Causes, Diagnosis, and Practical Solutions

Short Description

A motor can overheat during VFD operation even when the displayed current appears normal. This guide explains the effects of low-speed cooling, mechanical load, motor parameters, carrier frequency, PWM voltage, cable length, and operating conditions.

Introduction

Motor temperature is determined by a thermal balance. The heat produced inside the motor must remain lower than the heat that the motor can remove through its frame, cooling fan, and surrounding air.

A normal current reading does not always confirm safe motor operation.

A motor may operate near its nameplate current and still overheat because of:

• Reduced cooling at low speed
• Continuous high-torque operation
• Mechanical overload or excessive friction
• Incorrect motor parameters
• Frequent acceleration, stopping, or reversing
• Excessive DC Injection Braking
• Unsuitable Carrier Frequency settings
• PWM-related electrical losses
• Long motor cables
• High ambient temperature
• Blocked ventilation

Effective troubleshooting should evaluate the complete motor, VFD, cable, load, and operating environment.

Why Current Alone Is Not Enough

Motor heating depends on both electrical loading and cooling capacity.

The electrical loading includes:

• Copper losses in the windings
• Iron losses in the motor core
• PWM-related additional losses
• Starting and braking duty
• Mechanical torque demand

The cooling capacity depends on:

• Motor speed
• Cooling-fan airflow
• Ambient temperature
• Motor cleanliness
• Installation clearance
• Ventilation
• Motor design

The motor may therefore operate at an acceptable current while its cooling capacity is insufficient.

Low-Speed Motor Cooling

Many standard TEFC motors use a cooling fan mounted directly on the motor shaft.

When the VFD reduces motor speed, the cooling fan also slows down. This reduces the airflow across the motor frame.

At the same time, a constant-torque application may continue to require significant motor current.

This creates a difficult operating condition:

• Motor losses remain relatively high
• Cooling airflow is reduced
• Motor temperature gradually increases

Higher-Risk Applications

Continuous low-speed operation can be especially demanding in:

• Conveyors
• Mixers
• Extruders
• Positive-displacement equipment
• High-friction machinery
• Constant-torque applications

Lower-Risk Applications

Variable-torque loads often require less torque as speed decreases.

Examples include:

• Centrifugal fans
• Blowers
• Centrifugal pumps

These applications may produce less motor heat at reduced speed, but overheating can still occur because of poor ventilation, incorrect settings, or mechanical problems.

Practical Low-Speed Solutions

Set an Appropriate Minimum Speed

Set a minimum operating frequency that considers:

• Motor cooling capability
• Required torque
• Operating duration
• Process requirements
• Motor manufacturer recommendations

There is no single minimum-frequency value suitable for every motor.

Use an External Cooling Fan

An independently powered cooling fan can maintain airflow regardless of motor speed.

Forced ventilation should be considered when:

• Continuous low-speed torque is required
• The motor operates for long periods below its base speed
• The process does not allow a higher minimum speed
• Motor temperature rises despite acceptable current
• The motor manufacturer requires independent cooling

The external fan may also be connected to an interlock that prevents loaded motor operation if the cooling fan fails.

Apply Motor Derating

Motor derating means reducing the permitted load when operating conditions reduce the motor’s cooling capacity.

Derating may be required because of:

• Low operating speed
• High ambient temperature
• High installation altitude
• Enclosed installation
• Frequent starting and stopping
• Insufficient airflow
• Demanding duty cycles

The required derating should follow the motor manufacturer’s data.

Mechanical Load Problems

Motor overheating is not always caused by the VFD.

Possible mechanical causes include:

• Damaged bearings
• Incorrect shaft alignment
• Excessive belt tension
• Blocked pump or fan
• Gearbox problems
• Excessive friction
• Product accumulation
• Process overload

Possible Warning Signs

Mechanical overload may cause:

• High motor current
• Slow acceleration
• Inability to reach the required speed
• Excessive vibration
• Abnormal sound
• Repeated overload trips
• Current-limit operation
• High motor and driven-equipment temperatures

The driven equipment should be inspected before replacing the motor or VFD.

Incorrect Motor Parameters

The VFD uses the motor nameplate data for motor control and electronic thermal protection.

Important parameters include:

• Rated voltage
• Rated current
• Rated frequency
• Rated speed
• Rated power
• Motor type
• Power factor, when required

Entering a motor current higher than the actual nameplate value may weaken the electronic thermal protection.

Incorrect motor data can also affect Vector Control and may cause:

• Excessive current
• Unstable torque
• Weak low-speed performance
• Motor overheating
• Unsuccessful or inaccurate autotuning

Recommended Checks

• Copy the motor nameplate data accurately
• Confirm Star or Delta connection
• Verify that the motor voltage matches the VFD system
• Select the correct motor type
• Perform the manufacturer-specified autotuning procedure
• Do not copy settings from another motor without verification

Never increase the motor-current setting simply to prevent overload trips. This may hide the actual problem and allow damaging temperatures.

Carrier Frequency and Motor Temperature

Carrier Frequency is the rate at which the VFD power semiconductors switch to produce the PWM output.

Increasing Carrier Frequency normally:

• Reduces audible motor noise
• Reduces current ripple
• Produces smoother motor current

However, it also increases the number of switching events per second.

Possible effects include:

• Increased VFD switching losses
• Higher VFD temperature
• Possible VFD derating
• Increased cable charging current
• Increased leakage current
• Increased common-mode current
• Greater stress on some filters and cables

Does a Higher Carrier Frequency Increase Motor Temperature?

The effect on the motor is not always the same.

A higher Carrier Frequency may reduce some current ripple and related motor losses. However, it may also increase high-frequency electrical effects, leakage current, and cable-related losses.

Motor temperature may therefore:

• Decrease slightly
• Increase slightly
• Remain almost unchanged

The result depends on the motor, VFD, cable, load, switching frequency, and installation.

Recommended Practice

Start with the VFD manufacturer’s default Carrier Frequency.

Increase it only for a defined reason, such as reducing audible motor noise, and then verify:

• VFD temperature
• Motor temperature
• VFD derating requirements
• Motor cable limits
• Leakage current
• Output-filter compatibility

PWM Voltage and Long Motor Cables

The VFD output voltage consists of fast PWM pulses rather than a pure sinusoidal voltage.

Long motor cables can increase:

• Reflected-wave voltage peaks
• Motor-insulation stress
• Leakage current
• Common-mode current
• Electromagnetic interference
• Ground-fault trips
• Bearing-current risk

These effects may occur without a clear increase in the average motor current.

Recommended Actions

• Use a cable suitable for VFD applications
• Review the maximum permitted cable length
• Ground the motor frame correctly
• Terminate the cable shield correctly
• Separate motor cables from signal cables
• Evaluate an Output Reactor
• Evaluate a dV/dt Filter
• Evaluate a Sine Wave Filter when necessary

The correct solution depends on the complete VFD, motor, cable, and application design.

Environmental and Ventilation Conditions

Even a correctly selected motor can overheat if it cannot release heat into the surrounding environment.

Check for:

• High ambient temperature
• Direct sunlight
• Nearby ovens or hot equipment
• Blocked cooling-fan cover
• Dust, oil, or contamination
• Restricted airflow
• Insufficient installation clearance
• Enclosed motor installation
• High altitude

The motor surface and fan cover should remain clean, and the cooling air must be able to enter and leave freely.

Frequent Starting, Stopping, and Braking

Every acceleration cycle requires additional torque to increase the speed of the motor and load.

Frequent operating cycles can increase average motor temperature, especially when combined with:

• Short Acceleration Times
• High-inertia loads
• Frequent reversing
• Jogging
• Repeated stopping
• DC Injection Braking
• Rapid load changes

The complete Duty Cycle should be reviewed instead of evaluating only the current during steady operation.

DC Injection Braking

DC Injection Braking applies DC current to the motor windings to produce braking torque.

The braking energy is converted into heat inside the motor.

Excessive DC Injection current or duration may cause:

• Winding overheating
• Motor overload
• Reduced insulation life
• Excessive heat during repeated stops

Review:

• DC Braking Current
• Injection Time
• Start Frequency
• Number of braking cycles
• Motor temperature

DC Injection Braking should not be used as a substitute for a correctly designed Dynamic Braking or mechanical-braking system.

Standard Motor vs Inverter-Duty Motor

Standard Motor

A standard motor may operate with a VFD within the limits specified by the motor and VFD manufacturers.

Possible limitations include:

• Reduced continuous torque at low speed
• Lower tolerance to repetitive PWM voltage
• Limited thermal protection
• Greater sensitivity to bearing currents
• Need for external cooling or derating

Inverter-Duty Motor

Depending on its design, an Inverter-Duty Motor may include:

• Improved winding insulation
• Wider constant-torque speed range
• Internal temperature sensors
• Independent cooling
• Insulated bearings
• Shaft-grounding provisions
• Improved operation with Vector Control

The actual capabilities must be confirmed from the motor datasheet. The term “Inverter Duty” alone does not define the full operating range.

Motor Temperature Protection

PTC Thermistor

A PTC thermistor changes resistance rapidly near a specified temperature.

It can be connected to:

• A dedicated VFD input
• A thermistor relay
• A motor-protection relay
• A PLC

It is commonly used to generate an alarm or trip.

PT100 Sensor

A PT100 provides a temperature-dependent resistance that allows continuous temperature measurement.

It can be used for:

• Temperature trending
• Warning alarms
• Motor shutdown
• Early detection of cooling deterioration
• Winding-temperature monitoring

Thermal Imaging

A thermal camera can help compare temperatures across:

• Motor frame
• Bearings
• Cable terminals
• Coupling
• Driven equipment

Surface temperature does not always represent the internal winding temperature, so internal sensors provide better thermal protection where available.

Troubleshooting Procedure

1. Record Operating Conditions

Record:

• Output Frequency
• Motor current on all phases
• Motor speed
• Motor temperature
• Ambient temperature
• Carrier Frequency
• Starts and stops per hour
• Braking cycles
• Fault history

2. Verify Motor Data

Confirm:

• Nameplate information
• Star or Delta connection
• Rated current
• Rated voltage
• Base frequency
• Rated speed
• Control mode
• Autotuning status

3. Inspect the Mechanical Load

Check:

• Bearings
• Alignment
• Belt tension
• Gearbox
• Pump or fan
• Mechanical blockage
• Process load

4. Inspect Motor Cooling

Check:

• Cooling-fan condition
• Fan-cover cleanliness
• Motor surface
• Airflow
• Installation clearance
• External cooling fan
• Ambient temperature

5. Review the VFD and Cable

Review:

• Carrier Frequency
• Acceleration Time
• Deceleration Time
• DC Injection settings
• Minimum speed
• Motor cable length
• Cable type
• Shielding and grounding
• Output-filter requirements

6. Test Under Controlled Conditions

When safe and practical:

• Reduce the mechanical load
• Increase motor speed
• Monitor current and temperature
• Compare loaded and unloaded operation
• Observe whether improved airflow reduces temperature

If the motor temperature falls when speed increases, reduced self-cooling may be a major cause.

Engineering Checklist

Before replacing the motor or VFD, confirm that:

• Motor Rated Current is entered correctly
• Current is measured on all phases
• Motor connection is correct
• Minimum operating speed is justified
• Mechanical load is acceptable
• Bearings and alignment are healthy
• Motor fan and ventilation openings are clean
• Ambient temperature is within limits
• Carrier Frequency is appropriate
• Acceleration and Deceleration Times are realistic
• DC Injection settings are not excessive
• Motor cable length and type are reviewed
• Grounding and shielding are correct
• Control mode is appropriate
• Autotuning is completed correctly
• Motor derating requirements are applied
• PTC or PT100 protection is used when required
• External cooling is considered for continuous low-speed duty

Technical Conclusion

Motor overheating during VFD operation is rarely caused by one parameter alone.

A complete diagnosis should consider:

• Motor current
• Motor speed
• Cooling airflow
• Mechanical load
• Motor parameters
• Carrier Frequency
• PWM effects
• Motor cable
• Ambient conditions
• Starting and braking Duty Cycle

Continuous low-speed operation with a constant-torque load is one of the most demanding conditions because motor losses may remain high while self-cooling airflow is reduced.

In these applications, the correct solution may involve:

• Increasing the minimum operating speed
• Reducing the load
• Applying motor derating
• Installing an external cooling fan
• Adding temperature protection
• Using a suitable Inverter-Duty Motor

Need Help Diagnosing Motor Overheating?

PowerWadi provides technical support for:

• VFD selection
• Motor thermal assessment
• Motor and load evaluation
• Parameter configuration
• Carrier Frequency review
• Motor cable and output-filter assessment
• Commissioning
• Fault diagnosis
• Industrial motor-control applications

Contact the PowerWadi technical team and provide the VFD model, motor nameplate data, load type, operating speed, cable length, temperature readings, and operating Duty Cycle.