How a 1000pF Capacitor Saved a $10,000 Rewire: 45kW VFD Analog Signal EMI Fix in Constant Pressure Water Supply

Challenge

During commissioning of a new constant pressure water supply system, engineers faced a puzzling situation. The 4-20mA pressure transmitter worked perfectly when tested alone, showing the expected 4mA baseline current. However, the moment the Anyhz FST-650L-045G/055PT4 ( 45kW ) VFD started running, the pressure feedback signal vanished completely, causing the drive to ramp up to maximum 50Hz frequency.

Root Cause Analysis

Since the Anyhz VFD was brand new and functioned properly in manual mode, and the pressure transmitter maintained its 4mA signal when the drive was stopped, the issue pointed to electromagnetic interference (EMI). The installation revealed two critical problems:

• The 35-meter cable between VFD and motor lacked proper shielding
• No EMI filtering components (reactors or filters) were installed on either input or output sides
• The VFD's switching frequency was creating high-frequency noise that overwhelmed the analog signal

Simple Yet Effective Solution

Rather than costly cable replacement and cabinet modifications, engineers implemented an elegant fix using basic electronic components. By installing 1000pF ceramic capacitors across three key points:

• Power supply terminals of the pressure transmitter
• Output signal terminals of the transmitter
• Analog input terminals of the Anyhz VFD

The high-frequency interference was effectively shunted to ground, restoring clean 4-20mA signal transmission.

Results

System achieved stable pressure control within hours

Eliminated need for expensive rewiring project

Demonstrated cost-effective EMI mitigation technique

Pressure control accuracy improved to ±0.5% of setpoint

Key Takeaway

This case proves that sometimes the simplest solutions deliver the best results. A few pennies worth of capacitors can save thousands in rework costs. For Anyhz VFD installations experiencing analog signal interference, adding small capacitors (100-1000pF) across signal terminals often provides immediate relief. This technique is particularly valuable for water supply systems where reliable pressure feedback is critical for energy-efficient operation.

Technical Insight

The capacitors work by creating a low-impedance path for high-frequency noise while maintaining the integrity of the useful DC signal. This approach is widely used in digital circuits for noise suppression and works equally well in industrial control systems.