When a variable frequency drive reports an overtemperature alarm, the first reaction is often to check whether the motor current has exceeded the VFD’s rated current.

High output current can certainly increase heat generation. However, a VFD may still overheat even when the displayed motor current appears normal. This is because the drive’s internal temperature is affected by many factors other than motor current.

Carrier frequency, ambient temperature, cabinet ventilation, cooling fan condition, dust accumulation, installation spacing, altitude, and load cycle can all influence the temperature of the power components.

Understanding these factors helps engineers identify the real cause of overheating instead of simply replacing the VFD with a larger model.

Where Does Heat Inside a VFD Come From?

A VFD contains rectifier components, a DC bus, capacitors, power semiconductor modules, control circuits, and cooling components.

During operation, current passing through the rectifier and power modules creates conduction losses. At the same time, the output semiconductor devices switch on and off rapidly to generate the PWM waveform required to control the motor.

Every switching event produces a small amount of energy loss. The heat generated by a single event may be limited, but thousands of switching events occur every second. The accumulated switching loss can therefore become an important part of the VFD’s internal heat.

Additional heat may also come from the charging circuit, DC bus components, internal braking unit, control power supply, and surrounding equipment inside the electrical cabinet.

Why Does Carrier Frequency Affect VFD Temperature?

Carrier frequency determines how frequently the VFD’s output semiconductor devices switch.

A higher carrier frequency can reduce audible motor noise and may improve the smoothness of the motor current waveform. For this reason, some users increase the carrier frequency when the motor produces an obvious high-frequency sound.

However, increasing the carrier frequency also increases the number of switching events per second. More switching events generally mean higher switching losses and additional heat inside the VFD.

The motor current may remain below the rated value while the VFD temperature continues to rise. In this situation, the current display alone does not show the complete thermal condition of the drive.

Carrier frequency should therefore be selected according to the motor, application, cable length, noise requirements, ambient temperature, and VFD manufacturer’s recommendations. Setting it unnecessarily high may reduce the available output capacity or require derating.

How Does Ambient Temperature Affect Cooling?

A VFD transfers heat from its power components to a heat sink and then releases that heat into the surrounding air.

If the air entering the VFD is already hot, the temperature difference between the heat sink and the surrounding environment becomes smaller. The cooling system can no longer remove heat as effectively.

This situation is common in enclosed control cabinets, outdoor installations exposed to sunlight, factories with ovens or heating equipment, and environments where several high-power devices are installed close together.

The temperature inside an electrical cabinet can be significantly higher than the room temperature. Measuring only the workshop temperature may therefore lead to an incorrect conclusion. The temperature should be checked near the VFD’s air inlet under actual operating conditions.

Why Is Cabinet Ventilation Important?

A cooling fan can move air only when there is a clear path for airflow. If the cabinet inlet, outlet, filter, or VFD air duct is blocked, hot air may circulate inside the cabinet instead of being discharged.

Common installation problems include:

· Insufficient clearance above or below the VFD

· Several VFDs installed too closely together

· Hot air from a lower drive entering the drive above it

· Cabinet air inlets that are too small

· Incorrect fan direction

· Dust-clogged filters or heat sinks

· Cables blocking the VFD ventilation path

· Cabinet doors kept closed without sufficient ventilation

In dusty environments, filters and heat sinks require regular inspection and cleaning. A filter may appear acceptable from the outside while its internal airflow has already been significantly reduced.

Can a Cooling Fan Cause Intermittent Overheating?

Yes. A cooling fan may still rotate but no longer provide sufficient airflow because of bearing wear, dust, reduced speed, or unstable power supply.

This can create intermittent overheating. The VFD may operate normally when the load or ambient temperature is low but report an overtemperature alarm during longer production cycles or warmer periods.

Maintenance personnel should check not only whether the fan rotates, but also whether the airflow is strong and stable. Fan operating time and replacement recommendations should also be considered.

Why Does Altitude Require Derating?

At higher altitudes, air density decreases. Lower air density reduces the amount of heat that can be removed by the same airflow.

Even if the ambient temperature is not particularly high, a VFD operating at high altitude may have lower cooling capacity than the same model installed near sea level.

The insulation and cooling requirements of electrical equipment can also change with altitude. When an installation exceeds the manufacturer’s standard operating altitude, the applicable derating requirement should be confirmed.

What Does VFD Derating Mean?

Derating means operating the VFD below its nominal output capacity when environmental or operating conditions are more demanding than the standard rated conditions.

Derating may be required because of high ambient temperature, high altitude, elevated carrier frequency, demanding load cycles, or installation inside a poorly ventilated cabinet.

For example, a VFD with a certain rated current under standard conditions may need to operate at a lower continuous output current in a hot environment. Alternatively, a higher-power VFD may be selected to provide sufficient thermal margin.

Derating is not an indication that the VFD is defective. It is an engineering method used to maintain reliability and service life under non-standard conditions.

How Should an Overheating Problem Be Investigated?

When a VFD reports an overtemperature alarm, engineers should check:

· Actual output current and load cycle

· Ambient temperature near the VFD air inlet

· Cabinet internal temperature

· Carrier frequency setting

· Cooling fan operation and airflow

· Heat sink and filter cleanliness

· Installation clearance

· Cabinet inlet and exhaust design

· Altitude

· Nearby heat-producing equipment

· Frequency and duration of acceleration, deceleration, and braking

The alarm history should also be reviewed. If overheating occurs only at a particular time of day, production stage, or load condition, that pattern may help identify the cause.

Using VEIKONG VFDs in High-Temperature Applications

VEIKONG provides VFD solutions for a wide range of industrial motor applications. Correct selection, installation, ventilation, parameter settings, and maintenance are essential for reliable operation.

For applications involving high ambient temperatures, enclosed cabinets, high altitude, elevated carrier frequency, or continuous heavy loads, customers should provide the relevant operating conditions during product selection.

VEIKONG’s sales and engineering teams can help evaluate the application, recommend a suitable model, and determine whether additional power margin or derating should be considered.

A normal motor current reading does not always mean that the VFD’s thermal condition is normal. By examining the complete cooling environment and operating conditions, engineers can solve overheating problems more accurately and protect the long-term reliability of the drive system.

Related words:

VFD overtemperature alarm

VFD carrier frequency and heat

VFD derating in high ambient temperature