Output filter (LC filter)

01

Match drive voltage, current and switching frequency

Schaffner describes the FN 5040 HV as an LC sine wave filter that converts the rectangular PWM output voltage of motor drives into a smooth sine wave with low residual ripple, addressing motor damage from high dv/dt, overvoltages, cable ringing, overheating and eddy current losses, reducing bearing currents, electromagnetic emissions and acoustic noise, and eliminating pulse reflections in the motor cable. [1]

Its selection table runs from the FN 5040HV-13 (13 A, 7.5 kW at 690 V/50 Hz, 11.7 mH, 4.7 µF, 170 W loss, 14 kg) to the FN 5040HV-1320 (1,320 A, 1,200 kW, 0.12 mH, 816 µF, 4,700 W, 945 kg). The minimum drive switching frequency is 2 kHz up to the 300 A model and 1.5 kHz above it; capacitors are Y-connected. Schaffner requires the drive to run in scalar (V/Hz) mode at a fixed switching frequency at or above the minimum, and cautions that wrong drive settings may damage the filter. [1]

VEIKONG's sine wave filters are rated 3 × 380 V ±10%, 5–120 Hz operating frequency, 2–16 kHz switching frequency, 1.5 to 630 kW and 3.3 to 1,209 A; its table lists minimum switching frequencies of 6 kHz for the smallest models, falling to 4 kHz at 24–32 A. [2]

Editorial calculation: a simple per-phase estimate f = 1/(2π√(LC)) gives about 679 Hz for 11.7 mH with 4.7 µF, 644 Hz for 1.3 mH with 47 µF, and 509 Hz for 0.12 mH with 816 µF, each below the 1.5–2 kHz minimum switching frequency; the datasheet does not state a resonant frequency, so ask the maker. [1]

MTE Corporation's selection table for its High Frequency SineWave Guardian, an open-panel filter for 380 V and 480 V drives feeding high-frequency motors, shows the link between switching frequency and the motor frequency a filter can pass. Its 24 listed models fall into three groups: 2 kHz models from 200 A to 1,200 A for a 6–200 Hz system frequency, 3.5 kHz models from 200 A to 650 A for 6–240 Hz, and 5 kHz models from 80 A to 600 A for 6–300 Hz. MTE says to select by the motor's full-load amps and, if the FLA exceeds the filter rating, to use the next larger filter; capacitors are included, and the product page lists a three-year warranty. MTE states that its SineWave Guardian range protects motors on leads out to 15,000 ft. [4]

Editorial calculation: in MTE's table the highest system frequency is one-tenth of the switching frequency at 2 kHz (200 Hz) and about one-seventeenth at 5 kHz (300 / 5,000), so a drive that must run a motor above 200 Hz needs a filter designed for the higher switching frequency; 15,000 ft is about 4,570 m, against the 2,000 m cable limit Schaffner gives for the FN 5040 HV. [4] [1]

Give the supplier the drive make and model, system voltage, rated and overload current, switching frequency and control mode, motor frequency range, motor cable length and type, number of motors in parallel, any step-up transformer, ambient temperature, enclosure and mounting, and the ripple, voltage drop and loss limits.

02

Check ripple, voltage drop, overload and cable length

Schaffner's FN 5040 HV is specified for 3 × 690 VAC ±10%, 0–70 Hz motor frequency (up to 200 Hz with derating), rated currents of 13–1,320 A at 45 °C, motor cables up to 2,000 m, impedance of 8–10% at 690 V, 50 Hz and rated current, residual ripple voltage under 5%, overload of 1.5 × rated current for 1 minute once per hour, IP00 protection, acoustic noise under 70 dB(A) at 1 m, insulation class EIS 200 and design to IEC 61558-2-20. With reduced motor cable length the maximum switching frequency is 16 kHz. [1]

VEIKONG lists residual ripple THD under 5%, a standard voltage drop of 4% at rated current and 50 Hz, maximum permanent overload of 1.2 × and transient overload of 2.0 × rated current, IP00 as standard, natural ventilation, floor mounting, ambient −25 to +40 °C, and recommends its filters for motor cables up to 500 m. It notes that IGBT switching can produce voltage peaks reaching 1,300 V or more at the motor terminals. [2]

Editorial calculation: 4% of 380 V is about 15.2 V. [2]

The US Department of Energy's motor systems tip sheets set out why such filters are fitted. Tip Sheet #14 (November 2012) cites NEMA MG 1-2011 Part 31: insulation for definite-purpose low-voltage (≤ 600 V) inverter-duty motors should withstand 3.1 times rated line-to-line voltage — 1,426 V peak at the terminals of a 460 V motor — with rise times of at least 0.1 µs, and such motors can run without filters or load reactors provided overshoots stay within that limit. It reports spikes as high as 2,150 V in a 480 V system at 10% overvoltage, and says that "damaging reflected waves are generally not a problem when the distance between the motor and the drive is less than 15 feet". Tip Sheet #15 adds that with modern IGBT switches "cable lengths of 50 feet or more almost always need mitigation", and lists line inductors or load reactors at the drive end of the cable, or dV/dt filters, where the drive cannot sit close to the motor. [3]

Editorial calculation: 3.1 × 460 V = 1,426 V, and the 2,150 V spike DOE reports is about 1.5 times that limit (2,150 ÷ 1,426); a sine wave filter removes the steep PWM edges that cause such overshoots, which is why Schaffner rates the FN 5040 HV for motor cables up to 2,000 m. [3] [1]

03

Find output LC filter manufacturers

The companies in the manufacturer list below show this product on their own websites. Confirm the legal seller, factory and scope for your order.

Ask any supplier for rated voltage and current, inductance and capacitance, minimum and maximum switching frequency, motor frequency range and derating, residual ripple, voltage drop or impedance, losses, overload, maximum cable length, insulation class and test voltages, protection rating and cooling, weight and dimensions, and drive settings required.

04

Trace what each part does

This is a functional reference built from the sources cited; it is not one maker's specification.

05

Fill the specification fields with values for your order

An output filter quotation needs values for your drive and motor. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the drive, motor and filter data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Drive: [make; model; control mode]
Supply: [voltage; frequency]
Current: [rated; overload]
Switching: [kHz; fixed]
Motor: [power; frequency range; motors in parallel]
Cable: [length; type; step-up transformer]
Limits: [ripple; voltage drop; losses]
Installation: [IP; cooling; ambient]

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