Motor Tripping on VFD Under Load – Step-by-Step Diagnosis

 

In many industrial facilities, including cement, steel, and water treatment plants, one of the most common and challenging issues is motor tripping on VFD under load. This problem typically occurs only when the motor carries its full operational load, making it difficult to reproduce during no-load testing. Frequent trips can disrupt production, reduce equipment life, and cause unnecessary downtime.

Diagnosing motor trips under load requires a structured approach, combining electrical, mechanical, and parameter analysis. This guide provides a step-by-step diagnosis procedure that field engineers use to identify the root cause of motor trips and implement effective corrective measures.

1) What Does Tripping Under Load Actually Mean?

A motor trip under load occurs when the Variable Frequency Drive (VFD) shuts down the motor due to a protection limit being exceeded while carrying mechanical torque. Unlike no-load faults, these trips happen during active operation, when current, torque, or voltage exceed safe limits.

Typical trip types include:

• Overcurrent (OC)

• Overload (OL)

• Overvoltage (OV) / Undervoltage (UV)

• Ground Fault (GF)

Understanding the difference between instantaneous overcurrent and thermal overload is critical to isolating the cause.

2) Confirm the Exact VFD Trip Code (Critical First Step)

Before performing any mechanical or electrical inspections, check the VFD’s fault and alarm logs.

Key steps:

• Review alarm history for OC, OL, OV, UV, or GF codes

• Check timestamped events to correlate trips with operational conditions

• Identify whether the trip occurs at start-up, during ramp-up, or at full load

Different VFD manufacturers (ABB, Danfoss, Siemens, Schneider, VACON) have unique fault codes, but recognizing the exact code significantly reduces troubleshooting time.

3) Check Mechanical Load Conditions Before Touching the VFD

Mechanical issues are the root cause of most trips under load.

Common mechanical causes:

• Pump blockage or partially clogged piping

• Bearing failure or increased friction

• Belt or coupling misalignment

• Sudden load increase due to process variations

• Material jamming in mixers, conveyors, or crushers

If the torque demand exceeds motor capacity, the VFD triggers OC or OL protection immediately.

Read About: How to Build a Reliable Motor Control System

4) Verify Motor Parameters Entered Into the VFD (Critical)

Incorrect motor parameters can trigger false trips. Always confirm:

• Full Load Amps (FLA)

• Rated voltage and frequency

• Nominal RPM and slip compensation

• Power factor and motor efficiency

• Thermal class of the motor

• Auto-tuning settings in vector-controlled VFDs

Example: Entering a lower FLA than actual causes the drive to interpret normal current as overload.

5) Inspect Motor Current Under Different Load Levels

Monitoring motor current is essential:

• Measure phase currents (A/B/C) under 25%, 50%, 75%, and 100% load

• Identify imbalances between phases

• Compare VFD-displayed current vs. clamp meter readings

• Look for sudden spikes correlated with load increase

High current spikes indicate either excessive load, mechanical friction, or voltage drops.

6) Overcurrent (OC) Trip Root Causes Under Load

OC trips are the most frequent type of trip under load.

Typical causes:

• Sudden mechanical load surges

• Inadequate acceleration time

• Current limit configured too low

• Torque boost set incorrectly

• Weak upstream supply or voltage dips

• DC bus instability

OC trips generally occur instantly when the motor cannot meet torque demand.

7) Overload (OL) Trip Diagnosis

Overload trips are thermal and accumulate over time.

Root causes include:

• Motor overheating

• Long-term high torque operation

• Impeller oversized for pump applications

• High ambient temperatures (>50°C)

• Blocked ventilation or cooling issues

Unlike OC trips, OL trips often occur after minutes of operation.

8) Check Motor & Load for Mechanical or Thermal Stress

Inspect both motor and driven equipment:

• Shaft alignment and coupling wear

• Bearings and lubrication status

• Vibration and noise levels

• Cooling fan operation and airflow

• Environmental temperature

High mechanical or thermal stress can trigger both OC and OL trips.

9) Evaluate VFD Output Voltage & Frequency Stability

At high loads, voltage drops or frequency instability can cause trips:

• Voltage drop due to long cable runs

• Carrier frequency or IGBT switching noise

• Output voltage lower than required for torque demand

If the motor cannot reach rated torque due to low voltage, current spikes follow, leading to trip.

10) Power Supply Issues Leading to Load Trips

Power supply instability is often overlooked. Check for:

• Undervoltage or brownouts

• Phase imbalance

• Weak upstream transformers

• Voltage flicker during peak loads

• Loose or undersized cables

These issues cause the VFD to draw higher current and trip under load.

11) Motor Insulation or Earth Leakage Problems

High-load operation stresses insulation. Causes of trips include:

• Moisture ingress in motor windings

• Damaged or degraded insulation

• Partial discharge

• Ground faults

• High leakage current that triggers protective functions

Inspect insulation resistance and perform megger tests for accurate diagnosis.

12) Check Acceleration & Deceleration Profiles

Improper ramp settings can lead to trips under load:

• Too short acceleration or deceleration times

• Flying start disabled

• Torque boost improperly configured

Adjusting ramp times ensures smooth current increase and reduces mechanical stress.

13) Evaluate Torque Limits and Load Characteristics

Torque limits and V/f or vector settings are critical:

• Sensorless vector control requires correct tuning

• Incorrect torque limit or overload factor triggers trips

• Review load inertia and process dynamics

Ensuring torque limits match application prevents unnecessary trips.

14) Inspect Cooling Fans, Filters, and Heat Dissipation of the VFD

Thermal protection may trip the drive if:

• Cooling fans fail or filters are blocked

• Heat sinks are covered in dust

• Ambient temperature exceeds rated limits

Regular maintenance prevents temperature-related trips.

15) Run a Controlled “Test Under Load” to Isolate the Problem

Perform step-wise load tests:

• Gradually increase mechanical load while monitoring current and DC bus voltage

• Compare measured values to baseline

• Isolate whether electrical or mechanical factors dominate

Controlled testing reduces downtime while identifying root cause.

16) Analyze Trips Occurring Only at Specific Times

Some trips occur under specific production conditions:

• Peak load shifts

• High ambient temperature periods

• Upstream voltage fluctuations

Correlate trip times with plant operations to identify patterns.

17) Use Advanced Diagnostics Tools

Field engineers often use:

• Oscilloscopes to view voltage/current waveforms

• Thermal cameras to detect hotspots

• VFD trend logs for repeated patterns

• Harmonic analyzers for distorted supply

Advanced tools provide definitive evidence of root cause.

18) Root Cause Classification: Electrical vs Mechanical vs Parameter Error

After investigation, categorize trips into:

1. Electrical – supply issues, insulation, VFD output

2. Mechanical – load resistance, coupling, bearings

3. Parameter/configuration errors – FLA, torque limits, ramp times

Structured classification speeds corrective action.

19) Preventive Measures to Avoid Future Load Trips

• Perform periodic load analysis

• Verify motor and VFD parameters after each maintenance

• Implement preventive maintenance for bearings, fans, and cooling

• Monitor harmonic levels and voltage stability

• Ensure proper ventilation and ambient temperature control

20) When to Consider Motor or VFD Replacement

Replacement may be required if:

• Motor exhibits repeated thermal or insulation failures

• VFD reaches end-of-life, causing frequent OC/OL trips

• Process efficiency drops

• Retrofitting with a modern VFD improves performance and reliability

Conclusion

Motor tripping on VFD under load is a complex problem with electrical, mechanical, and parameter-related causes. A structured step-by-step diagnosis—starting with the VFD trip code, analyzing mechanical load, verifying parameters, inspecting supply, and using advanced diagnostics—ensures rapid identification of root causes. Implementing preventive maintenance, correct VFD configuration, and proper mechanical alignment minimizes trips, maximizes uptime, and protects critical industrial assets.