TECHNICAL SUPPORT
Published 2026-07-02
Quick Answer
To reset aservomotor phase loss alarm, you must first identify and correct the root cause—typically a missing or unstable input power phase, a blown fuse, a loose connection, or a faulty drive component. Simply cycling power or pressing a reset button without addressing the underlying issue will likely result in the alarm returning. The standard reset procedure involves verifying all three phases at the drive input, checking internal DC bus voltage, clearing the fault via the drive's keypad or control signal, and then performing a controlled restart. Always consult the specific drive manual, as reset logic varies by manufacturer and model.
Introduction
Picture this: a production line stops. The screen on yourservodrive flashes a phase loss alarm. The operator presses reset. The machine starts again—for ten minutes. Then it stops again. This cycle costs you production time, creates scrap, and forces your maintenance team into reactive mode. In many automated environments, aservomotor phase loss alarm is not a rare event, but it is often misunderstood.
The real problem is not the alarm itself. The alarm is a symptom. The underlying cause—whether it is a damaged contactor, a blown input fuse, a loose terminal, or an unbalanced power supply—will not go away by pressing a button. Yet many teams treat it as a nuisance rather than a signal. This article helps you understand what the alarm means, why it happens, and exactly how to reset it correctly without risking further damage to your drive or motor.
Table of Contents
1. What Does a Phase Loss Alarm Mean?
2. Common Causes of Phase Loss in Servo Systems
3. Why a Simple Reset Often Fails
4. Step-by-Step Reset Procedure
5. Key Specifications to Check Before Restarting
6. When the Drive Will Not Clear the Alarm
7. Questions Buyers Often Ask About Phase Loss Alarms
8. Making a Better Long-Term Decision
1. What Does a Phase Loss Alarm Mean?
A phase loss alarm indicates that the servo drive's input power is missing one of its three phases, or that the voltage on one phase has dropped below the drive's acceptable threshold. Most industrial servo drives require three-phase input to maintain a stable DC bus voltage. When a phase is lost, the remaining two phases must carry the full load, which causes increased current ripple, higher thermal stress on the bus capacitors, and unstable torque output.
In many drives, the alarm is triggered by monitoring the DC bus ripple frequency. If one phase is missing, the ripple pattern changes. The drive detects this and stops to protect itself and the motor. This is not a false alarm. It is a protective function designed to prevent capacitor failure, drive damage, or uncontrolled motor behavior.
2. Common Causes of Phase Loss in Servo Systems
Understanding what causes the alarm is the first step in resetting it correctly. The most common causes include:
Blown input fuses or tripped circuit breakerson one phase
Loose or corroded terminal connectionsin the main power path
Faulty contactor or relaynot closing all three poles
Unbalanced utility poweror a dropped phase from the transformer
Internal drive failuresuch as a damaged input rectifier or capacitor bank
In many cases, the cause is external to the drive. This is important because it means a reset without inspection will not solve the problem. You may clear the alarm once, but it will return as soon as the drive tries to draw power again.
3. Why a Simple Reset Often Fails
A common mistake is to reset the alarm and restart the machine immediately without checking the power supply. This approach works only if the lost phase was temporary—for example, a momentary dip caused by a large motor starting elsewhere. But in most industrial settings, phase loss is caused by a permanent or intermittent hardware fault.
When you reset without addressing the root cause, the drive attempts to pre-charge the DC bus again. If a phase is still missing, the bus voltage will not reach the required level, and the alarm will re-trigger. Repeated resets under these conditions can stress the drive's input circuits and reduce component life.
A proper reset procedure must include verification of all three input phases at the drive terminals, using a true RMS multimeter, before any reset attempt is made.
4. Step-by-Step Reset Procedure
Follow this sequence to safely reset a servo motor phase loss alarm. Do not skip steps.
Step 1: Isolate and lock out power
Before touching any terminals, disconnect main power and verify zero voltage at the drive input. Use a lockout tagout procedure.
Step 2: Inspect all input power components
Check fuses, circuit breakers, contactors, and terminal blocks for each phase. Look for signs of overheating, arcing, or mechanical damage.
Step 3: Measure voltage at the drive input
With power restored and under safe conditions, measure line-to-line and line-to-neutral voltage at the drive's input terminals. All three phases should be within the drive's rated voltage range, typically within ±10%.
Step 4: Check the DC bus voltage
If the drive has a DC bus test point, measure the voltage. On a 400 VAC system, the DC bus should be approximately 560 VDC. A significantly lower value may indicate a missing phase or failing capacitors.
Step 5: Clear the fault
Use the drive's keypad or software to clear the phase loss alarm. The exact procedure varies by brand. In many drives, this requires navigating to the fault log menu and selecting “clear faults” or “reset.”
Step 6: Perform a controlled restart
Start the drive in a low-load condition. Monitor the drive display for any recurring alarms. If the alarm does not return, gradually increase the load to normal operating levels.
Step 7: Log the event
Record the alarm code, the time, the actions taken, and the measured voltages. This log helps identify patterns and supports predictive maintenance.
5. Key Specifications to Check Before Restarting
Before you consider the reset successful, verify these parameters:
If any parameter falls outside the acceptable range, do not restart. Investigate the cause first.
6. When the Drive Will Not Clear the Alarm
Sometimes the alarm persists even after you have verified the input power. This usually points to internal damage. A failed input rectifier, a shorted IGBT, or degraded DC bus capacitors can cause the drive to detect a phase loss even when the input is normal.
In such cases, resetting via software or cycling power will not help. The drive requires repair or replacement. A competent technician can test the rectifier and bus capacitors with a multimeter and an ESR meter. However, if the drive is still under warranty, contact the manufacturer or yourservo motor supplierfor a formal evaluation.
If you are evaluating a new drive purchase, ask the supplier whether the drive offers configurable phase loss detection thresholds. Some drives allow you to adjust sensitivity, which can help reduce nuisance alarms in applications with marginal power quality.
7. Questions Buyers Often Ask About Phase Loss Alarms
Q: Can a phase loss alarm be caused by a bad motor?
No. Phase loss alarms monitor input power, not the motor. A bad motor typically triggers a different alarm, such as overcurrent or encoder fault.
Q: Will cycling main power clear the alarm permanently?
Only if the underlying cause is temporary. If the phase loss is due to a hardware fault, cycling power will not fix it.
Q: How do I know if the alarm is real or a false positive?
Measure all three input phases with a multimeter. If voltages are balanced and within range, the alarm may be a drive fault. If one phase is low or missing, the alarm is real.
Q: Can I bypass the phase loss detection?
Some drives allow disabling this protection, but it is not recommended. Running a drive on two phases significantly increases stress on the DC bus capacitors and can lead to catastrophic failure.
Q: What is the most common root cause in production environments?
Loose terminal connections and corroded contactor contacts are the most frequent causes. These are often intermittent and easy to miss.
Q: Should I replace the drive after a phase loss event?
Not necessarily. If the drive was only exposed to a brief phase loss and passed all electrical tests, it can continue operating. However, repeated phase loss events reduce drive life.
Q: Does phase loss damage the servo motor?
Indirectly, yes. Unstable DC bus voltage can cause erratic torque output, which may damage the motor shaft, bearings, or connected mechanical components over time.
Q: How fast does the drive detect phase loss?
Most drives detect phase loss within one to three AC cycles. This is fast enough to prevent damage in most applications.
Q: Can a phase loss alarm occur during a power dip?
Yes. A momentary voltage sag on one phase can trigger the alarm. In such cases, the alarm may clear automatically once voltage returns to normal, but the event should still be logged.
Q: Do all servo drives handle phase loss the same way?
No. Reset logic, detection thresholds, and alarm behavior vary between manufacturers. Always refer to the drive's manual for specific fault handling.
8. Making a Better Long-Term Decision
A phase loss alarm is not just a maintenance nuisance. It is a signal about your power system's health. Each time you ignore the root cause or rely on repeated resets, you increase the risk of drive failure, unplanned downtime, and production loss.
The most effective approach is to treat each alarm as an event worth investigating. Build a checklist, log every occurrence, and compare voltage readings over time. If you find that a single drive or a single line experiences frequent phase loss alarms, consider installing power monitoring equipment or upgrading the input protection components.
If your current drive is no longer reliable or if you are planning a new system, work with a supplier who understands your application. A knowledgeable partner can help you specify drives with appropriate input protection, configurable thresholds, and clear diagnostic feedback.
When you need support with a recurring alarm or are evaluating a replacement drive, contactkpowerservofor a technical review of your current setup. Send your drive specifications and fault history, and our engineering team can help you determine the correct next step—whether that is a repair, a replacement, or a system upgrade.
Update Time：2026-07-02
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