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