The right system components are critical to maximizing the energy-saving potential of VFDs
Cabling—like other components—must be vetted for its ability to withstand the harshest operating conditions.
By Peter Cox
Investment in VFD technology has proven time and again to pay off—often dramatically—not only in terms of reducing energy use and costs, but also in cutting carbon emissions and extending the service life of costly capital equipment. To ensure optimum performance, however, it is important to correctly specify and install the entire VFD system, including the cables.
Figure 1. These VFD cables are designed to meet different system requirements.
It is important to carefully match the motor and drive components based on the operating requirements, which typically include the load, torque, speed, and power. Cabling requires an equally thorough evaluation.
Matching the cable to the challenge
Cable selection plays a very important role in mitigating application and performance issues. A cable should never be the weak link in a VFD system; the cable selected must have the ability to withstand the harshest operating conditions in order to maintain the health of the other system components. In addition, noise emission is likely the single most significant problem associated with VFD systems today. Unless a proper cable shielding design is present to control it, noise emission from a drive system cable can disrupt plant and factory operations.
It is important to understand that there is no standard for what manufacturers call a “VFD cable.” The market carries a diverse mix of products with a variety of designs and performance levels. That said, there are four specific issues that need to be addressed to ensure a robust and reliable cabling solution:
- Common mode current (CMC)
- Capacitive coupling
- Reflected wave voltages
- Overall installation reliability.
Common mode current (CMC) is any current that flows between the drive and motor on any path other than the three primary motor leads. It is a function of inductive and capacitive coupling of the drive output waveform. Although VFD cable does not really change the magnitude of CMC, cabling that can contain the CMC and return it to the drive is important to the overall success of the system because it prevents this type of current noise (CMC) from interfering with other sensitive signals, instruments, or networks.
Capacitive coupling and cable charging currents are currents lost to interactions with other cables and cable systems. For long runs with smaller drives, cable charging currents can become a significant performance issue. In some pipe and wire systems where multiple motor lead sets are run together in a high-capacitance product such as thermoplastic high heat-resistant nylon-coated (THHN) insulation material, the cable charging currents rob the motors of torque. The results are false trips of drives and the potential safety hazard of induced voltage in other cable systems. Both lower capacitance and effective shielding act to reduce charging currents, resulting in more current delivered to the motor to do work.
Reflected wave voltages can cause in-service motor failures. They are mitigated by the right VFD cable in two ways. First, a properly designed VFD cable can reduce stress on motor insulation in many applications because it will have a lower capacitance (stored energy) and thus increase the critical distance required to generate the peak reflected wave voltages. Second, VFD cables themselves should be designed with an insulation system that can withstand the effects of the peak reflected waves. The wrong cable can allow voltage waves to reflect back from the motor toward the VFD, producing peak voltages of at least two times the drive bus voltage in the cable's conductors. There is the possibility of long-term cable damage under high voltage, and this problem is of even greater concern with VFDs operating at 575 V. Peak reflected wave voltages can be very close to the corona inception voltages for THHN. Age, moisture, or even stresses of installation can easily weaken THHN to the point where it can no longer withstand the reflected wave peaks, which then leads to cable failure.
High voltage levels in the cable can sometimes cause a corona discharge between the conductors. Corona discharge can damage the cable, motor, motor bearings, and drive, leading to system failure, production downtime, and costly repairs or component replacements.
Overall installation reliability will be addressed in more detail below. Needless to say, careful selection of VFD cable and sound installation practices will result in a system that is both safer and more reliable.
Attributes of a well-designed VFD cable
The spectrum of products described as VFD cables ranges from copper tape shielded building wire (construction grade VFD), conforming to minimum agency standards and with the smallest allowable ground conductor(s), to high-frequency IGBT-specific designs with braided shielded constructions and high-flex stranding with significant upgrades to the ground potential copper. This segment will discuss those attributes and how they influence the performance of the VFD system.
The objective of a VFD system is to save energy and reduce waste. As mentioned above, one of the liabilities of an improperly specified cable is not only that EMI can reach unacceptable levels, but also that current can radiate from the cable, causing machine inefficiency or other issues.
Proper shielding for noise immunity: The optimal cable design is not the same for all systems. In general, smaller drives are proportionally more problematic than larger ones; they can contain far less output filtering and common mode current control. They also tend to lack bus inductors, and they use much faster switching devices.
In addition, the grounds and shields in a VFD cable can be very active in conducting CMCs; they must be designed to deliver optimum performance. Smaller drives require a much larger percentage of copper at ground potential.
Noise radiated from a VFD cable is proportional to the amount of varying electric current within it, as well as cable length; more current and greater length mean more radiated noise. Belden's research suggests that shielding systems including copper tape and combination foil/braid types are the most appropriate for VFD applications, due to the low impedance path they provide for common-mode noise to return to the drive.
Foil/braided constructions offer the highest performance and flexibility with superior high-frequency noise conduction. Dual tapes, contra-helically layered, offer the next most effective conduction of high-frequency noise and currents. Single copper tape is the least attractive option; it tends to be less flexible than dual tapes and its smaller surface area reduces the ability to conduct high-frequency noise. Foil shields are not robust enough to restrict the volume of noise generated by VFDs.
Figure 2. This comparison illustrates how shield types conduct high-frequency noise and current.
Sturdy thermoset insulation for stable electrical performance: Industrial-grade XLP (XLPO or XLPE) insulation far surpasses PVC as an insulator for VFD cables. It provides a more stable electrical performance as well as a lower capacitance. Its dielectric constant is low, thus reducing voltage reflections from the motor back to the drive.
Lower cable capacitance also supports longer cable runs, reduces peak motor terminal voltages to extend motor life, and greatly reduces the likelihood of corona discharge. It also reduces the magnitude of standing waves and increases the efficiency of power transfer from drive to motor.
XLP insulation’s high impulse voltage breakdown rating significantly reduces the risk of failure in case of reflected wave voltage spikes resulting from cable-to-motor impedance mismatch. It allows engineers to more closely match the impedance of the drive to the motor to increase energy efficiency because it reduces reflected voltage, delivering more energy to be converted into useful rotational energy in the motor.
Thermoset insulation materials such as XLP reduce the likelihood of either the cable or the motor voltage reaching its corona inception voltage (CIV). A corona discharge produces extremely high temperatures, which can melt insulation materials such as PVC, causing premature cable burnout or a short circuit. Thermoset insulation materials do not melt.
There is a “2 kV Myth” arising from the historic failures in PVC/nylon. Many specifiers believe a 2000 V cable is required to reliably withstand the reflected wave voltages. Belden’s extensive research, coupled with experience in insulating materials, has proven that dielectric failures of properly constructed XLP 600 V VFD cable are not an issue. The lightest insulation available has a safety factor of at least 3 for corona inception voltages, as compared to the peaks obtainable. On the other hand, 600 V THHN cabling may eventually fail because peak reflected wave voltages can often exceed the corona inception voltages.
Correct grounding configuration and termination: A properly grounded VFD cable avoids the problem of uncontrolled current contaminating the ground plane and creating noise-related issues within the system. The recommended approach for grounding a cable is to use a suitable ground conductor, terminated both at the motor and at the drive. The shield surrounding the circuit conductors should be tied both physically and electrically to the insulated ground at the point where the cable enters the motor housing or drive proper. Introducing intermediate shield or ground connections, such as a bonding or conductive cable gland, will lead to the unintended release of CMC noise, often close to sensitive equipment. Grounding to an enclosure ground before grounding to the drive can also have the unintended consequence of releasing captured current noise. If the drive is mounted in an enclosure with other equipment, best practice is to leave the cable jacket in place until the cable enters the drive itself. Intermediate terminations can render the cable system ineffective in CMC containment, operating much the same as pipe and wire.
For smaller constructions, typically drives of 50 hp or less, Belden’s research indicates that having excess copper at ground potential is the most important factor in reducing the magnitude of CMC forced to return to the drive as noise in the ground plane. A cable with full-sized ground and a large drain and braid can have ground potential copper equivalent to as many as three circuit conductors. It is important to note that many cables described as “VFD” carry only the minimum grounds as required by the National Electrical Code (NEC): sometimes less than a single full-sized ground equivalent.
Figure 3. Noise impedance decreases with motor size.
As horsepower increases, cables become larger and carry more current. The relative size of the ground conductors within the cable can be reduced as the ratio of common mode to working current declines.
For motors above 50 hp, the internally induced ground currents begin to be of concern, and symmetric grounds become an important design consideration. In an asymmetric cable, conductor-to-ground spacing is not the same for all conductors, thus there is a net current flow in the ground. This current flow will lead to a potential difference between the motor and the drive, and result in current flow in the ground plane. This induced current has the potential to return to ground through the motor bearings and contribute to eventual failure.
For more information on proper termination, see Belden’s “Unarmored Variable Frequency Drive (VFD) Cable Termination Guide” at http://is.gd/termguide.
Appropriate stranding: VFD cables are available with two basic stranding constructions. Construction-grade VFD cables typically use Class B stranding, the same as used in commercial building wire. High-performance VFD cables use higher strand-count conductors that are frequently tinned. Tinned copper offers advantages over bare copper in terms of increased corrosion resistance and improved thermal stability. A tinned connection is much less likely to oxidize and degrade at hot spots.
High-performance VFD cables contain from four to eight times the surface area of construction-grade stranding. High strand-count conductors enhance cable flexibility and flex life in applications with motion or flexibility; in a VFD cable the high strand-count offers greater affinity for high-frequency circuits generated by the drive. This results in significantly more attractive return paths for the high-frequency noise currents and a significant reduction in the current forced to flow in the ground plane as harmful noise. It also results in reduced cable heating as the high-frequency resistance is much lower.
Industrial hardening: VFD cables must be reliable and rugged enough to handle the harsh industrial environments in which they are placed. It is important to choose industrial-grade cabling that can withstand humidity, grit, sunlight, oil and other conditions that can break down less robust materials.
Tested and certified: Because there are no standards for VFD drives, Belden recommends selecting VFD cables that have been tested and certified to fully comply with all industry certifications and safety standards appropriate for the specific VFD equipment, application, and/or installation site. Belden VFD cables, for example, are approved for use with ac drives by most leading drive manufacturers.
Sidebar: The case for VFDs as energy savers
Industry accounts for over 40% of worldwide energy consumption. Of this, more than 65% of industrial power demand comes from electric motor-driven systems. Reducing energy consumption in motor-driven systems by using variable frequency drives (VFDs) for motor control is most likely the “low-hanging fruit” for industrial energy conservation. Briefly, VFDs control the rotational speed of an ac electric motor by controlling the frequency of electrical power supplied to the motor. Although they have been available for more than two decades, today they are gaining increased popularity as industry looks for ways to control costs.
Older motor controls weren’t precise enough to support variable speeds, but technology has improved to the point where precision control for three-phase ac electric motors is available. Today, VFDs are used to alter the speed of a motor whenever the behavior of the motor’s load changes its demands on the motor. Motors driving devices such as pumps, fans, conveyor belts, or lathes can be adjusted as variables such as temperature, pressure, and force change.
Additional benefits from VFDs
VFDs make significant energy savings possible—for example, at half maximum speed using a pulse width modulation (PWM) VFD, a motor consumes roughly one quarter of the energy required to run at full speed. In addition, VFDs reduce wear-and-tear, resulting in lower maintenance costs and longer motor life. Because VFDs can control motor speeds to within 0.1% tolerance, they also contribute to less variation in the finished product and reduce material usage and scrap.
Technically, VFDs control the rotational speed of ac electric motors by controlling the frequency of the electrical power supplied to the motor. A series of capacitors, semiconductors (diodes and IGBTs), and an embedded computer chip allow the drive to moderate speed while still delivering the full torque of power to the motor. The drive not only varies the frequency, but can also regulate the voltage that is being sent to the motor.
Sidebar: Six steps for choosing the right VFD cable
With a good understanding of what makes an effective VFD cable, engineers can use this six-step checklist for selecting the right VFD cable for the job.
1. Select the most appropriate cable design. Consider only cables with cross-linked insulation suitable for the voltage demands and peaks. When the drive, cabling, or motor is close to sensitive equipment or networks, consider using a high-performance VFD cable with foil braid or dual copper tape shielding. If you are in a less sensitive environment where only reflected wave and capacitive coupling are concerns, consider a more economical construction grade product.
2. Match the cable ampacity to the motor full load amps (FLA). A higher motor horsepower translates to a higher current flow through the cable. Additional factors such as ambient temperature, altitude, or length of run may require additional derating. It is critical to follow the NEC guidelines for cable rating and applicable derating factors, and note that the cable ampacity must be a minimum of 125% of motor FLA.
3. Make sure the cable is able to support the voltage rating of the VFD itself. Enough said.
4. Make sure the cable is capable of providing a good seal if sealing is required. A round cable is the easiest to seal as it passes through circular openings and connection glands. Always choose the most suitable cable glands for the environment, and avoid the use of conductive glands as they will lead to unintended release of CMC noise.
5. Identify the possible impact of radiation on neighboring circuits. In noise-sensitive environments, cable with foil and braid shielding or dual copper tapes provides extra protection against EMI. Follow good practices for cable installation and avoid parallel runs in close proximity to networks and sensitive circuits. Keep the cable shields and jacket intact when routing them in enclosures with sensitive equipment or networks.
6. Determine the need for additional lines carrying signals between the drive and the motor. If signals are required (e.g., brake signals to slow or stop a motor if necessary), it can be more efficient to choose cable that has a signal pair integrally packaged inside the same outer jacket as the drive cable
One of the major advantages of a VFD system is its ability to save energy and reduce waste. Failing to match the cabling solution to the special needs of a VFD system may very well end up costing considerably more than choosing the right solution to begin with. Improper cable choices can lead to:
- Reliability and safety issues leading to increased troubleshooting and repair costs. For example, when current noise causes false trips of sensitive devices such as dual channel safety relays.
- Reduced equipment efficiency due to current leakage, which can also cause a safety hazard when service personnel experience electrical shocks or other surprises.
- Expensive downtime and lost operations due to premature cable failure, or worse, damage to the motor or drive.
Significant distinctions exist between cables described by the manufacturers as “VFD” cables. In addition, the correct cable must be properly installed to be effective. Investing in the right VFD cables and installation technique supports uptime and reliability of the VFD system as a whole, and protects the sensitive instrumentation and control systems adjacent to it.
Peter Cox is a project manager at Belden. He has worked in the cable, drives, and automation industries or as a consultant to those industries and markets for 27 years. He has extensive firsthand exposure to many of the common application issues in systems drives. He has a bachelor’s degree in engineering from Carleton University in Ottawa, Ontario, and is a licensed PE.
This article appeared in the February 2013 Industrial Energy Management special section to Control Engineering, CFE Media.