Building a reliable VFD system

A variable frequency drive (VFD) regulates the speed of a three-phase AC electric motor by controlling the frequency and voltage of the power it delivers to the motor. Today, these devices (also known as variable speed drives) are becoming prevalent in a wide range of applications throughout 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 worth mentioning include the following: they can maintain torque at levels to match the needs of the load, improve process control, reduce mechanical stress on three-phase induction motors by providing a ‘soft start’ and improve an electrical system’s power factor. What’s more, legacy systems that now use throttling devices to regulate motor speed can be retrofitted with VFDs to make speed regulation much more efficient and precise.

However, VFDs are not without drawbacks. For example, the very fast voltage rise times associated with IGBT technology contribute to precise motor speed control but can also lead to voltage spikes that damage cables of poor quality, or ones that are improperly insulated.

Figure 1: Schematic of a VFD set-up.

Other possible concerns with the use of VFDs are the potential for acoustic motor noise and motor heating when currents, induced by pulse width modulated switching, flow in improperly grounded motor shafts. The result can be damaged bearings. In addition, the purchase cost for a new VFD can be steep, though this must be balanced with the fact that the payback period can be a matter of just a few months to under three years.

Special consideration must be given to the proper installation and operation of the overall system that comprises the VFD, the motor it controls and the cable that connects them. See Figure 1 for a schematic of a generalised VFD system. 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.

Evaluation of cable types used for VFDs

In order to better understand the variables involved with the cables that are a key part of any VFD system and to formulate a useful guide to cable selection, the most commonly recommended cables for VFD applications have been studied by Belden, in both a lab and working application. The use of PVC-nylon insulated, PVC jacketed tray cables was also studied. 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. The PVC-nylon designs (PVC-nylon/PVC Type TC and PVC-nylon/PVC Foil Shield Type TC) were evaluated in both unshielded and foil shielded versions.

The commonly specified cables evaluated in the study included the following:

1. XLPE (cross-linked polyethylene) insulated, foil/braid (85%) shielded, industrial PVC jacketed cable designed for VFD applications
   • Four conductor (three conductors plus green/yellow ground)
   • XLPE insulation (1.15 mm wall) 100% foil + 85% tinned copper braid shield
   • Full-size tinned copper drain wire and full-size insulated tinned copper ground conductor
   • Industrial PVC jacket
   • 600 V/1000 V rated
2. XLPE-insulated, continuously welded aluminium armoured, industrial PVC jacketed cable designed for VFD applications
   • Three conductor #12 AWG
   • XPLE insulation (0.76 mm wall) with continuously welded aluminium armour and three symmetrical #16 AWG bare ground conductors
   • Industrial PVC jacket
   • 600 V MC rating
3. XPLE-insulated, dual-copper tape shielded, industrial PVC jacketed cable designed for VFD applications
   • Three conductor #12 AWG
   • XLPE insulation (0.76 mm wall) with two 0.05 mm copper tapes spiral wrapped with 20% overlap and three symmetrical #16 AWG bare ground conductors
   • Industrial PVC jacket
   • 600 V Rated
4. Nylon/PVC Foil Shield Type TC
5. PVC-Nylon/PVC Type TC

The cables investigated were used to interconnect a VFD to an AC motor. All testing was conducted using a current generation, IGBT-based, 480 VAC, 3.75 kW VFD, an inverter duty-rated AC motor and relevant lab equipment, such as an LCR meter to characterise 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 to result in a doubling of voltage level, which could damage the cable or the components inside the drive.

Because cable length is mostly determined by the layout of the application, while rise times vary with the VFD output semiconductor, and the voltage of the VFD is determined by the application, the impedance of the cable relative to the motor will be the primary mechanism outlined in this article. First, let’s look at estimated motor impedance relative to motor size in kW over a range of horsepower ratings, as indicated in Figure 2. Note that the cable impedance for 0.75 kW motor/drive combinations would need to be roughly 1000 Ω to match the corresponding motor’s impedance. Unfortunately, a cable with such high characteristic impedance would require conductor spacing in excess of 2 m. Obviously this would be both impractical and very expensive.

Figure 2: Motor impedance relative to motor size.

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 36.5 m cable lengths.

Table 1: Impedance impact on motor terminal voltage using 36.5 m of cable.

Cable type	Impedance	Voltage at motor terminals
Continuous aluminium armoured cable	87 Ω	1080 V
Belden foil/braid VFD cable 2950X Series	78 Ω	1110 V
Cu-tape shielded Belden VFD cable	58 Ω	1150 V
Unshielded PVC-Nylon/PVC	58 Ω	1150 V
Shielded PVC-Nylon/PVC	38 Ω	1260 V

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 Z_c = characteristic impedance, L = cable inductance, and C = cable capacitance.

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). If the CIV is reached, insulation failure can occur in the windings of the motor.

If the insulation system of the cable is a thermoplastic material such as PVC, the corona discharge can cause premature cable burnout or a short circuit due to a gradual, localised melting of the insulation.

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

Understanding radiated noise in VFD applications

Noise radiated from a VFD cable is proportional to the amount of varying electric current within it. As cable length grows, so does the magnitude of reflected voltage. This transient over-voltage, combined with the high amplitude 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 article, relative shielding effectiveness was observed by noting the magnitude of noise coupled to 3 m of parallel unshielded instrumentation cable for each VFD cable type examined. The results of the shielding effectiveness testing are documented in Figure 3.

Figure 3: Noise coupled from VFD cables to unshielded instrumentation cable.

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 80 V to unshielded communication cables, and in excess of 10 V 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 and control cables in the surrounding area. Maintain as much separation as possible between such cables and VFD cables. A minimum of 30 cm separation for shielded instrumentation cables, and 1 m for unshielded instrumentation cables, is recommended. If the cables must cross paths, try to minimise the amount of parallel runs, preferably crossing the instrument cable perpendicularly with the VFD cable.

If noise issues persist after these precautions are taken, use a non-metallic, vertical-tray flame rated fibre-optic cable and media converters or direct-connect fibre communication equipment for the instrumentation circuit. Other mitigation techniques may also be required, such as, but not limited to, the use of band-pass filters or 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 capacitively couple from the windings of the motor to the motor frame, and then to ground.

Common mode noise can also capacitively couple from unshielded motor leads in a conduit to ground via conduit ground straps, supports or other adjacent, unintentional grounding paths.

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.

Figure 4: Noise coupled from VFD cables to unshielded instrumentation cable.

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 shield system, can provide the noise with an easier way to get back to the drive than using the building ground grid, steel or equipment.
In the study presented in this article, 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

A cable should never be the weak link in a VFD system. It 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.

Cables employing a heavy wall of thermosetting insulation are recommended because of the proven electrical benefits and improved high-temperature stability it offers. Shielding systems, including copper tape, combination foil and braid, and continuous armour types, are the most appropriate for VFD applications because of the low impedance path they provide for common mode noise to return to the drive.

When VFD cables are installed in close proximity to low-level communications cables and other susceptible devices, shielded instrumentation cable should be used. It would also be prudent to limit the run length of VFD cable parallel to instrumentation cables to 3 m or less to reduce the likelihood of radiated noise issues.

Belden Australia Pty Ltd
www.belden.com.au