A motor does not always stop immediately when the VFD output is removed.<\/p>\n\n\n\n
Fans, pumps, centrifuges, and other high-inertia loads may continue rotating because of stored mechanical energy, airflow, fluid movement, or external forces.<\/p>\n\n\n\n
If the VFD applies a normal start command while the motor is still rotating, the output frequency may not match the actual motor speed. This can cause high current, torque shock, reverse torque, or a VFD trip.<\/p>\n\n\n\n
The Flying Start function helps the VFD detect the motor\u2019s existing speed and direction before taking control.<\/p>\n\n\n\n
Flying Start is a VFD function designed to connect safely to a motor that is already rotating.<\/p>\n\n\n\n
Depending on the manufacturer, the function may also be called:<\/p>\n\n\n\n
Catch-on-the-Fly<\/p>\n\n\n\n
Speed Search<\/p>\n\n\n\n
Flying Restart<\/p>\n\n\n\n
Spin Start<\/p>\n\n\n\n
Search Start<\/p>\n\n\n\n
The VFD searches for the motor\u2019s approximate speed and direction, synchronizes its output with the rotating motor, and then accelerates or decelerates smoothly toward the commanded speed.<\/p>\n\n\n\n
The exact detection method depends on the VFD model and control mode.<\/p>\n\n\n\n
The motor may continue rotating after the VFD stops because of:<\/p>\n\n\n\n
High mechanical inertia<\/p>\n\n\n\n
Airflow through a fan or blower<\/p>\n\n\n\n
Fluid movement through a pump<\/p>\n\n\n\n
A short power interruption<\/p>\n\n\n\n
Coasting after a stop command<\/p>\n\n\n\n
Movement caused by another connected machine<\/p>\n\n\n\n
Reverse rotation caused by airflow or fluid backflow<\/p>\n\n\n\n
The VFD must account for this existing motion before applying controlled torque.<\/p>\n\n\n\n
During a normal start, the VFD usually assumes that the motor is stationary and begins from zero frequency.<\/p>\n\n\n\n
During Flying Start, the VFD first searches for the motor\u2019s present speed.<\/p>\n\n\n\n
After synchronization, the VFD takes control and moves the motor toward the required operating speed.<\/p>\n\n\n\n
This can reduce:<\/p>\n\n\n\n
Starting-current peaks<\/p>\n\n\n\n
Mechanical shock<\/p>\n\n\n\n
Overcurrent trips<\/p>\n\n\n\n
Reverse torque<\/p>\n\n\n\n
Stress on couplings, belts, shafts, and gearboxes<\/p>\n\n\n\n
Recovery time after a short power interruption<\/p>\n\n\n\n
Flying Start is commonly considered for:<\/p>\n\n\n\n
Fans and blowers<\/p>\n\n\n\n
HVAC systems<\/p>\n\n\n\n
Centrifugal pumps<\/p>\n\n\n\n
Cooling towers<\/p>\n\n\n\n
Centrifuges<\/p>\n\n\n\n
High-inertia machines<\/p>\n\n\n\n
Applications affected by airflow or fluid movement<\/p>\n\n\n\n
Processes requiring rapid recovery after a short voltage dip<\/p>\n\n\n\n
It can be particularly useful when the motor may still be coasting when the run command returns.<\/p>\n\n\n\n
No.<\/p>\n\n\n\n
The function should only be used when restarting a rotating motor is safe and appropriate for the process.<\/p>\n\n\n\n
Special attention is required for:<\/p>\n\n\n\n
Conveyors that may move in reverse<\/p>\n\n\n\n
Hoists and cranes<\/p>\n\n\n\n
Elevators<\/p>\n\n\n\n
Machines that must start from a confirmed stationary position<\/p>\n\n\n\n
Applications with mechanical brakes<\/p>\n\n\n\n
Systems where unexpected movement creates a safety risk<\/p>\n\n\n\n
Processes where the product could be damaged by sudden motion<\/p>\n\n\n\n
Safety requirements and machine operating logic must take priority over automatic recovery.<\/p>\n\n\n\n
Some VFDs can search for a motor rotating in either direction. Other models may only search in the commanded direction or within a defined speed range.<\/p>\n\n\n\n
If reverse rotation is possible, confirm that:<\/p>\n\n\n\n
The VFD can detect reverse rotation<\/p>\n\n\n\n
Reverse operation is permitted by the machine<\/p>\n\n\n\n
The driven equipment will not be damaged<\/p>\n\n\n\n
The VFD will not apply excessive braking torque<\/p>\n\n\n\n
The motor cannot reverse the process unexpectedly<\/p>\n\n\n\n
Always check the manual for the exact VFD model.<\/p>\n\n\n\n
Flying Start and Automatic Restart are different functions.<\/p>\n\n\n\n
Flying Start identifies and synchronizes with a rotating motor.<\/p>\n\n\n\n
Automatic Restart causes the VFD to start again automatically after power returns or after a selected fault is cleared.<\/p>\n\n\n\n
The two functions may be used together, but doing so can cause the motor to restart without a new manual command.<\/p>\n\n\n\n
Before enabling Automatic Restart, review:<\/p>\n\n\n\n
Personnel safety<\/p>\n\n\n\n
Machine guarding<\/p>\n\n\n\n
Emergency-stop operation<\/p>\n\n\n\n
Run-command status<\/p>\n\n\n\n
Process conditions<\/p>\n\n\n\n
Restart delay<\/p>\n\n\n\n
Maximum restart attempts<\/p>\n\n\n\n
Fault-reset logic<\/p>\n\n\n\n
A safe Flying Start function does not make an unsafe automatic restart acceptable.<\/p>\n\n\n\n
Check the following:<\/p>\n\n\n\n
Confirm that the VFD supports Flying Start<\/p>\n\n\n\n
Review the manufacturer\u2019s instructions<\/p>\n\n\n\n
Enter the motor nameplate data correctly<\/p>\n\n\n\n
Select the correct motor control mode<\/p>\n\n\n\n
Complete Auto-Tuning when required<\/p>\n\n\n\n
Confirm the permitted direction of rotation<\/p>\n\n\n\n
Check the minimum and maximum search-speed range<\/p>\n\n\n\n
Review current-limit and torque-limit settings<\/p>\n\n\n\n
Confirm acceleration and deceleration times<\/p>\n\n\n\n
Check the process and mechanical condition<\/p>\n\n\n\n
Review Automatic Restart settings<\/p>\n\n\n\n
Make sure emergency stopping remains effective<\/p>\n\n\n\n
Keep personnel away from moving equipment during testing<\/p>\n\n\n\n
The VFD may fail to catch the motor because of:<\/p>\n\n\n\n
Incorrect motor data<\/p>\n\n\n\n
Incorrect control mode<\/p>\n\n\n\n
Poor Auto-Tuning results<\/p>\n\n\n\n
Motor speed outside the search range<\/p>\n\n\n\n
Very low residual motor voltage<\/p>\n\n\n\n
Motor speed changing rapidly during the search<\/p>\n\n\n\n
Unexpected reverse rotation<\/p>\n\n\n\n
A mechanical brake remaining applied<\/p>\n\n\n\n
An output contactor opening during operation<\/p>\n\n\n\n
Long motor cables or output filters affecting detection<\/p>\n\n\n\n
Current or torque limits set too low<\/p>\n\n\n\n
Incorrect Flying Start parameters<\/p>\n\n\n\n
A motor that is already nearly stopped<\/p>\n\n\n\n
If synchronization fails, the VFD may trip or return to a normal start, depending on its programmed behavior.<\/p>\n\n\n\n
A contactor between the VFD and motor can interfere with Flying Start.<\/p>\n\n\n\n
The contactor must be closed before the VFD begins searching for the motor.<\/p>\n\n\n\n
It should not open while the VFD is producing output unless the manufacturer specifically permits the sequence.<\/p>\n\n\n\n
Where an output contactor is necessary, its control must be coordinated with the VFD ready, run, and output states.<\/p>\n\n\n\n
A controlled test should be performed before the function is placed into normal operation.<\/p>\n\n\n\n
Start the motor at a safe test speed.<\/p>\n\n\n\n
Remove the VFD run command and allow the motor to coast.<\/p>\n\n\n\n
While the motor is still rotating, issue a new run command.<\/p>\n\n\n\n
Observe the detected direction and speed.<\/p>\n\n\n\n
Monitor motor current and DC-bus voltage.<\/p>\n\n\n\n
Check for mechanical shock, vibration, and unusual noise.<\/p>\n\n\n\n
Repeat the test at different safe speeds and directions when permitted.<\/p>\n\n\n\n
Confirm the behavior after a short power interruption.<\/p>\n\n\n\n
Record the final settings and test results.<\/p>\n\n\n\n
Testing must be performed by qualified personnel in a controlled and isolated area.<\/p>\n\n\n\n
Flying Start helps a VFD restart a motor that is already rotating by detecting its speed and direction before applying controlled torque.<\/p>\n\n\n\n
It is especially useful for fans, blowers, pumps, centrifuges, and high-inertia loads that may continue moving after the VFD stops.<\/p>\n\n\n\n
The function can reduce current peaks, torque shock, and nuisance trips, but it must not be enabled without reviewing machine safety, reverse rotation, restart logic, and process requirements.<\/p>\n\n\n\n
The parameter name, search method, supported direction, and operating limits vary between VFD models. Always follow the manufacturer\u2019s manual for the exact drive.<\/p>","protected":false},"excerpt":{"rendered":"
How a VFD Safely Restarts a Motor That Is Already Spinning A motor does not always stop immediately when the VFD output is removed. Fans, pumps, centrifuges, and other high-inertia loads may continue rotating because of stored mechanical energy, airflow, fluid movement, or external forces. If the VFD applies a normal start command while the […]<\/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":[152,201,134,126,105,140],"class_list":["post-2681","post","type-post","status-publish","format-standard","hentry","category-vfd","tag-catch-on-the-fly","tag-flying-start","tag-industrial-automation","tag-motor-control","tag-vfd","tag-vfd-commissioning"],"blocksy_meta":[],"yoast_head":"\n