AC Line Filter Selection for Allen-Bradley Kinetix Servo Drives

Overview: What an AC Line Filter Does on a Servo Drive

An AC line filter (also called a line-side EMI filter, RFI filter, or input filter) is a passive LC network installed between the AC mains supply and the drive's rectifier/ input stage. Its job is to block high-frequency conducted noise generated by the drive's switching action from propagating back into the facility power wiring, where it can disturb other sensitive equipment such as encoders, analog signal conditioners, communication nodes, and other VFDs.

For an Allen-Bradley Kinetix servo drive fed from a 460–480 V three-phase supply, the typical device in scope is the Bulletin 2090-UXLF-HV series line filter. The example referenced in this article is catalog number 2090-UXLF-HV323, rated 23 A continuous at 460 V, three-phase, designed for the Ultra-series Kinetix input stage. The 23 A rating is roughly 2× the 11 A drive rating, which is the standard derating philosophy Rockwell Automation publishes for these filters when used in switching-converter applications.

Input-Side vs. Output-Side Filtering

Servo system filtering must be split into two distinct functions, because they protect different things:

• Input (line-side) filter – Stops drive-generated EMI/RFI from exiting onto the plant AC bus. This is the 2090-UXLF-HV323 device in the application under discussion. It is the filter that addresses the question "will the drive disturb other equipment on the same feeder?"
• Output (load-side) filter – A dV/dt or sine filter placed between the drive and the motor. It slows the IGBT edge rate and reduces voltage reflection at the motor terminals, extending winding insulation life and lowering bearing currents. It is not an AC line filter in the sense the user is asking.

This article addresses only the input-side filter.

When an AC Line Filter Is Required

A line filter is required in any of the following situations:

1. CE / EMC compliance – Kinetix drives used in 400 V-class installations sold into the European Union must meet EN 61800-3 (Adjustable speed electrical power drive systems – EMC product standard). Conducted emissions on the AC port must be met when the drive is installed per the manufacturer's guidance, which generally includes a specified Bulletin 2090 line filter. Without the filter, the drive will fail the first harmonic and broadband conducted-emissions tests.
2. Sensitive equipment on the same AC bus – If the same feeder that powers the drive also powers low-level instrumentation (4–20 mA loops, RTDs, thermocouples, weighing scales, vision systems, communication gateways), and that equipment has shown noise-related symptoms after the drive was commissioned, a line filter is the first countermeasure.
3. Long shared conduit runs – When the drive's input feeder shares a long conduit or cable tray with signal wiring, capacitive coupling from the drive's switching noise into the signal conductors is amplified. A line filter plus physical separation is the standard fix.
4. Drive-to-drive hunting or DC bus instability – In multi-drive panels, line-side noise from one drive can disrupt the input sampling of its neighbors. Filters on each drive isolate them from each other.

None of those conditions applied to the plant in the original question (50 ft motor lead, 1–2 trapezoidal moves per minute, no reported interference symptoms, no CE marking requirement stated). In that situation, the filter is best classified as optional insurance rather than a hard requirement.

When an AC Line Filter Is Not Required

A line filter can be omitted when all of the following are true:

• The installation is in North America on an industrial (non-residential) feeder, where FCC Part 15 Class A is the governing rule and is generally met by the drive and motor combination alone.
• No other equipment on the shared AC bus has shown interference symptoms after the drive was energized.
• The drive's AC feeder is physically separated from signal wiring (different trays, different conduits, or a 150 mm / 6 in minimum separation).
• The system is not being CE-marked or otherwise third-party EMC-certified.

The contributor's experience ("we have lots of servos and none of them have line filters") is a valid empirical basis for omitting the filter in their specific plant. A factory environment with many drives that all work unfiltered is a strong signal that the bus impedance and installation practices are tolerant of the drive's conducted emissions.

Catalog Sizing for the 460 V Kinetix Family

Rockwell Automation publishes Bulletin 2090-UXLF-HV line filters for the 400/460/480 V Kinetix input stage. Continuous current, voltage class, and phase count are the only sizing parameters. The filter's continuous current must be greater than or equal to the drive's continuous input current, with a 1.5× to 2× margin recommended because servo drives draw highly distorted current at the rectifier stage (typical crest factor 1.4–1.6, and short-term peaks during regen events).

Catalog Number	Continuous Current	Voltage Class	Phase	Typical Kinetix Drive
2090-UXLF-HV323	23 A	460 V	3	~11–15 A input drives (e.g., Kinetix 6000, 6200, 6500 11 A ratings)
2090-UXLF-HV336	36 A	460 V	3	Larger 460 V Kinetix input stages
2090-UXLF-HV350	50 A	460 V	3	27 A+ Kinetix input stages

The 2090-UXLF-HV323 is rated 23 A continuous. If the 11 A in the application is single-phase input current, the 23 A rating provides a 2× margin. If 11 A is three-phase line current, the 23 A rating still provides a 2× margin. In either case, the filter is appropriately sized for the drive. Confirm whether "11 A" is per-phase line current or a single-phase reading on the drive's nameplate before final selection.

The original contributor's reading: a 5 A, 480 V servo motor on an 11 A drive, with the 2090-UXLF-HV323 (23 A) filter recommended. The filter's 23 A continuous rating is appropriate for an 11 A drive input regardless of whether the 11 A is a per-phase or total figure, because the standard derating practice is to oversize the filter by a factor of two for servo rectifier stages.

Sharing Rule: One Filter Per Drive

A common cost-driven question is whether a single AC line filter can be paralleled across multiple servo drives to save money. The official answer from Rockwell Automation Knowledge Base answer ID 59007 is unambiguous:

"No, it is not recommended to place more than one servo drive/converter onto a single line filter. The filters we recommend for any given drive have been tested."

The technical reason is that the filter is part of a tested, characterized impedance network. Two drives on one filter present a different input impedance profile than the test condition, and the filter's insertion loss curve is no longer valid. Paralleling drives on a single filter can also create circulating currents between the DC buses if the drives share a common DC link, and can re-couple noise from one drive's input to another's. The 2090-UXLF-HV323 must therefore be dedicated to a single drive.

Effects of 50 ft Motor Cable on Filter Selection

The 50 ft (≈15 m) motor cable length mentioned in the application is a consideration for the output filter, not the input filter. The rule of thumb is:

• Below 15 m (≈50 ft): no output filter is required for most Kinetix servo motors at 480 V. Voltage reflection at the motor terminals is below the critical value of twice the DC bus voltage.
• 15–30 m (50–100 ft): a dV/dt output filter (Bulletin 2090 series) becomes advisable to keep peak motor terminal voltage below winding-insulation ratings.
• Above 30 m: a sine filter or a Kinetix-compatible reactor is typically recommended.

At 50 ft the user is on the boundary. The 480 V class pushes the application into the "consider a dV/dt filter" category, but the 1–2 moves/min duty cycle (very low switching stress) means peak motor terminal voltage events are infrequent. The input-side 2090-UXLF-HV323 is a separate decision and is not affected by the motor cable length.

Installation Considerations

When a 2090-UXLF-HV323 is installed, follow these mechanical and wiring rules:

1. Mount the filter inside the cabinet, on the same panel as the drive, with the shortest possible lead length between the filter output and the drive input terminals. Long leads between filter and drive defeat the filter's high-frequency performance.
2. Use shielded VFD-rated cable from the filter input to the AC feeder. Ground the shield at the cabinet entry only (single-point grounding).
3. Keep the filter's input wiring physically separated from the drive's motor output wiring. Cross them at 90° if they must intersect.
4. Bond the filter's chassis ground terminal to the cabinet back-panel with a short, heavy braid or wire (6 AWG or larger). Paint must be removed at the contact area.
5. Verify that the filter's input current rating is not exceeded during inrush. Servo drives draw a large inrush at power-on as the DC bus capacitors charge. The 2090-UXLF-HV323 is rated to handle this internally; do not bypass it with a pre-charge contactor.

Cost-Benefit Assessment

The 2090-UXLF-HV323 lists at $329 in the distributor's package. Across a fleet of 10–20 drives, that becomes $3,290–$6,580. The cost of the filter is justified only when one of the conditions in the "When Required" section above is met. In a benign plant environment where no interference symptoms exist, the filter's value is purely insurance against a future failure mode that may never occur. A reasonable engineering decision is to omit the filter on all drives and stock one spare for field installation if a noise complaint arises. This is the approach the original contributor effectively adopted.

Verification After Installation

If a filter is installed, verify correct operation with the following checks:

1. Conducted emissions – Measure AC line current with a spectrum analyzer and current probe on the filter's input side. Compare against EN 55011 / EN 61800-3 Class A limits if CE compliance is required.
2. Drive input voltage – Confirm the line-to-line voltage at the drive's input terminals is within ±10% of nominal at full load. Excessive voltage drop across the filter wiring indicates undersized conductors.
3. Thermal check – After 1 hour at full load, the filter case temperature should not exceed the manufacturer's published limit (typically 75 °C rise above ambient).
4. No new faults – Drive fault log should remain clean. Common fault codes that can be triggered by improper filter installation include DC bus overvoltage (F03 / F04 on Kinetix), input phase loss (F05), and pre-charge timeout.
5. Bus noise symptom check – Other equipment on the shared AC bus should show no new interference symptoms after the filter is installed and the drive is running at full output.

Troubleshooting Matrix

Symptom	Likely Filter-Related Cause	Corrective Action
Other equipment resets or shows noise after drive starts	No input filter installed; high-frequency leakage onto AC bus	Install 2090-UXLF-HV-series filter at the drive input
Drive trips DC bus overvoltage on regeneration	Filter interaction with line impedance; regen energy not absorbed	Verify filter is correctly rated; check for regen resistor presence
Filter case runs hot	Filter undersized for the drive's RMS input current	Step up to the next size in the 2090-UXLF-HV family
CE compliance test fails conducted emissions	Filter omitted, wrong filter, or filter wiring too long	Install correct 2090-UXLF-HV-series filter with short leads to drive
Two drives on one filter cause each other to fault	Shared filter on paralleled drives	Provide a dedicated filter per drive per KB 59007
Filter has no measurable effect on noise	Noise source is radiated, not conducted; or the filter is on the wrong side	Re-check whether the issue is line-side or load-side EMI

FAQ

Is the Allen-Bradley 2090-UXLF-HV323 required for a 480 V Kinetix drive?

It is required only when the installation must meet CE / EN 61800-3 EMC compliance, or when conducted noise from the drive causes interference with other equipment on the shared AC bus. In a North American industrial environment with no reported interference symptoms, the filter is optional insurance rather than a hard requirement.

Can one AC line filter be shared between two Kinetix servo drives?

No. Rockwell Automation Knowledge Base answer 59007 states that the recommended filters have been tested for a specific drive and that more than one drive per filter is not recommended. Use one 2090-UXLF-HV filter per drive.

How is the 2090-UXLF-HV323 sized relative to the 11 A drive rating?

The filter is rated 23 A continuous at 460 V three-phase, which is roughly 2× the 11 A drive rating. This margin is standard for servo drives because of the high crest factor at the rectifier stage. The 23 A filter is appropriate for an 11 A drive regardless of whether the 11 A is single-phase or per-phase line current.

Does 50 ft of motor cable change the input-filter decision?

No. Motor cable length affects the choice of output (load-side) dV/dt or sine filter, not the input AC line filter. At 50 ft on a 480 V Kinetix system, an output dV/dt filter is on the boundary of advisable, but the input 2090-UXLF-HV323 is selected independently based on the drive's input current and the plant's EMC requirements.

What is the difference between an AC line filter and a dV/dt output filter?

The AC line filter (input side) blocks drive-generated EMI from escaping onto the AC power bus. The dV/dt output filter (load side, between drive and motor) slows the IGBT edge rate to limit peak voltage at the motor terminals and protect winding insulation. They serve different functions and are selected and installed independently.