Variable frequency drives (VFDs) are among the most common methods of controlling AC motors in industrial environments. They help equipment run at the right speed for the process, reduce mechanical stress during starts and stops, and often cut energy use in applications like pumps and fans.
They also introduce a layer of electronics and settings that can create confusion when problems show up. This guide breaks down what a VFD is, how it works, and the most common VFD issues we see in the field—plus how ICS approaches troubleshooting and repair.
What Is a Variable Frequency Drive (VFD)?
A variable frequency drive (VFD) is a motor controller that changes the frequency (and typically the voltage) supplied to an AC motor. By changing frequency, the drive changes motor speed.
You’ll also hear VFDs called:
- AC drives
- adjustable speed drives (ASDs)
- inverter drives
In most plants, “drive” usually means “VFD.”
What a VFD Controls
A VFD’s job is to control motor behavior in ways that a traditional across-the-line starter can’t. In practice, that usually means:
- Speed control
Instead of running at one fixed speed, the motor can run at the speed the process actually needs.
- Torque control
Many drives support control modes that help hold torque more consistently, especially during changes in load.
- Soft starts and controlled stops
VFDs ramp motors up and down rather than “slamming” them on and off. That reduces mechanical shock on couplings, belts, gearboxes, and driven equipment. - Potential energy savings
In variable-torque loads (like many fans and pumps), slowing the motor can reduce power draw significantly compared to throttling flow mechanically.
How Variable Frequency Drives Work
Most modern VFDs follow the same three-stage architecture:
1) Rectifier (AC → DC)
Incoming AC power is converted to DC. Many common industrial drives use a six-pulse rectifier arrangement.
2) DC Bus (Filter/Storage)
The DC bus smooths the rectified DC and stores energy (often using capacitors). This section is also where many “age-related” drive problems show up over time.
3) Inverter (DC → “Variable” AC)
The inverter uses switching devices (commonly IGBTs) to create a controlled output waveform. The output “looks like” AC to the motor, but the drive varies the frequency and voltage to control speed and torque. This is typically done using PWM (pulse width modulation).
That’s the core idea: fixed-frequency AC in → controlled-frequency AC out.
Common VFD Problems (And What They Usually Mean)
When a VFD faults, the fault code is rarely the whole story. It’s a clue. Here are common VFD issues and the kinds of root causes they point to.
Overcurrent (OC) / Motor Overload
Often tied to:
- mechanical binding or sudden load changes
- acceleration/deceleration ramps that are too aggressive
- incorrect motor data or control mode
- shorted motor leads or motor issues
Overcurrent faults can be settings-related, but they can also indicate a real electrical or mechanical problem.
Overvoltage (OV)
Often occurs during deceleration or when the load is driving the motor (regenerative conditions). Common contributors include:
- decel ramps too fast
- missing/incorrect braking hardware (resistor/regenerative unit)
- incoming power instability
Undervoltage (UV)
Common causes include:
- supply dips, brownouts, or loose connections
- undersized feeder wiring
- upstream contactors/issues
- drives sharing a weak supply during starts
Ground Fault (GF) / Earth Fault
Ground faults are a safety concern and frequently point to:
- motor insulation breakdown
- damaged motor leads/cabling
- moisture/contamination
- incorrect termination/grounding practices
These issues require careful isolation and testing to avoid further damage.
Overtemperature (OH/OT)
Common causes include:
- clogged filters, blocked airflow, failed fans
- high ambient temps or poor enclosure ventilation
- heat sinks loaded with dust/oil mist
- excessive switching/load conditions
Communication Faults
Often tied to:
- network wiring/shielding issues
- incorrect settings or node addressing
- noise coupling from power wiring
- termination problems on certain networks
“It Runs… But Not Right”
Some of the most expensive problems are subtle:
- unstable speed control
- hunting/oscillation
- inconsistent torque
- nuisance trips that only happen on changeover or certain products
These usually require a system-level view (drive + motor + wiring + load + controls logic).
How ICS Diagnoses VFD Problems
When a VFD is down, the fastest fix is rarely guessing at parameters. ICS approaches VFD troubleshooting with a structured workflow:
1) Confirm the symptom and fault history
We look at fault logs, operating conditions, and what changed before the problem started.
2) Separate drive faults from system faults
Many “drive faults” originate outside the drive (motor condition, cabling, grounding, load, cooling, supply power). Ground faults and overcurrent faults are common examples.
3) Validate the control side
Command sources, interlocks, permissives, and logic behavior can create symptoms that look like a drive problem. When controls are part of the root cause, we can support related troubleshooting through industrial automation and control services.
4) Repair vs. replace decision support
Sometimes the best outcome is repair and validation. Sometimes it’s modernization, especially when failures repeat, and the surrounding system needs attention.
If the issue is urgent, ICS provides emergency VFD repair support 24/7/365.
Preventing Repeat VFD Failures: Maintenance and Setup Best Practices
Most repeat VFD failures come from a handful of preventable conditions. A few helpful best practices:
- Keep drives cool and clean
Check filters, fans, airflow paths, enclosure temperature, and contamination.
- Verify wiring, grounding, and shielding
Especially on motor leads and feedback/communication cabling. Small wiring issues can create large noise problems.
- Confirm motor data and parameters
Incorrect motor nameplate data, control mode, or ramps can cause nuisance faults and stress the system.
- Review fault history periodically
Fault logs tell a story. Repeating faults are often early warning signs.
- Plan upgrades before downtime forces the decision
If a drive is aging, obsolete, or repeatedly failing, a planned upgrade is almost always cheaper than an emergency scramble.
For installation, upgrades, and ongoing support, explore ICS’s variable frequency drive services.
When to Call ICS
If you’re seeing any of the following, it’s usually time to get support:
- repeated trips with no clear root cause
- ground faults, insulation concerns, or motor lead issues
- overheating problems that return after “cleaning it once”
- instability at certain speeds/products
- drive faults that started after a changeover or upgrade
ICS supports VFD troubleshooting, repair coordination, and system-level fixes across many industrial environments. To learn more about all the operations we support, explore other industries we serve.
If production is down, request emergency VFD repair now.
