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How to Identify and Fix Power Quality Issues That Damage VFDs and PLCs
Power quality issues are one of the most common—and most misunderstood—causes of equipment failure in industrial automation. Modern VFDs and PLCs are efficient but far more sensitive than their predecessors. Even small distortions in the electrical supply can cause nuisance trips, CPU resets, I/O instability, motor failures, and unpredictable downtime.
This guide focuses on the two most important capabilities engineers need today:
1. Identifying power quality issues in VFD and PLC systems
2. Fixing power quality issues using proven engineering controls
Unlike generic advice found online, this article delivers field-ready procedures, real thresholds, industry examples, and specific solutions supported by the Siemens, Mitsubishi, WEG, and Yaskawa products Industrial Automation Co. sells.
Why Power Quality Matters More Today
VFDs rectify incoming three-phase AC into a DC bus before generating variable-frequency output to the motor. That DC bus—and its electrolytic capacitors—must remain stable. PLCs depend on clean, ripple-free 24VDC power, often generated from sensitive switch-mode supplies.
When voltage sags, spikes, or distorts, the internal electronics of this equipment react instantly. Drives trip. PLCs reset. HMIs freeze. Motors overheat.
Yet power quality is often the last thing maintenance teams consider during troubleshooting. A trip is treated as a drive problem. A reset is treated as a PLC problem. A communication dropout is treated as a network problem.
In reality, unstable power often sits at the center of all of these symptoms.
Real-World Example #1: VFD Undervoltage Faults Caused by Hidden Voltage Sags
A packaging facility reported repeated undervoltage faults on Siemens SINAMICS G120 drives. Maintenance checked motors, wiring, and fuses—everything looked normal. But a 72-hour power quality recording told the real story: every morning when multiple conveyors started, the supply voltage dipped to 77% of nominal for 40 ms.
No drive malfunctioned. The power did.
This is a classic symptom of plant-wide sags—not individual equipment failure.
Real-World Example #2: PLC Resets Caused by Excessive Harmonics
A machining line saw intermittent resets on Siemens S7-1200 PLCs during coolant pump startup. Engineers suspected firmware bugs or failing modules. Power measurement revealed 14% THD—far exceeding what PLC power supplies can tolerate.
After installing line reactors on the pump drives and replacing generic 24V supplies with Siemens SITOP PSU8600 units, the resets stopped entirely.
Harmonics—not PLC hardware—were the cause.
1. Identifying Power Quality Issues in VFD and PLC Systems
Power quality issues rarely announce themselves directly. Instead, they leave a breadcrumb trail of scattered symptoms across drives, PLCs, HMIs, and motors. The key is interpreting these symptoms correctly.
Power Quality Troubleshooting for VFDs
VFDs are sensitive to variations in the incoming waveform. The most common PQ-related symptoms include:
• DC bus undervoltage trips (often during acceleration or shift changes)
• Overvoltage faults during deceleration or utility switching
• Overcurrent faults with no mechanical cause
• Drive overheating even at partial loads
• Input diode failures following storms or capacitor bank switching
A pattern across multiple drives—especially in the same time window—strongly indicates a PQ issue.
How Harmonics Damage PLC Power Supplies
While VFDs trip noisily, PLCs fail quietly.
Harmonic distortion causes:
• Overheating of switching regulators
• Unstable 24VDC output
• Disappearing analog signals
• CPU watchdog resets
• Profinet or Modbus dropouts during electrical disturbances
PLC resets during compressor starts or heavy load events almost always point to sag or harmonic distortion—not a software or module fault.
Step 1: Use Equipment Logs as Diagnostic Clues
VFD and PLC logs provide the earliest warning signs of power issues.
Look for:
• Repeated undervoltage alarms (G120 F30005, Mitsubishi LV)
• PLC CPU resets with no programming changes
• Communication timeouts when nearby motors start
• DC bus ripple warnings
• HMI restarts during line disturbances
These clues reveal the pattern behind the failures, which is more important than any single event.
Step 2: Measure Voltage Distortion and Imbalance With Real Data
Handheld meters are not fast enough to capture damaging sags or transients.
A 24–72 hour power quality recording is the only reliable method.
Voltage Sag Depth & Duration:
• Drives trip below 85% nominal voltage
• PLC supplies struggle when sags drop DC output below 19–20 VDC
• Severe stress occurs when sags reach <70% nominal
Total Harmonic Distortion (THD):
• <5% = Ideal
• 5–8% = Acceptable
• 8–12% = Harmful to VFD capacitors
• >12% = Critical — affects control systems, power supplies, and transformers
Voltage Imbalance:
• >2–3% imbalance doubles motor heating and stresses VFD front ends
Step 3: Perform a Grounding & Shielding Audit
Some of the most disruptive symptoms—PLC dropouts, encoder noise, analog jitter—stem from EMC issues rather than waveform distortion.
Inspect:
• 360° shield terminations on all VFD motor cables
• Bonding integrity across machine frames
• Proper segregation of power and control cabling
• Ground bars bonded directly to building steel
A major percentage of “PLC instability” is actually noise-related.
2. Fixing Power Quality Issues for VFDs and PLCs
Fixing Power Quality Issues in VFDs
Line Reactors: The First Line of Defense
A 3%–5% reactor reduces harmonic current, protects rectifiers, and stabilizes the DC bus.
It is one of the highest-ROI upgrades you can make.
Use line reactors when:
• THD > 8%
• Multiple drives share the same transformer
• Drives trip during facility-wide startup
• Drive input components fail prematurely
Available from Industrial Automation Co.:
Siemens SINAMICS line reactors
Mitsubishi FR-series compatible reactors
DV/DT or Sinewave Filters for Long Motor Cables
When cable runs exceed 100–150 ft, motor terminal voltage peaks can exceed insulation ratings due to reflected wave phenomena. This creates:
• Motor overheating
• Insulation breakdown
• Bearing pitting
• Overcurrent faults
DV/DT filters protect the drive; sinewave filters protect the entire motor system.
Fixing Power Quality Issues in PLCs
Regulated 24VDC Power Supplies (Siemens SITOP)
Most PLC resets are not software problems—they are caused by voltage dips.
SITOP power supplies provide:
• Brownout buffering
• Stable voltage under load
• Selectivity modules that isolate overloads
• High immunity to ripple and harmonics
If a PLC resets “at random,” replacing the 24V supply with a SITOP unit is often the fix.
Industrial UPS for Control Power
A UPS bridges deeper sags, generator transfers, and utility switching events. It protects:
• PLC CPUs
• HMIs
• Industrial PCs
• Network switches
Surge Protection Devices (SPD)
Surges silently destroy VFD rectifier sections.
A Type 2 SPD is essential in environments exposed to:
• Lightning
• Utility capacitor switching
• Large motor starting/stopping
• Regenerative loads
SPDs dramatically extend the lifespan of VFDs and PLC power supplies.
Power Quality Troubleshooting Diagram
┌──────────────────────────────────────────┐
│ START: EQUIPMENT INSTABILITY │
│ (VFD trips, PLC resets, motor heating) │
└──────────────────────────────────────────┘
|
v
┌──────────────────────────────────────────────┐
│ 1. COLLECT SYMPTOMS & LOGS │
│ • VFD faults (UV, OV, OC, DC ripple) │
│ • PLC watchdog resets / I/O dropouts │
│ • HMI or network instability │
└──────────────────────────────────────────────┘
|
v
┌──────────────────────────────────────────────┐
│ 2. INSTALL POWER QUALITY RECORDER │
│ • Measure sags, swells, THD, imbalance │
│ • 24–72 hr capture for accurate diagnosis │
│ • Compare to thresholds (e.g., THD > 8%) │
└──────────────────────────────────────────────┘
|
v
┌──────────────────────────────────────────────┐
│ 3. GROUNDING & SHIELDING AUDIT │
│ • Verify bonding to plant ground │
│ • Check 360° shield terminations │
│ • Correct routing of power vs control cables │
└──────────────────────────────────────────────┘
|
v
┌──────────────────────────────────────────────────────────┐
│ DECISION POINT: WHAT TYPE OF PQ ISSUE WAS IDENTIFIED? │
└──────────────────────────────────────────────────────────┘
/ | \
/ | \
v v v
┌─────────────────┐ ┌──────────────────────┐ ┌──────────────────────┐
│ VOLTAGE SAGS │ │ HARMONIC DISTORTION │ │ TRANSIENTS / SURGES │
└─────────────────┘ └──────────────────────┘ └──────────────────────┘
| | |
v v v
┌──────────────────┐ ┌────────────────────────┐ ┌────────────────────────────┐
│ Install SITOP │ │ Add 3%–5% Line Reactors│ │ Install Type 2 SPDs │
│ Regulated PSU │ │ or Passive Harmonic │ │ Surge filtering at MCC │
│ Add Industrial UPS│ │ Filters │ │ Protect VFD input stages │
└──────────────────┘ └────────────────────────┘ └────────────────────────────┘
| | |
v v v
┌──────────────────────────────────────────────────────────┐
│ LONG MOTOR CABLES? ( >100–150 ft ) │
│ → Install dv/dt or Sinewave Filters to protect motors & │
│ reduce reflected-wave voltage peaks │
└──────────────────────────────────────────────────────────┘
|
v
┌─────────────────────────────────────────────┐
│ FINAL STEP: RE-MEASURE PQ AFTER FIXES │
│ • Confirm THD reduction │
│ • Confirm stable DC bus voltage │
│ • Baseline PQ data for future maintenance │
└─────────────────────────────────────────────┘
|
v
┌─────────────────────────────────────────┐
│ SYSTEM STABILIZED — REDUCED VFD/PLC │
│ FAILURES & IMPROVED RELIABILITY │
└─────────────────────────────────────────┘
Conclusion: Power Quality Is the Foundation of Automation Reliability
If your facility is experiencing repeated VFD faults, PLC resets, or premature motor failures, power quality is one of the first—and most cost-effective—areas to investigate.
Industrial Automation Co. provides proven power-quality solutions:
• Siemens SINAMICS line reactors & filters
• Siemens SITOP regulated 24VDC power supplies & UPS modules
• Mitsubishi FR-series compatible reactors and dv/dt filters
• Surge protection devices and PQ-enhancing components
To stabilize your automation systems:
👉 Check stock for the components listed above
👉 Request parts matching for your specific VFD or PLC model
👉 Contact IAC for a facility-wide power quality upgrade evaluation
Improving power quality isn’t just about reducing faults—it’s one of the highest-ROI reliability improvements you can make.