How to Select Cables for Reliable VFD Motor Control and Sensor Feedback

Industrial automation and efforts to improve energy efficiency are increasing the use of variable-frequency drives (VFDs) in motor systems, such as conveyors, pumps, and industrial robots. Selecting cables for these motors is more complex than just choosing a wire gauge based on load current and an insulation rating for the operating voltage.

Modern VFD motor systems use switched-mode power electronics that generate pulse-width-modulated (PWM) drive signals with very fast edges. These fast transition times can exacerbate reflections caused by impedance mismatches between the cable and the motor terminals, leading to standing waves that raise voltage stress along the cable. Additionally, line-to-line and line-to-ground cable capacitance can influence drive performance and increase charging currents. Because VFD PWM signals are rich in high-frequency harmonics, motor cables must be effectively shielded to reduce electromagnetic interference (EMI).

This article offers a brief overview of VFDs and explores the challenges designers face when choosing VFD motor cables to ensure proper functionality, reliability, and safety. It then introduces VFD cables from LAPP and demonstrates how they can be used to deliver stable power and control signals while reducing EMI emissions and susceptibility in harsh environments.

Introduction to VFDs

Industrial automation requires reliable, efficient motors capable of operating in either direction across a full speed range. VFDs, sometimes called adjustable-speed drives, are motor controllers that regulate the speed and torque of an AC induction motor (ACIM) by varying the motor's power input frequency, voltage, and duty cycle. They operate by rectifying the AC input and using the DC output to generate PWM signals to drive the motor. By adjusting the frequency, width, and amplitude of these pulses, the motor's speed and output torque can be controlled across a wide range of motor-driven systems.

To fulfill its function, a VFD consists of three main components (Figure 1): a rectifier to change AC to DC, an inverter to turn DC into a PWM stream, and a VFD controller.

Figure 1: A VFD rectifies its AC input and uses DC to generate the PWM signals to control the motor’s speed and output torque. (Image source: Art Pini)

The controller monitors motor operation through sensors, including resolver/encoder feedback devices, tachometers, and temperature and vibration sensors, to manage key motor parameters.

The rectifier uses conventional diodes followed by a filter. The inverter employs power field-effect transistors (FETs) or insulated-gate bipolar transistors (IGBTs). These are driven with isolated high-voltage gate drivers managed by the VFD controller.

VFD operation differs from traditional three-phase AC operation in that the signals driving the motor are not sine waves but rather PWM pulses (Figure 2).

Figure 2: A VFD’s PWM pulses generate a sinusoidal current response in the motor windings. (Image source: LAPP)

The PWM signal frequency generally ranges from 2 kHz to 20 kHz. The inverter alternates connecting the motor to the positive and negative DC bus, as well as to the DC common voltage. The DC bus voltage is close to the peak AC mains voltage. The VFD PWM waveform applied creates a sinusoidal current response that controls the motor’s speed and torque.

The need for special cables to connect the VFD to the motor arises from the characteristics of the PWM waveform. This waveform, as a rectangular pulse, has a wide frequency spectrum rich in harmonics. VFD cables are designed to reduce radiation of these high-frequency signals. Additionally, to minimize switching losses in the inverter switches and maximize efficiency, the pulse transitions are made as fast as possible. This leads to pulse edges with high rates of voltage change (dV/dt). These features, combined with fast edges and high spectral content, can cause high levels of EMI. Fast edges can also produce transmission-line reflections at impedance changes along the cable. Reflections create standing waves in the cable, increasing the voltage along its length and requiring a higher voltage rating for the VFD cable.

Cable capacitance between metallic conductors is another concern. When the inverter switches connect the cable to the DC bus, a current surge occurs, charging the cable’s capacitances. This can raise the instantaneous current levels, potentially causing cable damage. This common-mode current may flow between phases or from a phase to ground. It can also find a ground return through the motor frame, passing through the motor’s bearings. Currents through the bearings can cause pitting on their surfaces, reducing motor life. These issues are generally seen with VFDs operating at higher voltages, with higher motor horsepower (HP) ratings, and longer run lengths.

As with all wire and cable, power can be lost due to current flowing through the cable’s DC resistance. Additionally, because of the broader spectral bandwidth of PWM signals, the cable’s resistance may increase because of the skin effect. These resistance effects change with cable length.

VFD cables directly address connection challenges

The LAPP ӦLFLEX VFD 2XL with Signal cable family (Figure 3) is designed for VFD service in industrial environments, fixed installations, and for applications that call for occasional flexing. They address many of the problems found in VFD service.

Figure 3: Shown are two views of a typical ӦLFLEX VFD 2XL with Signal cable that illustrate the key design features related to VFD applications. (Image source: Art Pini, based on material from LAPP)

The most fundamental feature is the structure of the power conductors. Cable stranding influences the cable's flexibility and current-carrying capacity. This LAPP VFD cable family meets North American and European Class 5 cable stranding standards. Class 5 conductors consist of multiple, very thin, tinned copper wires arranged to create highly flexible cables. The circular mil area (CMA) can exceed that of the equivalent American Wire Gauge (AWG) sizes. This leads to lower DC resistance, a smaller voltage drop across the cable, and reduced power loss. The signal wire pair has a smaller diameter and conforms to Class K stranding.

Each cable includes three black insulated power wires marked with a printed phase label, a green/yellow striped ground wire, and a shielded twisted pair signal cable with two conductors.

The wires inside the cable are individually insulated with cross-linked polyethylene (XLPE), a thermoset plastic that resists heat, moisture, and chemicals. XLPE has excellent thermomechanical properties that let it withstand the heat from overcurrent conditions. It also has a lower dielectric constant, which reduces cable capacitance and helps minimize charging and common-mode currents. Additionally, the lower dielectric constant allows for closer spacing between insulated conductors, decreasing the cable's diameter while increasing the maximum operating voltage.

The outer jacket of the cable is made from a specially formulated thermoplastic elastomer (TPE). TPE is a flexible and durable material that combines the qualities of both plastic and rubber. It offers excellent resistance to heat, oil, chemicals, ultraviolet light, and ozone, making it suitable for industrial settings.

The ӦLFLEX VFD 2XL family features metallic shielding to minimize radiated EMI. The primary shield is a tri-laminate foil tape that provides 100% coverage. The secondary shield consists of a tinned copper braid with 85% coverage. A barrier tape shields the insulated core wires beneath the shield layers. When properly grounded, these shields offer EMI protection by preventing external interference from entering the cable and reducing radiation from the cable itself. A shield-ground drain wire runs along the entire length of the cable, offering flexible grounding options.

The twisted signal pair with a smaller wire gauge is used for control or sensor connections, such as brake control or temperature sensors. The signal pair is also shielded with foil tape and has its own drain wire.

Selecting LAPP ӦLFLEX VFD cables

The overall performance of a VFD motor system largely depends on choosing the correct VFD cable. The ӦLFLEX 2XL VFD with Signal family offers a variety of wire gauges to suit different motor sizes. It includes cables from 16 AWG (1.5 mm²) to 2 AWG (33.7 mm²), with intermediate gauges of 14 AWG, 12 AWG, 10 AWG, 8 AWG, 6 AWG, and 4 AWG. These cables feature four power conductors (three phase lines and one ground line) and a two-wire twisted-pair signal cable. All cables are rated to handle up to 2,000 V_AC rms, in accordance with the Underwriters Laboratories (UL) Tray Cable standard. The appropriate wire gauge depends on the motor HP, which relates to the full-load current (FLC), as well as the run length and the acceptable voltage drop along the cable (Figure 4).

Figure 4: Shown is the VFD cable wire gauge required for a specific motor HP. (Image source: LAPP)

Wire sizes are listed as AWG or as area in thousands of circular mils (KCMIL). Circular mils are used for wire gauges above 0 AWG.

For example, consider the LAPP 700710, a VFD cable with four 16 AWG power conductors and two 18 AWG signal wires. It is the smallest VFD cable in the ӦLFLEX VFD 2XL family, with a diameter of 0.652" (16.6 mm). The cable diameter determines the minimum bend radius, which is 7.5 times the cable diameter, or 4.9" (124 mm). These cables also specify an approximate weight of 200 pounds (lb) per thousand feet. The cable weight is important when designing support structures, such as cable trays. According to the chart, this cable can be used for motors in the ½ HP to 2 HP range for all three line voltages. It can also be used with 3 HP to 5 HP motors on 460 V and 575 V lines.

The LAPP 700713 is a six-conductor (four 10 AWG, two 18 AWG) VFD cable with a diameter of 0.798" (20.3 mm). It is suitable for 15 HP to 20 HP motors operating at 460 V, 20 HP motors at 575 V, and 7½ HP to 10 HP motors at 230 V.

The largest cable in the series is the LAPP 700717, a six-conductor (four 2 AWG, two 14 AWG) VFD cable. It has a diameter of 1.4" (35.6 mm) and weighs 1580 lb per thousand feet. It is compatible with a 50 HP motor operating at 230 V, a 100 HP motor at 460 V, or a 125 HP motor at 575 V.

Conclusion

As VFD deployments accelerate, designers must carefully choose the appropriate connection cable to ensure project success. The LAPP ӦLFLEX VFD 2XL with Signal family of cables supports a broad range of VFD drive and motor applications. Its multi-shield design guarantees reliable performance in noisy industrial environments, while its durable industrial-grade outer jacket resists water, oil, and harsh chemicals.