What is Phase Loss Error?
What Is a Phase Loss Fault on a VFD?
A phase loss fault means that one or more expected phases in a three-phase electrical system are missing or severely unbalanced.
With a VFD, however, one distinction is essential:
Input Phase Loss: One of the R, S or T supply phases feeding the VFD is missing.
Output Phase Loss: One of the U, V or W motor-side phases is open or carrying abnormally little current.
The names sound similar, but these faults occur on opposite sides of the drive and have very different diagnostic paths.
For the complete fault tree, see the OtomasyonAVM VFD Fault Codes guide.
Input Phase Loss · Output Phase Loss · Troubleshooting · Phase protection relay · FAQ
Input and output phase loss are not the same fault
The easiest way to understand the difference is to follow the energy path through a VFD:
R-S-T Supply → Rectifier → DC Bus → Inverter → U-V-W Motor Output
An input phase fault happens before the DC bus.
An output phase fault happens after the inverter bridge.
What is Input Phase Loss?
Input Phase Loss occurs when one of the three supply phases feeding a three-phase VFD is missing.
If the S phase disappears from an R-S-T supply, the input rectifier is no longer being fed by a healthy balanced three-phase source.
Manufacturers may identify this condition as Input Phase Loss, Supply Phase Loss, IPL, SPI or another model-specific code.
INVT drives, for example, use SPI for input phase loss and SPO for output phase loss. OtomasyonAVM's INVT fault-code material also keeps these two faults separate.
Does losing an input phase immediately make the motor single-phase?
Not in the same way as a motor connected directly to the mains.
The motor is separated from the utility supply by the VFD power-conversion stages:
AC Input → Rectifier → DC Link → PWM Inverter → Motor
If one mains phase disappears, the inverter may still create three motor-side phases for a period of time from the DC bus.
The real problem begins on the rectifier and DC-link side: DC-bus ripple increases, current in the remaining input paths rises and the rectifier and DC-link capacitors can experience additional thermal stress.
The drive therefore normally detects the abnormal condition and trips before prolonged operation damages the equipment.
ABB describes its Input Phase Loss fault as a condition in which the intermediate DC circuit voltage oscillates because an input power-line phase is missing. :contentReference[oaicite:4]{index=4}
Common causes of Input Phase Loss
Blown input fuse: One of the three branch fuses has opened.
Circuit-breaker pole failure: The breaker can appear mechanically ON while one electrical pole is not conducting properly.
Loose input terminal: An overheated R, S or T connection may intermittently become open circuit.
Input contactor failure: One main contact has burned or developed excessive resistance.
Damaged incoming cable: One conductor between the distribution system and the VFD is open.
Utility or transformer problem: A real phase loss has occurred upstream.
Severe voltage imbalance: Some drives can interpret extreme supply imbalance as a phase-loss condition even before one phase falls completely to zero.
What does Input Phase Loss do to the DC bus?
Under normal three-phase operation, the rectifier charges the DC bus in a relatively regular pattern.
With one supply phase missing, the charging pattern changes and DC-link ripple increases.
Current stress in the remaining input paths can also rise.
Depending on severity and drive design, the VFD may report Input Phase Loss directly or may also encounter DC-link undervoltage, overcurrent or related protection events.
Where should Input Phase Loss be checked?
Think in this order:
Mains → Disconnect / Breaker → Fuses → Input Contactor → Cable → VFD R-S-T terminals
Find which incoming phase is missing and trace that phase upstream.
Live three-phase measurements involve hazardous voltage and should be carried out only by qualified personnel using appropriate equipment and the manufacturer's procedures.
What is Output Phase Loss?
Output Phase Loss means that one of the expected U, V or W motor-current paths is missing or severely unbalanced.
For example, if current is present in U and W while the V-phase current remains close to zero, the drive may interpret the condition as a motor phase loss.
Manufacturers may call the fault Output Phase Loss, Motor Phase Loss, OPL or SPO.
Common causes of Output Phase Loss
Open motor cable: One U, V or W conductor has broken.
Loose motor terminal: A terminal lug has loosened, burned or broken.
Open motor winding: One of the three motor winding paths is electrically open.
Output contactor problem: One pole of a contactor between the drive and motor is not conducting.
Connector or terminal problem: Oxidation, overheating or vibration creates an intermittent open phase.
Drive-side problem: Less commonly, an inverter power-device, gate-driver or current-measurement problem can create abnormal phase-current behaviour.
What happens to the motor during Output Phase Loss?
If one motor phase is genuinely open, the three-phase magnetic field becomes unbalanced.
Available torque can fall, phase currents can become unbalanced, vibration can increase and motor heating can rise.
Modern drives are intended to detect the abnormal current pattern and stop the output before prolonged operation causes serious thermal damage.
This is why a VFD-controlled motor does not necessarily continue visibly “single-phasing” for minutes before the fault is detected.
Output Phase Loss vs Output Short Circuit
They are nearly opposite electrical problems.
With an Output Short Circuit, an unwanted low-impedance current path appears and current rises very rapidly.
With Output Phase Loss, an expected current path has disappeared.
Short circuit: an unwanted path has appeared.
Phase loss: a required path has disappeared.
Output Phase Loss vs Ground Fault
A Ground Fault means motor current has found an unintended path to earth or the motor frame.
Output Phase Loss means an expected motor-phase current is absent or abnormally low.
Severe electrical imbalance can sometimes activate more than one protection algorithm, so the manufacturer's exact fault description remains important.
Can phase loss cause an Overcurrent fault?
Yes, under some conditions.
Loss of one motor-side phase can create severe current imbalance and may contribute to an OC / Overcurrent trip.
This is why some VFD fault tables list output phase loss among possible causes of overcurrent.
Phase loss vs phase imbalance
Phase loss means a phase is electrically missing or nearly absent.
Phase imbalance means all three phases are present, but their voltage or current values differ significantly.
Severe imbalance may eventually cross a manufacturer's phase-loss detection threshold, but the concepts are not identical.
Phase loss vs phase sequence
Phase sequence describes the order of the three supply phases.
All three phases can be present while the sequence changes from R-S-T to R-T-S.
On many conventional diode-input VFDs, changing input phase sequence does not directly reverse the motor because the motor output is independently generated by the inverter stage.
Other equipment in the plant can still depend on correct phase sequence.
Why is mains single-phasing different from VFD input phase loss?
On a motor connected directly to the three-phase mains, losing one supply phase immediately removes one motor phase. This is commonly called single phasing.
A VFD inserts a rectifier, DC link and inverter between the supply and the motor.
Input phase loss therefore first affects the VFD's supply and DC-link system rather than directly removing the corresponding motor-output phase.
How does a VFD detect Output Phase Loss?
Many drives compare the measured U, V and W phase currents. If one phase current remains abnormally small for a defined period, the drive can generate a motor-phase-loss fault.
Detection is not equally reliable under every possible operating condition.
For example, ABB notes that on some drives motor phase-loss supervision becomes active only above a defined minimum frequency, and very small motors connected to much larger drives can require special consideration because phase current may be close to the detection threshold. :contentReference[oaicite:5]{index=5}
Therefore it is not correct to assume that every VFD will detect every output phase-loss condition under all loads and speeds.
A practical phase-loss troubleshooting sequence
1. Determine whether the code says Input or Output Phase Loss. This immediately tells you which side of the drive to investigate first.
2. Record when the fault occurs. At power-up, when RUN is commanded, under load or intermittently?
3. For Input Phase Loss, inspect the R-S-T supply path. Check fuses, breaker, contactor, wiring and terminals.
4. For Output Phase Loss, inspect the U-V-W motor path. Check motor cable, terminal box, output contactor and motor windings.
5. Look for heat and intermittent connections. A loose terminal can operate normally when cold and open when current heats the connection.
6. Review the fault history. A simultaneous Undervoltage, overcurrent or other fault can provide an important clue.
7. Correct the cause before repeatedly resetting the VFD.
Should motor winding resistance be checked?
Comparing U-V, V-W and W-U winding resistance can be a useful first test.
An open winding may be obvious.
However, similar resistance values do not prove that motor insulation is healthy under operating voltage and temperature.
Winding resistance and insulation resistance are different tests.
Any insulation-resistance test should be carried out with the motor and cable disconnected from the VFD. Do not apply a megohmmeter test voltage through the drive output terminals.
Can a phase-protection relay detect phase loss?
A Phase Protection Relay can monitor a three-phase mains supply for phase loss, phase sequence and voltage imbalance.
It can therefore be useful on the input side of a VFD as part of the wider plant protection design.
Many modern VFDs already include their own input-phase-loss supervision, so an external relay is not automatically required in every system.
Can a normal phase-monitoring relay be connected to VFD output?
Not unless the device is specifically designed for PWM/VFD output.
The U-V-W waveform from a VFD is not a conventional clean 50 or 60 Hz mains sine wave. It contains high-frequency PWM switching pulses.
A conventional mains phase-monitoring relay can misinterpret that waveform or may not be electrically suitable for it.
Where external motor-side phase monitoring is required, use equipment explicitly approved for variable-frequency PWM output.
What about single-phase-input VFDs?
A VFD specifically designed for single-phase input does not use three-phase input-loss supervision in the same way as a three-phase-input drive.
A three-phase-input drive should not simply be supplied from single-phase power unless the manufacturer explicitly permits it and provides the necessary derating and installation instructions.
Editor's field note
The first question in a phase-loss fault should not be “which phase is missing?” It should be “where is the phase missing?”
If the problem is on R-S-T, think supply, fuse, contactor and DC bus.
If the problem is on U-V-W, think motor cable, motor terminal box, output contactor and motor winding.
That one distinction can save a great deal of unnecessary troubleshooting time.
Phase Loss FAQ
Are Input Phase Loss and Output Phase Loss the same fault?
No. Input Phase Loss is on the VFD supply side; Output Phase Loss is on the motor side.
If one input phase disappears, does the motor instantly lose one phase?
Not in a VFD system. The AC supply is first converted to DC and the inverter independently creates the motor-side three-phase PWM output. The drive input and DC-link system, however, can be heavily stressed and the VFD will normally trip.
Can Output Phase Loss overheat the motor?
Yes if the condition persists, although a modern drive normally attempts to detect the imbalance and stop operation before prolonged overheating occurs.
Can a blown input fuse cause Input Phase Loss?
Yes. One open fuse on a three-phase VFD supply is a classic cause.
Can a loose terminal create intermittent phase loss?
Yes. Connections can open only after heating or under vibration, making intermittent phase-loss faults difficult to find.
Is Output Phase Loss the same as a short circuit?
No. A short circuit creates an unwanted low-impedance path; phase loss removes an expected current path.
Can phase loss cause an OC fault?
Yes. Severe current imbalance associated with a missing motor phase can contribute to an overcurrent trip.
Does every VFD need an external phase-protection relay?
No. Many drives include their own phase-loss supervision. External monitoring depends on the overall plant-protection design.
Can I connect a normal phase relay to U-V-W?
Only if it is explicitly rated for VFD/PWM output.
If a phase-loss alarm disappears after reset, can it be ignored?
No. An intermittent open connection can be an early warning of a burned contact, loose terminal or damaged cable.
Related VFD fault topics
VFD Fault Codes · OC / Overcurrent Fault · Output Short Circuit · Ground Fault · Under Voltage