Factors to Consider in Cabling a Reliable VFD System

Factors to Consider in Cabling a Reliable VFD System


By Brian Shuman, RCDD,
Senior Product Development Engineer, Belden

A Variable Frequency Drive (VFD) regulates the speed of a 3-phase AC electric motor by controlling the frequency and voltage of the power it delivers to the motor. Today, these devices (also known as Adjustable Speed Drives or Variable Speed Drives) are becoming prevalent in a wide range of applications throughout the industry, from motion control applications to ventilation systems, from wastewater processing facilities to machining areas, and many others.
 


VFDs offer many benefits; principal among them the ability to save a substantial amount of energy during motor operation. In that sense, these devices represent both an attractive, “green” engineering solution, and an economical choice. Other benefits include the ability to:
  • maintain torque at levels to match the needs of the load
  • improve process control
  • reduce mechanical stress on 3-phase induction motors by providing a “soft start” 
  • improve an electrical system’s power factor
The way in which VFD-based systems are constructed and operated will have an impact on both the longevity and reliability of all the components of the system, as well as nearby or adjacent systems. This article focuses on the motor-supply cable in the VFD/motor system. It looks at some fundamental cable design considerations, and presents suggestions for installation.

Evaluation of Cable Types Used for VFDs

The most commonly recommended cables for VFD applications have been studied by Belden, in both a lab and a working application. (Reference #1) Some wiring methods were not examined, such as THHN building wire in conduit, since their use has been shown to have detrimental effects, as outlined in other studies. (References #2 and #3)

An exception to this exclusion was the use of PVC-Nylon insulated, PVC jacketed tray cables. These cables are the most commonly installed type of industrial control cable, and though they are often misapplied for use in VFD applications, they were included in the tests for purposes of comparison.

In the testing, the following five cable designs were evaluated: 
  • XLPE (cross-linked polyethylene) insulated, foil/braid (85%) shielded, Industrial PVC jacketed cable designed for VFD applications. (600V/1000V Rated)
  • XLPE insulated, dual-copper tape shielded, Industrial PVC jacketed cable designed for VFD applications (600V Rated)
  • XLPE insulated, continuously welded aluminum armored, Industrial PVC jacketed cable designed for VFD applications (600V MC Rating)
  • PVC-Nylon/PVC Type TC (unshielded)
  • PVC-Nylon/PVC Foil Shield Type TC
The cables investigated were used to interconnect a VFD to the AC motor. All testing was conducted using a current generation, IGBT-based, 480VAC, 5HP VFD, an inverter-duty rated AC motor, and relevant lab equipment, such as an LCR meter to characterize the cables and an oscilloscope to make voltage measurements.

Impact of Cable Design on Motor and Cable Life

Reflected waves caused by a cable-to-motor impedance mismatch are prevalent in all AC VFD applications. The magnitude of the problem depends on the length of the cable, the rise-time of the PWM (pulse width modulated) carrier wave, the voltage of the VFD, and the magnitude of the impedance difference between the motor and cable.

Under the right conditions, a pulse from the VFD can add to a pulse reflected back from the motor resulting in a doubling of voltage level, which could damage the cable or the components inside the drive. A solution is the use of an XLPE cable insulation, a material with high impulse voltage breakdown levels. This makes the system more immune to failure from reflected wave and voltage spikes in a VFD application than a PVC material which is not recommended in these applications.

The impedance of the cable relative to the motor will be the primary mechanism outlined in this article. This is done because cable length is mostly determined by the layout of the application, rise times vary with the VFD output semiconductor and the voltage of the VFD is determined by the application.

First, let’s look at estimated motor impedance relative to motor size in HP over a range of horsepower ratings, as indicated in Figure 2.



Note that the cable impedance for 1HP motor/drive combinations would need to be roughly 1,000 ohms to match the corresponding motor’s impedance. Unfortunately, a cable with such high characteristic impedance would require conductor spacing in excess of several feet. Obviously, this would be both impractical and very expensive.

In addition to other benefits, such as reduced capacitance, a more closely matched impedance can improve motor life. Table 1 lists the observed line-to-line peak motor terminal voltages, as well as the impedance of the cables under test. The voltage measurements were taken using 120 ft. cable lengths.

Table 1 shows typical impedance values for #12 AWG circuit conductors and is based on actual data. Cable impedance is influenced both by its geometry and materials used in its manufacture. The characteristic impedance of a cable is calculated using the following formula, where Zc = characteristic impedance, L = cable inductance, and C = cable capacitance: Zc = vL / C


Also in Table 1, note the inversely proportional relationship between the cable’s impedance and the peak motor terminal voltage: cables with higher impedance tended to result in lower peak motor terminal voltages. A cable’s design for impedance also impacts its useful life. Lower voltages across the motor terminals translate into the cable being exposed to lower voltages, increasing its life expectancy.

In addition, this reduces the likelihood of either the cable or the motor reaching its corona inception voltage (CIV). That’s the point at which the air gap between two conductors in the cable, or two windings on the motor, breaks down via arcing or a spark under the high potential difference. If the CIV is reached, insulation failure can occur in the windings of the motor. (See References, Number 3)

Corona discharge occurring between conductors of the cable can produce very high temperatures. If the insulation system of the cable is a thermoplastic material such as PVC, the phenomenon can cause premature cable burn-out or a short circuit due to a gradual, localized melting of the insulation. For this reason alone, thermoplastic insulations should not be used for VFD applications.

On the other hand, thermoset insulation systems such as those based on XLPE are ideal materials for these applications because of the high temperature stability they exhibit. In their case, the heat generated from corona forms a thermally-isolating charred layer on the surface of the insulation, preventing further degradation. All cables used for VFDs should use a thermoset insulation system as a precautionary measure.

Understanding and Mitigating Radiated Noise in VFD applications

Noise radiated from a VFD cable is proportional to the amount of varying electric current within it. As cable lengths grow, so does the magnitude of reflected voltage. This transient over voltage, combined with the high amplitudes of current associated with VFDs, creates a significant source of radiated noise. By shielding the VFD cable, the noise can be controlled.

In the tests presented in this paper, relative shielding effectiveness was observed by noting the magnitude of noise coupled to 10 ft. of parallel unshielded instrumentation cable for each VFD cable type examined. The results of the shielding effectiveness testing are documented in Figure 3.



As demonstrated by its trace in that figure, foil shields are simply not robust enough to capture the volume of noise generated by VFDs. Unshielded cables connected between a VFD and a motor can radiate noise in excess of 80V to unshielded communication wires/cables, and in excess of 10V to shielded instrumentation cables. Moreover, the use of unshielded cables in conduits should be limited, as the conduit is an uncontrolled path to ground for the noise it captures.

Any equipment in the vicinity of the conduit or conduit hangers may be subject to an injection of this captured, common-mode noise. Therefore, unshielded cables in conduit are also not a recommended method for connecting VFDs to motors.

If radiated noise is an issue in an existing VFD installation, care should be taken when routing instrumentation/control cables in the surrounding area. Maintain as much separation as possible between such cables and VFD cables/leads. A minimum of one foot separation for shielded instrumentation cables, and three feet for unshielded instrumentation cables, is recommended. If the cables must cross paths, try to minimize the amount of parallel runs, preferably crossing the instrument cable perpendicularly with the power/VFD cable.

If noise issues persist after these precautions are taken, use a non-metallic, vertical-tray flame rated fiber optic cable and media-converters or direct-connect fiber communication equipment for the instrumentation circuit. Other mitigation techniques may also be required, such as, but not limited to, use of band-pass filters/chokes, output reactors, motor terminators, and metallic barriers in cable trays or raceways.

Impact of Common Mode Noise in VFD Applications

Radiated noise from a VFD cable is a source of interference with adjacent systems that is often easier to identify and rectify than common mode noise. In the latter, high levels of noise across a broad frequency range, often from 60 Hz to 30 MHz, can capacitatively couple from the windings of the motor to the motor frame, and then to ground.

Common-mode noise can also capacitatively couple from unshielded motor leads in a conduit to ground via conduit ground straps, supports or other adjacent, unintentional grounding paths. This common-mode ground current is particularly troublesome because digital systems are susceptible to the high-frequency noise generated by VFDs.

Signals susceptible to common-mode noise include those from proximity sensors, and signals from thermocouples or encoders, as well as low-level communication signals in general. Because this type of noise takes the path of least resistance, it finds unpredictable grounding paths that become intermittent as humidity, temperature, and load change over time.

One way to control common-mode noise is to provide a known path to ground for noise captured at the motor’s frame. A low-impedance path, such as a properly designed cable ground/shield system, can provide the noise with an easier way to get back to the drive than using the building ground grid, steel, equipment, etc.

In the study presented here, tests were conducted on the five cable types to determine the ground path impedance of the shield and grounding system of each cable. The tests were conducted across a broad frequency spectrum. Results are outlined in Figure 4. Lower impedance implies a more robust ground path, and therefore relatively lower noise coupled to the building ground.
 
Lower building ground noise means a reduced need for troubleshooting of nearby adjacent systems and components.
 

 
 
 
Conclusion: Cable Selection Is Key to VFD Performance and Reliability

What this testing clearly illustrates is that a cable should never be the weak link in a VFD system. The cable must be able to stand up to the operating conditions, and maintain the life of other components in the system. Selecting an appropriate VFD cable can improve overall drive system longevity and reliability by mitigating the impact of reflected waves.

References:

1. Brandon L. Phillips and Eric J. Burlington, "Specifying Cables for VFD Applications," 2007
2. E. J. Bartolucci, B.H. Finke, “Cable Design for PWM Variable Speed AC Drives,” IEEE Petroleum and Chemical Industry Conference, Sept, 1998
3. E. Bulington, S. Abney, G. Skibinski, “Cable Alternatives of PWM AC Drive Applications,” IEEE Petroleum and Chemical Industry Conference, Sept, 1999
4. Evon, S., Kempke, D., Saunders, L., Skibinski, G., “Riding the Reflected wave - IGBT Drive Technology Demands New Motor and Cable Considerations,” IEEE Petroleum and Chemical Industry Conference, Sept, 1996

About the Author:
Brian Shuman, RCDD, is Senior Product Development Engineer for Belden, a leading provider of cabling, connectivity, switches, hardware and cable management solutions for a wide range of markets, including industrial, enterprise networking, broadcasting, audio/video, security and more.