The rated power shown on a Variable Frequency Drive does not always represent the power that will be continuously available under every operating condition.
High ambient temperature, installation altitude, increased switching frequency, restricted ventilation, and the installation of multiple drives inside the same enclosure can reduce the continuous output current that the VFD can safely provide.
This reduction is known as derating.
Ignoring derating may cause repeated overtemperature trips, reduced service life, unexpected shutdowns, or premature failure of the drive’s electronic components.
What Is VFD Derating?
VFD derating is the intentional reduction of the drive’s allowable output current or power when it operates outside its standard rated conditions.
A VFD may be rated for a specific output current under defined conditions such as:
A specified ambient temperature
A maximum installation altitude
A standard switching frequency
Adequate ventilation
Required clearances around the drive
A defined load duty
When the actual operating conditions exceed these limits, the drive may no longer be able to deliver its full nameplate current continuously.
The available current must therefore be calculated using the derating data provided by the manufacturer.
Rated Power Does Not Always Mean Available Power
Two VFDs with the same kilowatt rating may not provide the same usable output under real installation conditions.
The correct selection should be based primarily on:
Motor nameplate current
Load type
Required overload capacity
Ambient temperature
Installation altitude
Switching frequency
Panel ventilation
Installation clearances
Number of drives inside the enclosure
The motor current must remain within the drive’s available output current after all required derating factors have been considered.
Why Does a VFD Generate Heat?
A VFD contains power electronic components that generate losses during operation.
The main sources of heat include:
Rectifier losses
DC bus losses
Inverter transistor switching losses
Conduction losses
Control circuit losses
Internal fan and auxiliary losses
The generated heat must be transferred from the internal components to the surrounding air.
When the surrounding air becomes hotter or less dense, the cooling system becomes less effective and the drive may need to operate at a reduced current.
- Ambient Temperature Derating
Every VFD has a specified ambient temperature range.
The manufacturer may allow the drive to operate at full rated current up to a certain temperature, such as 40°C or 50°C, depending on the model and installation method.
Above the specified full-load temperature, the allowable output current may need to be reduced.
For example, a manufacturer may require the current to be reduced by a certain percentage for every degree above the rated temperature.
The exact derating rate is not universal and must be taken from the drive’s technical manual.
Why Does High Temperature Reduce VFD Capacity?
Higher ambient temperature reduces the temperature difference between the drive’s heat sink and the surrounding air.
This decreases the ability of the heat sink and cooling fans to remove heat.
As the internal temperature rises, thermal stress increases on components such as:
IGBT or power transistor modules
DC bus capacitors
Rectifier modules
Control boards
Cooling fans
Insulation materials
Continuous operation at high temperature can significantly reduce component life, especially the life of electrolytic capacitors and cooling fans.
Practical Actions for High-Temperature Locations
Provide adequate panel ventilation
Use filtered ventilation fans when appropriate
Install an enclosure air conditioner when necessary
Maintain the clearances specified by the manufacturer
Avoid installing the VFD close to transformers, braking resistors, or other heat sources
Remove dust from filters and heat sinks
Measure the actual temperature inside the panel
Apply the manufacturer’s temperature derating curve
Select a larger VFD when the available current becomes lower than the motor requirement
- Altitude Derating
Air density decreases as altitude increases.
Lower air density reduces the cooling capability of the surrounding air and may also affect electrical insulation performance.
Many drives can operate at their full rating up to a specified altitude, often around 1,000 metres above sea level, although the exact limit depends on the manufacturer.
Above this altitude, output current derating may be required.
Why Does Altitude Affect the VFD?
At higher altitude:
The air removes less heat from the heat sink
Fan cooling becomes less effective
Internal component temperatures may rise
Electrical clearances and insulation performance become more critical
The exact altitude derating percentage must be taken from the drive manual.
Some manufacturers specify a current reduction per additional 100 metres above the rated altitude, while others provide a derating curve or table.
Additional High-Altitude Considerations
Check the maximum permitted installation altitude
Review both thermal and insulation limitations
Check whether input voltage restrictions apply
Apply the manufacturer’s altitude derating factor
Consider the combined effect of altitude and ambient temperature
Verify the performance of panel fans and cooling equipment
- Switching Frequency Derating
The switching frequency determines how frequently the VFD output transistors switch to create the motor voltage waveform.
A higher switching frequency can reduce audible motor noise and improve current waveform quality in some applications.
However, increasing the switching frequency also increases switching losses inside the VFD.
Effects of Increasing the Switching Frequency
Higher internal heat generation
Reduced continuous output-current capability
Possible need for drive derating
Increased common-mode and leakage currents
Higher stress on the motor insulation
Greater sensitivity to motor cable length
Possible increase in electromagnetic interference
When Is a Higher Switching Frequency Used?
Noise-sensitive applications
Applications requiring smoother motor current
Low-speed operation where audible noise is important
Specific motor-control requirements
The switching frequency should not be increased simply to make the motor quieter without checking the drive derating table.
In many drives, the full output current is only available at the standard factory switching frequency.
- Installation Inside Electrical Panels
The temperature around the drive is not always equal to the room temperature.
For example, the room temperature may be 35°C while the temperature inside the electrical panel reaches 50°C or more.
The VFD must be selected according to the actual temperature surrounding the drive inside the enclosure.
Factors That Increase Panel Temperature
Multiple VFDs installed in the same panel
Insufficient ventilation
Blocked air filters
Small enclosure dimensions
Direct sunlight
High ambient temperature
Braking resistors installed inside the panel
Transformers or reactors installed close to the drives
Incorrect drive spacing
Hot air recirculation inside the enclosure
Recommended Installation Practices
Maintain the required top, bottom, and side clearances
Arrange the airflow from the bottom of the panel to the top
Prevent hot exhaust air from one VFD from entering another
Install braking resistors outside the enclosure when appropriate
Separate major heat-producing components
Use thermostatically controlled fans
Calculate the total panel heat losses
Measure the internal temperature during full-load operation
- Installing Multiple VFDs Side by Side
Some VFDs can be installed side by side without spacing, while others require a minimum separation.
Side-by-side installation may reduce the ability of the drive to dissipate heat.
The manufacturer may therefore require:
Additional spacing
Reduced output current
Lower ambient temperature
Reduced switching frequency
Forced ventilation
The installation manual must be reviewed before using a compact side-by-side arrangement.
- Dust, Contamination, and Cooling Performance
Dust accumulation can reduce cooling performance even when the room temperature is within the permitted range.
Dust may block:
Air filters
Cooling fan openings
Heat sink passages
Internal air channels
Conductive or corrosive contamination may also damage electronic components.
Maintenance should include:
Cleaning or replacing panel filters
Inspecting cooling fans
Cleaning heat sinks according to manufacturer instructions
Checking fan alarms
Measuring the panel temperature
Keeping ventilation openings unobstructed
- Combining Multiple Derating Factors
Temperature, altitude, switching frequency, and installation conditions may affect the same drive at the same time.
The factors must not be considered independently if the installation is affected by more than one condition.
The manufacturer may provide:
A combined derating table
Separate correction factors
A selection software tool
A requirement to choose the next drive size
Do not automatically multiply all derating factors unless the manufacturer’s instructions allow this method.
Practical Example
Assume that a VFD has a rated output current of 100 A.
The manufacturer specifies:
Temperature factor: 0.90
Altitude factor: 0.95
If the manufacturer permits the factors to be multiplied:
Available output current = 100 × 0.90 × 0.95
Available output current = 85.5 A
If the connected motor has a rated current of 90 A, this VFD would not provide sufficient continuous current under the stated conditions.
A larger drive or an improvement in the installation conditions would be required.
This example is for explanation only. Actual derating must always follow the manufacturer’s technical data.
- Derating Versus Oversizing
Derating and oversizing are related but different concepts.
Derating determines how much current the selected VFD can safely provide under the actual conditions.
Oversizing means selecting a drive with a higher current rating than the motor requires.
Oversizing should not be used randomly.
The correct process is:
Determine the motor rated current
Identify the load duty
Check the required overload capacity
Apply all manufacturer derating requirements
Calculate the available drive current
Select a drive whose derated current remains above the motor requirement
- Normal Duty and Heavy Duty Ratings
Many VFDs have more than one rating.
Normal Duty is generally intended for loads with lower overload requirements, such as many centrifugal pumps and fans.
Heavy Duty is generally intended for constant-torque or high-overload applications such as:
Conveyors
Mixers
Crushers
Extruders
Positive-displacement pumps
The same VFD model may have different kilowatt and current ratings under Normal Duty and Heavy Duty operation.
Always compare the motor current and load duty with the correct VFD rating.
- Common Selection Mistakes
Selecting the VFD by motor kilowatts only
Ignoring the motor nameplate current
Using room temperature instead of panel temperature
Ignoring installation altitude
Increasing switching frequency without checking derating
Installing several drives with insufficient spacing
Placing braking resistors inside a poorly ventilated panel
Ignoring blocked filters and failed fans
Using the Normal Duty rating for a Heavy Duty application
Multiplying derating factors without checking the manufacturer’s method
Selecting the next kilowatt size without checking its output current
- Information Required Before Selecting the VFD
Motor rated power
Motor nameplate current
Supply voltage
Motor frequency
Load type
Required starting torque
Required overload capacity
Minimum and maximum operating speed
Ambient temperature
Expected panel temperature
Installation altitude
Selected switching frequency
Motor cable length
Number of drives inside the panel
Enclosure dimensions
Ventilation or air-conditioning method
Installation clearances
Presence of reactors, filters, or braking resistors
- Practical Selection Procedure
Step 1
Record the motor nameplate current.
Step 2
Classify the load as Normal Duty or Heavy Duty.
Step 3
Determine the required overload capacity.
Step 4
Identify the maximum temperature around the VFD.
Step 5
Identify the installation altitude.
Step 6
Select the required switching frequency.
Step 7
Check enclosure ventilation and drive clearances.
Step 8
Apply the manufacturer’s derating data.
Step 9
Compare the derated VFD current with the motor rated current.
Step 10
Select a larger drive or improve the installation conditions when necessary.