What Support Products Does it Take to Maximize the Impact of Using VFDs and VSDs? - Part 1

Part 1 of this article series looks at what to consider when selecting motor connection cables, output reactors, braking resistors, line reactors and line filters. Part 2 continues by looking at the differences between VSDs/VFDs and servo drives, reviewing uses for AC and DC rotary and linear servo motors, considering where soft start-stop units fit into industrial operations, and Looking at how DC converters are used to power peripherals like sensors, human-machine interfaces (HMIs), and safety devices.

Using variable speed drives and variable frequency drives (VSDs/VFDs) is necessary to maximize industrial operations' efficiency and sustainability, but it’s not sufficient. To get the maximum benefit from VSDs/VFDs, additional components like high-performance cables, braking resistors, line filters, line reactors, output reactors, and more are needed.

Cabling is ubiquitous and critical. A poorly specified cable connecting the VSD/VFD to the motor can significantly degrade system performance. Other elements like braking resistors, filters, and reactors vary from installation to installation and can be very important to a successful deployment.

For example, some systems operate in areas where it’s necessary to control electromagnetic interference (EMI) and can benefit from using line filters that meet EN 61800-3 Category C2. Applications where rapid deceleration is required will need braking resistors. Line reactors can improve the power factor and boost efficiency, and output reactors can enable the use of longer cables.

This article begins with a look at some considerations when selecting motor connection cables and presents typical cabling options from LAPP and Belden. It then reviews factors that impact the selection of output reactors, braking resistors, line reactors, and line filters, including representative devices from ABB, Schneider Electric, Omron, Delta Electronics, Panasonic, and Siemens.

Motor cables are available in various configurations to meet specific application requirements. They typically have three main power conductors, often insulated with cross-linked polyethylene (XLPE). Some have uninsulated grounding wires. There can be various signal wires and numerous braided and foil shielding choices. The entire assembly is encased in an environmentally rugged outer jacket (Figure 1).

Figure 1: VFD motor cables come in a wide range of configurations. (Image source: Belden)

Even basic cables like Belden Basics part number 29521C 0105000 are complex assemblies of conductors, shielding, and insulation. These cables have three 14 American Wire Gauge (AWG) (7x22 strands) copper conductors covered with XLPE insulation and three 18 AWG (7x26 strands) uninsulated copper ground wires. These six wires are surrounded by dual helical tape shields that provide 100% coverage, and the entire cable assembly is encased in a polyvinyl chloride (PVC) jacket for environmental protection.

Belden Basic cables are suited for use in class 1 division 2 hazardous locations as defined in the National Electrical Code (NEC). Class 1 refers to facilities for handling flammable gases, vapors, and liquids. Division 2 specifies that these flammable materials are not ordinarily present in concentrations high enough to be ignitable.

Some cable series, like LAPP's ÖLFLEX VFD 1XL, are available with and without signal wires. Applications that benefit from having signal wires can turn to LAPP’s 701710 cable. It includes three power conductors, a ground conductor, and a pair of signal wires. The power conductors are 16 AWG (26x30 stranding) with XLPE (plus) insulation. The signal pair are individually foil shielded.

The entire assembly is shielded with barrier tape, triple-layer foil tape (100% coverage), and tinned copper braid (85% coverage). The outer jacket is a specially formulated thermoplastic elastomer (TPE) resistant to disinfecting solutions and is typically used in the food, beverage, chemical, and related industries.

In addition to reliably and efficiently handling power and signals, VFD cables need to be able to handle high voltage spikes and electromagnetic interference (EMI) noise levels resulting from the drive's high-frequency operation. While VFD cables are designed to contain and manage high voltage spikes and EMI, they have their limits (Figure 2). That’s when load reactors reduce high voltage spikes and EMI.

Figure 2: Uncontrolled high voltage spikes can pierce the insulation and result in cable failure. (Image source: LAPP)

For a more detailed discussion of VFD cable selection, see “Specifying and Using VFD Cables to Improve Reliability and Safety and Reduce Carbon Emissions.”

Load reactors

Load reactors, also called output reactors, are connected close to the drive's output to reduce the impact of high voltage spikes and EMI, and they protect wire insulation in both the cable and motor. VSDs/VFDs produce a high-frequency (usually between 16 and 20 kHz) output. The high-frequency switching results in voltage rise times of a few microseconds, causing high voltage spikes that can exceed the motor’s peak voltage rating, resulting in insulation breakdown.

Depending on the type of motor used, load reactors are often recommended if the VFD cable length exceeds 30 m (100 ft.). There are exceptions; for example, if the motor meets the NEMA MG-1 Part 31 standard, it may be possible to have a 90 m cable (300 ft) without using a load reactor.

Regardless of the motor type, a load reactor is generally recommended if the cable length exceeds 90 m. If the distance exceeds 150 m, a specially designed filter is usually recommended. In EMI-sensitive environments, using a load reactor for all applications is usually good practice.

Load reactors are often designed for use with specific drive models. For example, the Omron model 3G3AX-RAO04600110-DE load reactor is rated for 11 A and 4.6 mH and designed for use with 400 V three-phase 5.5 kW motors driven by the company’s 3G3MX2-A4040-V1 VFD.

Braking resistors and thermal overloads

In addition to a load reactor, a braking resistor and thermal overload shutdown device can be essential additions to the output side of a VSD/VFD. Braking resistors enable maximum transient braking torque by absorbing the braking energy. Most braking resistors dissipate the energy, while some are used as part of a regenerative braking system that captures and recycles the energy.

Dissipative braking resistors are rated for specific applications. The Schneider Electric VW3A7755 8 Ω braking resistor can dissipate up to 25 kW, while the Delta Electronics BR300W100 100 Ω braking resistor is rated for 300 W.

Braking resistor applications are defined using a percentage of energy dissipation (ED%). The defined ED% ensures the resistor can effectively dissipate the heat generated during braking. ED% is defined relative to the peak dissipation, the braking interval (T1), and the overall cycle time (T0) in Figure 3.

Figure 3: Definition of percentage of energy dissipation (ED%). (Image source: Delta Electronics)

Depending on the severity of the braking, ED% is specified to ensure adequate time for the brake unit and brake resistor to dissipate the heat generated by braking. If the brake resistor heats up due to inadequate thermal dissipation, its resistance increases, reducing the current flow and the brake torque absorbed.

Braking resistors can be defined by various dissipation cycles like:

• Light braking, where the braking power is limited to 1.5 times the nominal torque (Tn) for 0.8 s every 40 s. Used with machines with limited inertia, like injection molding machines
• Medium braking, where the braking power is limited to 1.35 Tn for 4 s every 40 s. Used with machines with high inertia, like flywheel presses and industrial centrifuges
• Severe braking where the braking power is limited to 1.65 Tn for 6 s and Tn for 54 s every 120 s. Used with machines with very high inertia, often accompanied by vertical movement, like hoists and cranes

In addition to a braking resistor, most systems include a thermal overload unit connected to the brake resistor as a safety precaution, like the ABB Control TF65-33 thermal overload relay. The thermal overload unit protects the resistor and drive system from too frequent or too strong braking. When a thermal overload is detected, the drive is turned off. Turning off the braking function only could result in serious damage to the drive.

Protection on the drive input

Line reactors and filters on the drive input limit low-frequency harmonics and high-frequency EMI, respectively (Figure 4). Line reactors help reduce harmonic distortion of the AC input power caused by the drive circuitry. They can be especially useful in applications that must meet the requirements of IEEE-519, “Harmonic Control in Power Systems.” Line reactors also smooth out disturbances on the mains power like surges, spikes and transients, increasing operating reliability, and preventing overvoltage shutdowns.

Figure 4: Line filters limit high-frequency EMC, while line reactors limit low-frequency harmonics. (Image source: Siemens)

Examples of line reactors include the DV0P228 2 mH inductor rated for 8 A that’s part of the Minas family of three-phase drives and accessories from Panasonic and Siemens’ 6SL32030CE132AA0 2.5 mH inductor rated for drives up to 1.1 kW that draw up to 4 A of input current and operate from 3-phase 380 V_AC -10% to 480 V_AC +10% power.

Line filters

Line filters are required to support electromagnetic compatibility (EMC) and provide EMI protection in most applications. Depending on the specific environment, two classifications of EMI filters, Class A and Class B, are used in industrial and commercial (building) environments, respectively. Class B demands a higher level of filtering than Class A because commercial environments (offices, administration, etc.) generally include electronic systems that are more sensitive to EMI.

The relevant EMC standards include EN 55011, which details emissions limitations for industrial, scientific, and medical equipment, and IEC/EN 61800-3, which relates specifically to adjustable speed drives.

VFDs/VSDs are available with and without integrated line filters. If they have a filter, it may be Class A or Class B. Depending on the environment and installation factors like cabling lengths, even a drive with an integrated filter may require additional filtering. A drive rated for operation in Class A environments can also be used in Class B environments with the addition of an optional filter.

IEC/EN 61800-3 defines EMC requirements based on Environments and Categories. Residential buildings are defined as the First Environment, and industrial installations connected to the medium-voltage distribution network through their transformers are the Second Environment.

The four Categories defined in EN 61800-3 include:

• C1 for drive systems for rated voltages < 1000 V for unlimited use in the first environment
• C2 for stationary drive systems for rated voltages < 1000 V for use in the second environment and possible use in the first environment
• C3 for drive systems for rated voltages < 1000 V for exclusive use in the second environment
• C4 special requirements for drive systems for rated voltages ≥ 1000 V and rated currents ≥ 400 A in the second environment

Generic line filters are available, but like line reactors, line filters are often designed for use with specific drive families. For example, the VW3A4708 line filter from Schneider Electric is rated for 200 A (Figure 5). It’s designed for the company’s Altivar VSDs and Lexium servo drives. It’s rated for mains voltages from 200 V_AC to 480 V_AC and has a protection index of IP20. Its EN 61800-3 rating depends on the motor cable length:

• Category C1 using up to 50 m of shielded cable
• Category C2 using up to 150 m of shielded cable
• Category C3 using up to 300 m of shielded cable

Figure 5: 200 A line filter rated for mains voltages from 200 V_AC to 480 V_AC. (Image source: Schneider Electric)

Conclusion

VSDs and VFDs are important systems for maximizing the efficiency of industrial operations and minimizing greenhouse gas emissions. These drives require several support components to ensure effective and reliable installations that meet the relevant international standards, including VFD cables, output reactors, braking resistors, line reactors, and line filters.