Drives & motors
VFD and motor problems: what's really going on
Variable-frequency drives save energy and control processes, but they also change how a plant's electrical system behaves. Here are the common symptoms and their usual causes.
Symptoms and likely causes
| Symptom | Common causes | First checks |
|---|---|---|
| Drives trip on overvoltage | Capacitor bank switching transients; regenerative braking energy; high supply voltage at light load | Check the trip log against capacitor step switching, and the DC bus voltage |
| Drives trip on undervoltage | Voltage sags from large motor starts, utility faults or weak supply | Log the voltage with a power quality analyzer |
| Ground-fault trips or GFCI nuisance trips | Leakage current through cable and motor capacitance at high switching frequencies | Cable length and type, carrier frequency, grounding, filter selection |
| Transformer or neutral overheating | Transformer heating can result from drive harmonics. Neutral heating is chiefly associated with triplen currents from single-phase loads on four-wire systems. | Measure harmonic current; check transformer K-rating or derating |
| Capacitor fuses blowing | Resonance between capacitors and supply at a drive harmonic | Harmonic measurement; consider a detuned bank |
| Motor insulation failures on long leads | Reflected-wave voltage peaks between drive and motor | Lead length vs the drive maker's limit; dV/dt or sine-wave filter; inverter-duty motor |
| Motor bearing damage (fluting) | Shaft voltages discharging through bearings | Shaft grounding, insulated bearings, cable and grounding practice |
| Interference with instruments or radios | Conducted and radiated emissions | Shielded motor cable, correct grounding, EMC filtering per the drive's category |
| Lights flicker | Usually voltage fluctuation from large motor starts, welders or cyclic loads, less often harmonics | Log voltage and flicker (Pst/Plt) and correlate with load events |
Drives, soft starters and power factor
- Induction motors started across the line run at a power factor that depends on load: reasonable near full load, poor when lightly loaded.
- A soft starter reduces starting current and torque during acceleration, then normally bypasses. Once bypassed, the motor's power factor is the same as across-the-line. A soft starter doesn't improve running power factor.
- A standard VFD (6-pulse diode front end) supplies the motor's magnetizing current from its own DC bus, so the supply sees a high displacement power factor. But its pulsed input current lowers true power factor through harmonics. Adding capacitors does not fix that.
- Never put power-factor capacitors on a drive's output. On the input side, capacitor banks near drives should be checked for resonance and are usually detuned (see power factor correction).
Choosing a soft starter or a drive
A soft starter suits fixed-speed loads that just need a gentler start, such as compressors, conveyors and some pumps: lower starting current and mechanical stress, simple, low heat once bypassed. A VFD suits loads where speed should follow demand. For centrifugal fans and pumps, power falls steeply as speed drops, so the energy savings can be large. VFDs cost more, add heat and harmonics, and need more attention to cabling and grounding. The right choice depends on the load, not the brand.
Standards that apply
- IEEE 519: harmonic limits at the point of common coupling with the utility (see our IEEE 519 page).
- IEC/EN 61800-3: EMC requirements for power drive systems. Categories C1–C4 depend on environment, voltage/current ratings and installation conditions.
- UL 61800-5-1: safety of adjustable-speed drives (it replaced the older UL 508C).
- UL/IEC 60947-4-2: semiconductor motor controllers and soft starters.
- NFPA 70 (NEC) Article 430: motors, motor circuits and controllers.
- NEMA MG 1 Part 31: motors intended for use on inverters.
Where to start
Collect the drive fault logs and the times the problem happened, then measure: a power quality survey correlates voltage, current, harmonics and events with what the plant was doing. Or tell us the symptoms and we'll suggest what to measure.
Working with us
Send us the drive model, the fault history and what the plant was doing when it tripped. Our engineers will help you narrow down the cause and the next check. From the bill to the fix →