You've hit upon a tricky but important concept in filter design, particularly when dealing with non-linear loads like rectifiers. Here's a breakdown of what's likely happening and why your component value change didn't work as expected

Understanding the Problem: Harmonics and Impedance
Harmonic Frequencies

Rectifiers (especially half-wave and some types of full-wave) are notorious for drawing current in short, sharp pulses. This pulsed current is mathematically equivalent to a fundamental frequency (60 Hz in your case) plus a series of harmonics (multiples of the fundamental frequency: 120 Hz, 180 Hz, 240 Hz, etc.). Your observation of a strong 180 Hz component confirms this.

Harmonics and Inverter Instability

Inverters are designed to output a clean sine wave. The harmonic currents from the UPS or hair dryer can distort the inverter's output voltage. Modern inverters use sophisticated feedback control to maintain a clean output, but excessive harmonic distortion can overwhelm this control system, leading to faults and shutdowns.

Shunt Filter Function

Your LC shunt filter is designed to present a low impedance path specifically at the 180 Hz harmonic frequency. Ideally, it should act like a short circuit at 180 Hz, "sucking up" the harmonic current and preventing it from reaching the inverter.

Why Your First Filter Worked

Your initial 4.83 mH inductor and 160 uF capacitor created a resonance close to 180 Hz. This low impedance at 180 Hz effectively reduced the harmonic current injected into the inverter. Even though the filter drew 16A, it was largely reactive and prevented the inverter from seeing the distorted current.

Why Changing L and C Didn't Work

While you maintained the same resonant frequency by doubling the inductance and halving the capacitance, you significantly changed the impedance of the filter at the resonant frequency. Here's the key: Impedance at Resonance

The impedance of an ideal LC parallel resonant circuit is theoretically infinite at resonance. In a real-world circuit, however, the impedance is limited by the resistance of the inductor (DCR - Direct Current Resistance) and the capacitor's ESR (Equivalent Series Resistance). A very high impedance means the filter presents a significant barrier to harmonic currents.

Impedance Change

By increasing the inductance and decreasing the capacitance while keeping the resonant frequency constant, you have increased the impedance of the individual inductor and capacitor at any given frequency (including 180 Hz). Although the circuit is still resonant at 180 Hz, it no longer provides a low impedance path for the harmonic current.

Analogy

Imagine trying to siphon water from a tank. Your initial filter was like a large diameter pipe – easy flow, even if the pressure (voltage) was small. The altered filter is like a very long, thin pipe. It's still connected (resonant), but it restricts the flow significantly.

Mathematical Explanation Impedance of Inductor (X_L)

X_L = 2πfL. Increasing L increases the impedance.

Impedance of Capacitor (X_C)

X_C = 1 / (2πfC). Decreasing C increases the impedance.At resonance, X_L = X_C. But what matters is that the individual values of X_L and X_C have increased. The circuit is still resonant, but it's a high-impedance resonant circuit.

Think in terms of Q-Factor

The Q-factor (quality factor) of the parallel resonant circuit is important. A higher Q-factor means a sharper, narrower resonance. In this case, you wanted a high current to be able to flow in the filter, which requires a low impedance at resonance. Changing L and C as you did likely decreased the Q-factor (because the increased impedance of L and C are more easily influenced by their parasitic resistances), but also significantly increased the impedance at resonance.

What to Do

Here's how to address the problem and potentially reduce the current flowing through your filter:1. Minimize Inductor Resistance (DCR)

The lower the DC resistance (DCR) of your inductor, the lower the impedance of the filter at resonance, and the more effective it will be at shunting harmonic currents. Use a larger gauge wire for your inductor or buy a high-current inductor with low DCR.2.

Minimize Capacitor ESR

Similarly, a capacitor with low ESR is important. Choose a capacitor type with inherently low ESR (e.g., polypropylene film capacitors are typically better than electrolytic for this purpose).3.

Slightly Detune

Very slight detuning can sometimes help spread the harmonic current across a wider frequency range, reducing the peak current in the filter. You might try slightly adjusting L or C to shift the resonant frequency a bit. Use your oscilloscope to observe the effect.4.

Consider a Damping Resistor

A small resistor in series with the inductor can help to damp the resonance, lowering the current, but it will also reduce the filter's effectiveness somewhat. Start with a small value (e.g., 1 ohm) and increase it gradually, monitoring the inverter's behavior and the filter current.5.

Try a Different Filter Topology

Consider a series LC filter in series with the load, rather than a shunt filter. A series filter presents a low impedance in series with the load, and a very high impedance to the harmonic frequencies. This prevents the harmonics from ever getting to the load in the first place. A series LC filter can be very effective. However, you will still need components that can handle significant current.6.

Measure Harmonic Current from Load

Ideally, you'd directly measure the 180Hz current being produced by the UPS before the filter. This will help you determine if your filter is truly removing the harmonic, or just shifting the problem. A clamp meter that can measure AC current at higher frequencies would be necessary.7.

Consider a Commercial Harmonic Filter

Commercial harmonic filters are available and are designed specifically for dealing with non-linear loads. They're more expensive but are typically more effective and safer than a DIY solution.8.

Check the Inverter Specifications

Confirm the inverter's harmonic distortion tolerance. Exceeding these limits will cause problems.

Safety Note: Working with mains voltage AC is dangerous. Be extremely careful and take all necessary safety precautions. Make sure all components (inductor, capacitor, resistor) are rated for the voltage and current they will be subjected to.* Use proper fusing to protect against overcurrent conditions.By understanding the impedance characteristics of your filter and carefully selecting components, you can fine-tune it to effectively mitigate the harmonic distortion and keep your inverter running smoothly. Remember to prioritize safety when experimenting with mains voltage. Flag for review