Passive Harmonic Filter for Single-Phase Loads using LC Trap Circuits

A passive harmonic filter for single-phase loads is a shunt panel of series inductor-capacitor branches connected across the supply feeding a nonlinear load. What does a passive LC trap filter do for a single-phase load? It gives each targeted harmonic current an easy path to circulate through a tuned branch instead of flowing back into the supply, so the current drawn from the 230 V, 50 Hz source contains less of the 3rd and 5th harmonic content produced by a rectifier-type load. Each trap branch is designed so that its series impedance is at a minimum near its tuned frequency, which is why the 150 Hz branch attracts mainly 3rd harmonic current and the 250 Hz branch attracts mainly 5th harmonic current, while both branches present a much higher impedance at the 50 Hz fundamental.

The filter works because a series inductor-capacitor branch has its lowest impedance at its resonant frequency, set by the component values through f = 1/(2π√LC). A rectifier-type load draws a pulsed current containing harmonic components at 150 Hz, 250 Hz and higher multiples of the 50 Hz fundamental, and those harmonic currents would otherwise flow back through the supply. Each trap branch is connected in parallel with the load, so it offers those harmonic currents an alternative path: at its tuned frequency the branch impedance falls to little more than its winding resistance, while at 50 Hz the same branch presents a much higher impedance and draws only a small fundamental current. The 150 Hz branch therefore diverts mainly 3rd harmonic current and the 250 Hz branch diverts mainly 5th harmonic current, leaving less of those two harmonics in the supply conductors, as panel meters and a current probe show.

1. The nonlinear load draws a pulsed current from the 230 V, 50 Hz supply, and the supply current is recorded with both trap branches bypassed as the unfiltered reference.
2. The 150 Hz trap branch is switched in, and the supply current and the branch current are recorded so the 3rd harmonic diversion can be compared with the reference.
3. The 250 Hz trap branch is switched in alongside the first branch, and the same readings are repeated to show the combined effect on the supply waveform.
4. Each branch is then switched out in turn, so the contribution of each trap can be separated in the lab record instead of being merged into one reading.
5. An oscilloscope or harmonic measurement on the supply current shows the change in the 150 Hz and 250 Hz content between the bypassed and filtered cases.
6. The branch inductor and capacitor values are checked against the tuning formula, and any difference between the design-target frequency and the measured resonance is noted as detuning in the report.

Builder note from the actual build: tune each branch with the load disconnected and verify its resonance with a signal source and a meter before connecting the panel to mains, because a branch tuned on paper can shift once it is wired. Keep trap-branch wiring short, since stray inductance in long leads adds to the branch inductance and moves the tuned frequency away from its design target. A trap is a frequency-selective shunt path, not a general clean-up device: it diverts the harmonics it is tuned for and leaves the rest of the spectrum largely unchanged.

Tech stack:

• Series LC trap branch for harmonic diversion
• Air-core inductor winding for trap branches
• AC film capacitor for trap branches
• MCB protection for panel input
• Analog panel ammeter and voltmeter
• Rectifier-type nonlinear test load
• Terminal blocks and selector switches
• Oscilloscope current measurement for verification

A passive harmonic filter earns its place in an electrical lab through four capabilities: two shunt trap branches tuned to the harmonics a rectifier load produces, a tuning method students can check with a formula before any mains connection, panel meters showing supply current before and after filtering, and a protected panel layout with short, labelled trap connections. The 150 Hz branch targets the 3rd harmonic and the 250 Hz branch targets the 5th harmonic, so a lab session can switch branches in and out and compare the supply waveform for each case. With no control software or switching converter to tune, the experiment stays focused on resonance, impedance and waveform measurement. Each branch can be isolated for its own resonance check, and the panel wiring makes the shunt connection visible: the trap sits across the supply, in parallel with the load, rather than in series with it.

• [Two tuned trap branches] One branch tuned near 150 Hz (design target) for the 3rd harmonic and one near 250 Hz (design target) for the 5th harmonic, each switchable independently so the effect of each branch can be measured on its own.
• [Formula-checked tuning] Each branch follows f = 1/(2π√LC), with the inductor and capacitor values chosen as design targets from that formula, so students can repeat the tuning arithmetic and check it against a resonance measurement.
• [Visible shunt connection] The trap branches connect across the supply in parallel with the load, with labelled terminals and short branch wiring, so the shunt filtering concept is visible on the panel instead of hidden inside a box.
• [Before-and-after current measurement] Panel ammeter and voltmeter terminals plus a meter connection point let students record the supply current with the filter bypassed and with each branch connected, using the same load and the same instruments.
• [Rectifier-type test load] A rectifier load board represents the nonlinear load that produces the harmonics, so the filter is tested against a real distorted current rather than a signal-generator imitation alone.
• [MCB protection and isolated terminals] An MCB guards the panel input and the load and filter connections use shrouded terminals, so a wiring fault trips protection and live parts are not left exposed on the bench.
• [Lab-ready labelled panel] Inductors, capacitors, switches, meters and terminals are mounted on one labelled bench panel with documented wiring, so the unit can be set up and demonstrated without loose bench wiring.

Implemented in the base build: the two trap branches tuned near 150 Hz and 250 Hz (design targets), branch selector switches, panel meters, MCB protection, and the rectifier-type test load connection. Optional on request: a third branch tuned near 350 Hz (design target) for the 7th harmonic, or a detuned reactor option for a capacitor-bank study. Future scope only, not in the base build: automatic harmonic spectrum logging to a computer.

• Assembled passive harmonic filter panel with two LC trap branches, selector switches, meters and MCB protection
• Rectifier-type test load board for the output terminals
• Tuning calculations for both branches showing the f = 1/(2π√LC) arithmetic
• Circuit and wiring documentation for the panel
• Component list with part ratings
• Lab experiment guide covering unfiltered reference readings, single-branch readings and combined-branch readings
• Project report PDF (background, tuning theory, methodology, observations format)
• PPT presentation for final review
• Viva Q&A preparation document covering resonance, trap impedance, harmonic orders and safety

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