The rated power shown on a Variable Frequency Drive does not always represent the power that will be continuously available under every operating condition.<\/p>\n\n\n\n
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.<\/p>\n\n\n\n
This reduction is known as derating.<\/p>\n\n\n\n
Ignoring derating may cause repeated overtemperature trips, reduced service life, unexpected shutdowns, or premature failure of the drive\u2019s electronic components.<\/p>\n\n\n\n
What Is VFD Derating?<\/p>\n\n\n\n
VFD derating is the intentional reduction of the drive\u2019s allowable output current or power when it operates outside its standard rated conditions.<\/p>\n\n\n\n
A VFD may be rated for a specific output current under defined conditions such as:<\/p>\n\n\n\n
A specified ambient temperature<\/p>\n\n\n\n
A maximum installation altitude<\/p>\n\n\n\n
A standard switching frequency<\/p>\n\n\n\n
Adequate ventilation<\/p>\n\n\n\n
Required clearances around the drive<\/p>\n\n\n\n
A defined load duty<\/p>\n\n\n\n
When the actual operating conditions exceed these limits, the drive may no longer be able to deliver its full nameplate current continuously.<\/p>\n\n\n\n
The available current must therefore be calculated using the derating data provided by the manufacturer.<\/p>\n\n\n\n
Rated Power Does Not Always Mean Available Power<\/p>\n\n\n\n
Two VFDs with the same kilowatt rating may not provide the same usable output under real installation conditions.<\/p>\n\n\n\n
The correct selection should be based primarily on:<\/p>\n\n\n\n
Motor nameplate current<\/p>\n\n\n\n
Load type<\/p>\n\n\n\n
Required overload capacity<\/p>\n\n\n\n
Ambient temperature<\/p>\n\n\n\n
Installation altitude<\/p>\n\n\n\n
Switching frequency<\/p>\n\n\n\n
Panel ventilation<\/p>\n\n\n\n
Installation clearances<\/p>\n\n\n\n
Number of drives inside the enclosure<\/p>\n\n\n\n
The motor current must remain within the drive\u2019s available output current after all required derating factors have been considered.<\/p>\n\n\n\n
Why Does a VFD Generate Heat?<\/p>\n\n\n\n
A VFD contains power electronic components that generate losses during operation.<\/p>\n\n\n\n
The main sources of heat include:<\/p>\n\n\n\n
Rectifier losses<\/p>\n\n\n\n
DC bus losses<\/p>\n\n\n\n
Inverter transistor switching losses<\/p>\n\n\n\n
Conduction losses<\/p>\n\n\n\n
Control circuit losses<\/p>\n\n\n\n
Internal fan and auxiliary losses<\/p>\n\n\n\n
The generated heat must be transferred from the internal components to the surrounding air.<\/p>\n\n\n\n
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.<\/p>\n\n\n\n
Every VFD has a specified ambient temperature range.<\/p>\n\n\n\n
The manufacturer may allow the drive to operate at full rated current up to a certain temperature, such as 40\u00b0C or 50\u00b0C, depending on the model and installation method.<\/p>\n\n\n\n
Above the specified full-load temperature, the allowable output current may need to be reduced.<\/p>\n\n\n\n
For example, a manufacturer may require the current to be reduced by a certain percentage for every degree above the rated temperature.<\/p>\n\n\n\n
The exact derating rate is not universal and must be taken from the drive\u2019s technical manual.<\/p>\n\n\n\n
Why Does High Temperature Reduce VFD Capacity?<\/p>\n\n\n\n
Higher ambient temperature reduces the temperature difference between the drive\u2019s heat sink and the surrounding air.<\/p>\n\n\n\n
This decreases the ability of the heat sink and cooling fans to remove heat.<\/p>\n\n\n\n
As the internal temperature rises, thermal stress increases on components such as:<\/p>\n\n\n\n
IGBT or power transistor modules<\/p>\n\n\n\n
DC bus capacitors<\/p>\n\n\n\n
Rectifier modules<\/p>\n\n\n\n
Control boards<\/p>\n\n\n\n
Cooling fans<\/p>\n\n\n\n
Insulation materials<\/p>\n\n\n\n
Continuous operation at high temperature can significantly reduce component life, especially the life of electrolytic capacitors and cooling fans.<\/p>\n\n\n\n
Practical Actions for High-Temperature Locations<\/p>\n\n\n\n
Provide adequate panel ventilation<\/p>\n\n\n\n
Use filtered ventilation fans when appropriate<\/p>\n\n\n\n
Install an enclosure air conditioner when necessary<\/p>\n\n\n\n
Maintain the clearances specified by the manufacturer<\/p>\n\n\n\n
Avoid installing the VFD close to transformers, braking resistors, or other heat sources<\/p>\n\n\n\n
Remove dust from filters and heat sinks<\/p>\n\n\n\n
Measure the actual temperature inside the panel<\/p>\n\n\n\n
Apply the manufacturer\u2019s temperature derating curve<\/p>\n\n\n\n
Select a larger VFD when the available current becomes lower than the motor requirement<\/p>\n\n\n\n
Air density decreases as altitude increases.<\/p>\n\n\n\n
Lower air density reduces the cooling capability of the surrounding air and may also affect electrical insulation performance.<\/p>\n\n\n\n
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.<\/p>\n\n\n\n
Above this altitude, output current derating may be required.<\/p>\n\n\n\n
Why Does Altitude Affect the VFD?<\/p>\n\n\n\n
At higher altitude:<\/p>\n\n\n\n
The air removes less heat from the heat sink<\/p>\n\n\n\n
Fan cooling becomes less effective<\/p>\n\n\n\n
Internal component temperatures may rise<\/p>\n\n\n\n
Electrical clearances and insulation performance become more critical<\/p>\n\n\n\n
The exact altitude derating percentage must be taken from the drive manual.<\/p>\n\n\n\n
Some manufacturers specify a current reduction per additional 100 metres above the rated altitude, while others provide a derating curve or table.<\/p>\n\n\n\n
Additional High-Altitude Considerations<\/p>\n\n\n\n
Check the maximum permitted installation altitude<\/p>\n\n\n\n
Review both thermal and insulation limitations<\/p>\n\n\n\n
Check whether input voltage restrictions apply<\/p>\n\n\n\n
Apply the manufacturer\u2019s altitude derating factor<\/p>\n\n\n\n
Consider the combined effect of altitude and ambient temperature<\/p>\n\n\n\n
Verify the performance of panel fans and cooling equipment<\/p>\n\n\n\n
The switching frequency determines how frequently the VFD output transistors switch to create the motor voltage waveform.<\/p>\n\n\n\n
A higher switching frequency can reduce audible motor noise and improve current waveform quality in some applications.<\/p>\n\n\n\n
However, increasing the switching frequency also increases switching losses inside the VFD.<\/p>\n\n\n\n
Effects of Increasing the Switching Frequency<\/p>\n\n\n\n
Higher internal heat generation<\/p>\n\n\n\n
Reduced continuous output-current capability<\/p>\n\n\n\n
Possible need for drive derating<\/p>\n\n\n\n
Increased common-mode and leakage currents<\/p>\n\n\n\n
Higher stress on the motor insulation<\/p>\n\n\n\n
Greater sensitivity to motor cable length<\/p>\n\n\n\n
Possible increase in electromagnetic interference<\/p>\n\n\n\n
When Is a Higher Switching Frequency Used?<\/p>\n\n\n\n
Noise-sensitive applications<\/p>\n\n\n\n
Applications requiring smoother motor current<\/p>\n\n\n\n
Low-speed operation where audible noise is important<\/p>\n\n\n\n
Specific motor-control requirements<\/p>\n\n\n\n
The switching frequency should not be increased simply to make the motor quieter without checking the drive derating table.<\/p>\n\n\n\n
In many drives, the full output current is only available at the standard factory switching frequency.<\/p>\n\n\n\n
The temperature around the drive is not always equal to the room temperature.<\/p>\n\n\n\n
For example, the room temperature may be 35\u00b0C while the temperature inside the electrical panel reaches 50\u00b0C or more.<\/p>\n\n\n\n
The VFD must be selected according to the actual temperature surrounding the drive inside the enclosure.<\/p>\n\n\n\n
Factors That Increase Panel Temperature<\/p>\n\n\n\n
Multiple VFDs installed in the same panel<\/p>\n\n\n\n
Insufficient ventilation<\/p>\n\n\n\n
Blocked air filters<\/p>\n\n\n\n
Small enclosure dimensions<\/p>\n\n\n\n
Direct sunlight<\/p>\n\n\n\n
High ambient temperature<\/p>\n\n\n\n
Braking resistors installed inside the panel<\/p>\n\n\n\n
Transformers or reactors installed close to the drives<\/p>\n\n\n\n
Incorrect drive spacing<\/p>\n\n\n\n
Hot air recirculation inside the enclosure<\/p>\n\n\n\n
Recommended Installation Practices<\/p>\n\n\n\n
Maintain the required top, bottom, and side clearances<\/p>\n\n\n\n
Arrange the airflow from the bottom of the panel to the top<\/p>\n\n\n\n
Prevent hot exhaust air from one VFD from entering another<\/p>\n\n\n\n
Install braking resistors outside the enclosure when appropriate<\/p>\n\n\n\n
Separate major heat-producing components<\/p>\n\n\n\n
Use thermostatically controlled fans<\/p>\n\n\n\n
Calculate the total panel heat losses<\/p>\n\n\n\n
Measure the internal temperature during full-load operation<\/p>\n\n\n\n
Some VFDs can be installed side by side without spacing, while others require a minimum separation.<\/p>\n\n\n\n
Side-by-side installation may reduce the ability of the drive to dissipate heat.<\/p>\n\n\n\n
The manufacturer may therefore require:<\/p>\n\n\n\n
Additional spacing<\/p>\n\n\n\n
Reduced output current<\/p>\n\n\n\n
Lower ambient temperature<\/p>\n\n\n\n
Reduced switching frequency<\/p>\n\n\n\n
Forced ventilation<\/p>\n\n\n\n
The installation manual must be reviewed before using a compact side-by-side arrangement.<\/p>\n\n\n\n
Dust accumulation can reduce cooling performance even when the room temperature is within the permitted range.<\/p>\n\n\n\n
Dust may block:<\/p>\n\n\n\n
Air filters<\/p>\n\n\n\n
Cooling fan openings<\/p>\n\n\n\n
Heat sink passages<\/p>\n\n\n\n
Internal air channels<\/p>\n\n\n\n
Conductive or corrosive contamination may also damage electronic components.<\/p>\n\n\n\n
Maintenance should include:<\/p>\n\n\n\n
Cleaning or replacing panel filters<\/p>\n\n\n\n
Inspecting cooling fans<\/p>\n\n\n\n
Cleaning heat sinks according to manufacturer instructions<\/p>\n\n\n\n
Checking fan alarms<\/p>\n\n\n\n
Measuring the panel temperature<\/p>\n\n\n\n
Keeping ventilation openings unobstructed<\/p>\n\n\n\n
Temperature, altitude, switching frequency, and installation conditions may affect the same drive at the same time.<\/p>\n\n\n\n
The factors must not be considered independently if the installation is affected by more than one condition.<\/p>\n\n\n\n
The manufacturer may provide:<\/p>\n\n\n\n
A combined derating table<\/p>\n\n\n\n
Separate correction factors<\/p>\n\n\n\n
A selection software tool<\/p>\n\n\n\n
A requirement to choose the next drive size<\/p>\n\n\n\n
Do not automatically multiply all derating factors unless the manufacturer\u2019s instructions allow this method.<\/p>\n\n\n\n
Practical Example<\/p>\n\n\n\n
Assume that a VFD has a rated output current of 100 A.<\/p>\n\n\n\n
The manufacturer specifies:<\/p>\n\n\n\n
Temperature factor: 0.90<\/p>\n\n\n\n
Altitude factor: 0.95<\/p>\n\n\n\n
If the manufacturer permits the factors to be multiplied:<\/p>\n\n\n\n
Available output current = 100 \u00d7 0.90 \u00d7 0.95<\/p>\n\n\n\n
Available output current = 85.5 A<\/p>\n\n\n\n
If the connected motor has a rated current of 90 A, this VFD would not provide sufficient continuous current under the stated conditions.<\/p>\n\n\n\n
A larger drive or an improvement in the installation conditions would be required.<\/p>\n\n\n\n
This example is for explanation only. Actual derating must always follow the manufacturer\u2019s technical data.<\/p>\n\n\n\n
Derating and oversizing are related but different concepts.<\/p>\n\n\n\n
Derating determines how much current the selected VFD can safely provide under the actual conditions.<\/p>\n\n\n\n
Oversizing means selecting a drive with a higher current rating than the motor requires.<\/p>\n\n\n\n
Oversizing should not be used randomly.<\/p>\n\n\n\n
The correct process is:<\/p>\n\n\n\n
Determine the motor rated current<\/p>\n\n\n\n
Identify the load duty<\/p>\n\n\n\n
Check the required overload capacity<\/p>\n\n\n\n
Apply all manufacturer derating requirements<\/p>\n\n\n\n
Calculate the available drive current<\/p>\n\n\n\n
Select a drive whose derated current remains above the motor requirement<\/p>\n\n\n\n
Many VFDs have more than one rating.<\/p>\n\n\n\n
Normal Duty is generally intended for loads with lower overload requirements, such as many centrifugal pumps and fans.<\/p>\n\n\n\n
Heavy Duty is generally intended for constant-torque or high-overload applications such as:<\/p>\n\n\n\n
Conveyors<\/p>\n\n\n\n
Mixers<\/p>\n\n\n\n
Crushers<\/p>\n\n\n\n
Extruders<\/p>\n\n\n\n
Positive-displacement pumps<\/p>\n\n\n\n
The same VFD model may have different kilowatt and current ratings under Normal Duty and Heavy Duty operation.<\/p>\n\n\n\n
Always compare the motor current and load duty with the correct VFD rating.<\/p>\n\n\n\n
Selecting the VFD by motor kilowatts only<\/p>\n\n\n\n
Ignoring the motor nameplate current<\/p>\n\n\n\n
Using room temperature instead of panel temperature<\/p>\n\n\n\n
Ignoring installation altitude<\/p>\n\n\n\n
Increasing switching frequency without checking derating<\/p>\n\n\n\n
Installing several drives with insufficient spacing<\/p>\n\n\n\n
Placing braking resistors inside a poorly ventilated panel<\/p>\n\n\n\n
Ignoring blocked filters and failed fans<\/p>\n\n\n\n
Using the Normal Duty rating for a Heavy Duty application<\/p>\n\n\n\n
Multiplying derating factors without checking the manufacturer\u2019s method<\/p>\n\n\n\n
Selecting the next kilowatt size without checking its output current<\/p>\n\n\n\n
Motor rated power<\/p>\n\n\n\n
Motor nameplate current<\/p>\n\n\n\n
Supply voltage<\/p>\n\n\n\n
Motor frequency<\/p>\n\n\n\n
Load type<\/p>\n\n\n\n
Required starting torque<\/p>\n\n\n\n
Required overload capacity<\/p>\n\n\n\n
Minimum and maximum operating speed<\/p>\n\n\n\n
Ambient temperature<\/p>\n\n\n\n
Expected panel temperature<\/p>\n\n\n\n
Installation altitude<\/p>\n\n\n\n
Selected switching frequency<\/p>\n\n\n\n
Motor cable length<\/p>\n\n\n\n
Number of drives inside the panel<\/p>\n\n\n\n
Enclosure dimensions<\/p>\n\n\n\n
Ventilation or air-conditioning method<\/p>\n\n\n\n
Installation clearances<\/p>\n\n\n\n
Presence of reactors, filters, or braking resistors<\/p>\n\n\n\n
Step 1<\/p>\n\n\n\n
Record the motor nameplate current.<\/p>\n\n\n\n
Step 2<\/p>\n\n\n\n
Classify the load as Normal Duty or Heavy Duty.<\/p>\n\n\n\n
Step 3<\/p>\n\n\n\n
Determine the required overload capacity.<\/p>\n\n\n\n
Step 4<\/p>\n\n\n\n
Identify the maximum temperature around the VFD.<\/p>\n\n\n\n
Step 5<\/p>\n\n\n\n
Identify the installation altitude.<\/p>\n\n\n\n
Step 6<\/p>\n\n\n\n
Select the required switching frequency.<\/p>\n\n\n\n
Step 7<\/p>\n\n\n\n
Check enclosure ventilation and drive clearances.<\/p>\n\n\n\n
Step 8<\/p>\n\n\n\n
Apply the manufacturer\u2019s derating data.<\/p>\n\n\n\n
Step 9<\/p>\n\n\n\n
Compare the derated VFD current with the motor rated current.<\/p>\n\n\n\n
Step 10<\/p>\n\n\n\n
Select a larger drive or improve the installation conditions when necessary.<\/p>\n\n\n\n
<\/p>","protected":false},"excerpt":{"rendered":"
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 […]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[100],"tags":[122,123,126,128,119,124,121,125,120,127],"class_list":["post-2673","post","type-post","status-publish","format-standard","hentry","category-vfd","tag-ambient-temperature","tag-installation-altitude","tag-motor-control","tag-panel-ventilation","tag-power-wadi","tag-switching-frequency","tag-variable-frequency-drive","tag-vfd-cooling","tag-vfd-derating","tag-vfd-selection"],"blocksy_meta":[],"yoast_head":"\n