supported and protected against mechanical stresses. Regular inspections and proper grounding are also key.

Reactors can provide another solution. Also known as motor chokes, reactors are large inductors that are installed at the output of a VSD. They absorb and smooth out electrical noise and interference that occurs as electrical signals travel along cables and, in doing so, ensure that the motor runs cooler and the system operates stably and efficiently. As a result, false overcurrent trips and cable overheating can be avoided.

Technidrive recommends reactors for cable runs over 80m in length when using a VSD. However, even shorter cable runs may require reactors if multiple motors are connected to a single VSD, as the cumulative length of cables can create electrical interference. Let’s look at an example of these technologies in use.

Noise absorbed

A food processing plant was experiencing frequent nuisance trips in its cooling system. Given that the cooling fans were essential for maintaining the right temperature for perishable items, the disruptions were a serious problem.

The food processor approached Technidrive to help solve the issue. The plant’s system involved multiple motors running from a single WEG CFW 11 VSD with long cable runs. However, electrical interference led the VSD to mistakenly detect overcurrent conditions. This caused the system to shut down unexpectedly and halt the cooling process.

To select the right reactor for this application, Technidrive’s specialists advised it was crucial to match the reactor’s current rating with the VSD’s maximum current. For instance, if the VSD is rated for a maximum current of ten amps, a reactor capable of handling this current should be chosen. This ensures the reactor can effectively absorb electrical noise and interference without being overstressed.

For the plant, Technidrive’s engineers recommended that a REO motor choke (CNW854) was installed at the output of the VSD. Its specifications include a voltage rise limitation of <200 volts per microsecond (V/µs), noise reduction capabilities, fully encapsulated design for protection up to IP 66. and the ability to support longer motor cable lengths without introducing additional interference. The result? The reactor absorbed the electrical noise and eliminated the reflective waveforms, preventing false trips.

David Strain is technical director at Technidrive

www.technidrive.co.uk