Power Quality Issues That Damage VFDs and PLCs: Causes, Signs, and Fixes

A voltage sag is a brief drop below normal voltage, usually lasting only a few cycles. On a VFD, a sag commonly trips an under voltage fault, drops the DC bus voltage, costs the motor torque mid task, and can shut the drive down entirely if it happens during acceleration.

On a PLC, the same sag often triggers an unexpected reboot, interrupts program execution partway through a cycle, and causes input and output modules to briefly malfunction. Voltage sags are one of the most frequent reasons behind PLC restarts and unplanned line stoppages.

A surge is a brief rise in voltage lasting only milliseconds. On a VFD, a surge can damage the IGBTs, blow the input rectifier, trigger an overvoltage fault code, and damage the control board itself.

On a PLC, the same surge frequently burns out the internal power module, causes an outright power supply failure, or corrupts memory that was mid write at the moment the surge hit.

Harmonic distortion is a departure from the standard sinusoidal waveform, usually caused by non linear loads elsewhere on the same system. On a VFD, harmonics show up as excessive heating, nuisance overcurrent trips, transformer and cable overheating, and reduced overall drive efficiency.

On a PLC, the same distortion introduces power supply noise, unstable input and output readings, and extra heating inside the control module itself.

Electrical or radio frequency interference is one of the harder power quality issues to trace, since it degrades signal integrity without necessarily touching the supply voltage at all. On a VFD, this typically appears as erratic motor speed, encoder misreadings, and random fault trips with no obvious cause.

On a PLC, the same noise shows up as fluctuating analog signals, false triggering on digital inputs, and lost communication over Ethernet, RS485, or Profibus links.

Phase imbalance occurs when voltage or current magnitude differs noticeably across the three phases. On a VFD, this drives motor overheating, pushes excess current onto one phase specifically, and reduces available torque and efficiency.

On a PLC, phase imbalance shows up less directly, mainly as an overheated power supply and a shortened lifespan for internal components exposed to the resulting stress.

A physically weak, corroded, or improperly installed ground creates an unstable electrical reference for the whole system, and it is one of the power quality issues most often missed during a routine inspection. On a VFD, this typically causes nuisance tripping, overcurrent or ground fault alarms, and noise finding its way into control circuits that should be isolated from it.

On a PLC, poor grounding shows up as communication dropouts, a frozen HMI, and generally erratic input and output behavior that seems to come and go without a clear pattern.

A transient is a fast, energetic voltage pulse, often triggered by nearby switching events like contactors or capacitor banks engaging. On a VFD, a transient can push the DC bus into overvoltage, damage the input bridge, and cause a sudden, unplanned drive shutdown.

On a PLC, the same transient can force a power supply reset, corrupt memory mid operation, and leave logic execution genuinely unpredictable until the unit is power cycled.