Optimum operational performance and safety are absolutely critical in industrial applications involving mission-critical areas. To achieve these goals, the industry seeks new ways to enable more flexibility, prevent common causes of system downtime and reduce costs.
One area where innovation continues to make great strides is in variable frequency drives (VFDs), also known as adjustable speed drives or variable speed drives. A VFD is a device that regulates motor speed by controlling the frequency and voltage of the power delivered to a motor. Depending on the motor’s load, the VFD allows the motor to modify its RPMs to match the load.
VFDs have become very popular across many industries and in many applications. For example, they are widely used in fans and pumps. This is due to the various benefits VFDs can provide, specifically around energy, wear and tear, and precise process control. By controlling a motor’s speed electrically through a VFD – instead of by mechanical means – manufacturers can achieve the operational efficiencies and flexibility they need.
Though it is still common for people to use thermoplastic, high heat-resistant, nylon-coated (THHN) wire, more commonly known as “building wire”, there are many application issues associated with this type of wiring. Fortunately, the use of a dedicated and purpose-designed VFD cable is becoming increasingly common. THHN wire is no longer the default choice. With the proper cable selection and installation methods, systems can maximise uptime without fear of failures due to low-quality wire or cable – and can extend the life of motors and other devices.
Since there is currently no standard to govern VFD cables, it is important to understand the key design elements of the VFD system. This article will describe the four most common challenges facing VFD applications today, as well as what drive manufacturers and specifying engineers should look for when selecting a VFD cable to avoid these issues.
Four common challenges in VFD applications
VFD applications face unique challenges when managing electrical current and other factors. Without proper planning, these applications can experience many issues, including disrupted network communication, inaccurate process performance, power loss, motor failure and cable destruction.
Common mode current
Common mode current, also known as current noise, is current that is induced in paths outside the primary motor phase conductors and returns to the drive through any ground path. This noise can compromise the surrounding systems, resulting in system downtime or erratic network communication. The errors and anomalies it can cause are often hard to diagnose and fix. It can even destroy the motor bearings and cause premature motor failure.
For example, an ice cream manufacturer upgrades the drive system on a large mixer. Even though the drive system performs as expected, over time the manufacturer might begin to notice deviations in the inventory of the ingredients. In this scenario, imagine the drive system is immediately adjacent to the drive on the first floor, with the motor wired by a short length of conduit and flexible conduit finishing at the motor junction box. The batch system for the ingredients is on the floor above. Despite the conduit, common mode current causes the batch scales to misread, and dump the incorrect blend into the mixer. Replacing the motor leads with VFD cable would effectively eliminate the scale error and reestablish the process quality.
A VFD cable’s role is to provide the best path for potentially harmful currents to return to the drive, preventing or mitigating a variety of potential system issues. A “real” VFD cable is designed with effective grounding to keep current within the cable and minimise the ground path flowing outside of the cable, thus, eliminating the disturbance to surrounding equipment, networks and/or instrumentation. The main way to prevent common mode current is to use VFD cabling with high amounts of copper content and surface area. More copper will more effectively keep all electrical noise within the cable. Cables with the equivalent of three full-size phase conductors – 300% ground – in the ground system will be most effective in containing common mode noise. Many VFD cables are made with less than one full-size phase conductor of copper in the ground system, complying with the NEC standards, but not effectively protecting the system.
Systems with smaller drives tend to be proportionally more likely to see issues with common mode current. This is due to faster switching devices, larger drive-cable mismatch and the reduction of output filtering in more basic drive products.
Capacitive coupling, or cable charging, occurs when energy leaves a drive, and is displaced or lost – and ends up charging the conductor or interacting with other cables outside of the intended electrical network. When this happens, it reduces the delivery of power from the drive to the motor. This can result in either false tripping or loss of motor torque and efficiency. Even worse, this capacitive interaction can induce voltage in adjacent cables, leading to the presence of voltage on de-energised circuits, causing safety concerns and shock hazards.
The resulting power losses are directly related to the length of the cable and the interactivity between conductors and motor leads. The worst-case scenario occurs when multiple sets of motor leads with high capacitance are run together in conduit.
There are two main ways to reduce the impact of capacitive coupling and cable charging. One is effectively shielding the motor lead sets from each other to prevent capacitive interaction. The second is to achieve the lowest cable capacitance possible through proper VFD-grade insulation materials and thicknesses. Both of these methods minimise the negative effects.
Reflected wave voltage
Reflected wave voltage is a high-voltage electrical pulse that originates at the motor. This voltage pulse has the potential to damage the motor or motor cabling. This is a common problem in VFDs with longer motor leads (the distance between the drive and the motor). Induced voltages and system failures as a result of unsuitable cable selection create risk to both personnel safety and operational reliability. To minimise or mitigate reflected wave voltage, use cables with the lowest capacitance or least ability to store an electric charge. This effectively increases the cable distance required to reach full reflected wave voltage and reduces the damaging energy in the reflected waves.
While VFD cables can’t completely eliminate reflected waves, they are the safest and most robust solution available to manage the waves and make longer cable runs possible. To accomplish this, look for cables with the proper insulation thickness and material, such as VFD grade XLPE. This will reduce the stress put on the motor and VFD cable.
System reliability and safety
While there are many drive systems operating today with THHN in conduit, these systems are often causing issues that are not recognised. Wet THHN, for example, has a very low dielectric breakdown voltage.
Under some circumstances, the reflected wave voltage may exceed the breakdown level of the insulation. Typically, this failure mode will not be catastrophic, rather it will cause nuisance trips, such as ground faults, that may occur infrequently, but can be extremely difficult to trouble shoot.
In some cases, while the system may seem to function initially as it should, the accumulation of system noise and bad practices will lead to more and more anomalies and failures over time, compared to a properly cabled system. The system failures and induced voltages that can occur from reflected wave voltage, common mode current and capacitive coupling put operations and personnel safety at risk.
In order to keep systems operating reliably and keep workers and equipment safe, it is important to select a properly designed cable system and use products that can withstand harsh conditions and electrical challenges.
The benefits of VFD cabling
VFD cable is critical for maximising uptime and increasing the life cycle of motor systems. But, not all cables are VFD cables. Cabling in a VFD system must carry power from AC drive systems to AC motors. As a result, the cables must not only handle high power current, but also the high voltage that can occur.
In the past, typical cabling solutions have included unshielded tray cables, singleconductor THHN wire or continuously welded armored cable (CCW). Not only do these products require complex, costly installation and introduce potential reliability problems, they also have proven to be ineffective in handling common mode current (noise) and voltage spikes or protecting against capacitive coupling. Most, including CCW, contain only the minimum ground copper required to comply with the NEC standards.
THHN wire, for example, uses a thinner insulation and has an inflexible nylon coating. While it is inexpensive, THHN wire isn’t built to withstand the challenging conditions facing VFDs, which often create the noisiest signals in an entire plant. When faced with noise, high-voltage pulses and other factors, THHN wire insulation breaks down over time, not only destroying the cable, but ultimately causing system trips or failures. For these reasons, the manufacturers of motors and drives do not recommend using THHN – as it can have detrimental performance and reliability implications.
True VFD cabling, on the other hand, is specifically built to meet these demands. It includes the necessary conductor, insulation, grounding/shielding and cable jacketing to minimise risk and provide maximum safety and uptime. Systems using high-quality VFD cables can also benefit from:
- Less downtime: Using cables specifically designed for VFD applications will significantly reduce downtime from both cable failure and the adverse effects of poor cable selection. A reliable and safe cable choice in combination with proper installation helps manufacturers avoid the expensive downtime and costly troubleshooting associated with false trips, reduced torque, disruption to other control systems, motor failure and much more. VFD cabling offers better process stability, more system uptime and overall improved reliability and safety.
- Extended motor life: Only VFD cable is designed to protect the expensive and critical plant equipment by extending the life of motors. By running motors at the required speed for the load, instead of continuously at full speed – and being able to change speed instantaneously – the motors do not wear out as quickly. VFD cabling also protects against wear on the motor bearings and helps motors avoid overheating (due to cable charging losses).
- Cost savings: When comparing the cost of cables, THHN wire may appear cheaper per foot than VFD cables; however, users must also purchase and install conduit that greatly increases overall installation costs. VFD cables do not require conduit or the associated installation costs. In addition, if the design or location changes, VFD cables can easily be moved. Other cables would require ripping out existing conduit or buying new conduit altogether.
Less downtime, as mentioned previously, also leads to cost savings. If a drive or motor fails, for instance, the repair and labour costs alone could be 15 – 20 times the cost of the VFD cabling itself. These indirect effects can send total downtime costs soaring to hundreds of thousands, even millions of dollars.
With recent advancements in VFD cable technology, it’s now easier than ever to bend VFD cables around tight turns or corners and in motor junction boxes – places that were previously hard to reach or had to be “designed around.” This added flexibility is the direct result of a more bendable thermoplastic elastomer (TPE) jacket and finely stranded tinned copper conductors – that can now have over to 2000 individual stands in a single cable.
Flexible VFD cables increase the ease of installation without sacrificing any crucial performance elements. They can also be mounted directly on a motor or drive. VFD cables with a tray cable-exposed run (TC-ER) rating give installers additional flexibility to put cabling in trays without conduit and drop them right into the motor, reducing installation costs.
Cables suitable for use in NFPA 79 applications, which are ideal for applications in need of high flexibility, should also be a consideration.
Installation is also critical. Even the correct cable must also be properly installed to be effective. Without proper specification and installation, it’s possible to emit noise and affect other equipment and systems; bring the entire drive system down; or affect product quality. Improper termination, such as through the use of conductive cable glands, increases the potential for such effects.
Some VFD applications are for automated manufacturing, which requires a high level of intense flexing. Others are designed for motors that are in constant motion since they are mounted directly on the machine.
VFD applications continue to grow due to strict energy efficiency standards and the need to retrofit existing motors. And, while there is no official standard for VFD cables, there are critical differences between true VFD cabling and other cables marketed as such. Any manufacturer can classify any cable as a VFD cable. It is incumbent on the user to understand the construction requirements needed for a given application to select a suitable cable. Only some VFD cabling is properly designed to extend the life of motors, while also increasing application safety and reliability. Choosing these designs can minimise disruption to other control systems, offer more consistent performance and reduce cable failures to nearly zero.
Contact Greg Pokroy, Jaycor, Tel 021 447-4247, firstname.lastname@example.org