what is flying start in vfd?
What is Flying Start in a VFD?
Flying Start, also called Catch on the Fly and often associated with Speed Search, is a function used by a variable frequency drive to take control of a motor that is already rotating. Its main purpose is not to accelerate the motor to full speed as quickly as possible. The important part is finding the coasting motor and taking it over without first forcing it to stop.
A large fan is an easy example. If power disappears for a few seconds, the fan does not instantly stop. When power returns, the motor may still be turning at several hundred rpm. Instead of treating that motor as stationary, the drive can use Speed Search to estimate the existing motion and bring its output into line with the rotating motor.
This makes flying start useful after short power interruptions and on fans, conveyors, centrifuges and other high-inertia machinery where the motor can keep moving after drive output has been removed.
Explore related topics: Speed Search · Auto-Tuning · Vector Control · PG Cards · Encoders · PLC & HMI
How does Flying Start work?
If a spinning motor is re-energised with an output frequency that is far away from its actual electrical speed, the transition can produce a large current and torque disturbance. Flying start tries to avoid that situation. The drive first searches for the rotating motor, then brings its own output frequency and voltage toward the motor's existing condition before taking control.
The exact method depends on the drive. Some drives estimate speed from current and motor electrical behaviour. Others can use actual feedback from an encoder. In feedback systems, a PG card can provide the interface between the encoder and the drive.
The result also depends on the selected motor-control method. Basic V/f operation, sensorless vector control and closed-loop encoder control do not have exactly the same information about the motor. Correct motor data and, where required, a proper auto-tuning procedure therefore matter.
Compact Flying Start demo
In the simplified example below, the motor begins by coasting without being controlled by the VFD. Pressing START represents the drive searching for the existing speed, synchronising with the motor and then taking it toward the target speed.
This is a simplified educational display. Actual speed-search methods depend on the drive and its control algorithm.
Are Flying Start and Speed Search the same thing?
In everyday VFD terminology, the names are often used for the same family of functions. Manufacturers may use different names and different search algorithms. Our Speed Search guide concentrates on the search process itself, while this page looks at the broader job of taking control of a spinning motor.
Why is Flying Start useful after a power interruption?
When VFD output disappears, a fan, conveyor or high-inertia machine may continue to coast for quite a long time. If power returns while the shaft is still moving, the drive is not starting a stationary motor. It is trying to reconnect to an active electromechanical system.
Once the motor has been found, the drive can continue toward the required speed using its normal acceleration profile. That means flying start and automatic ramping are related but not identical: one finds the moving motor, while the other controls how speed changes afterward.
Does Flying Start require an encoder?
No. Many general-purpose VFDs can perform a sensorless search using motor current and an internal motor model. Higher-performance systems may instead use an encoder to know the real shaft speed directly.
When encoder feedback is used, the subject naturally connects with PG cards, closed-loop vector control and ultimately high-performance servo systems.
Can a PLC or HMI control Flying Start?
Depending on the VFD, flying start may be enabled permanently, triggered by a digital input or requested as part of a restart sequence. A PLC can coordinate the run sequence, while an HMI can display motor speed, output frequency, alarms and the drive's operating state.
On networked machinery, those commands and values may travel over an industrial communications link rather than individual hardwired signals. Flying start therefore sits inside a much larger automation chain rather than being an isolated VFD parameter.
Editor's note: The simplest way to think about flying start is this: the drive does not tell a spinning motor to stop so that it can start it again. It first tries to understand what the motor is already doing and then takes over from there. This can make a major difference on large fans and high-inertia machinery, but it is not a parameter that should automatically be enabled on every application.
Where is Flying Start especially useful?
Large fans and blowers can keep rotating because of stored inertia or airflow. Conveyors may continue to move because of the mechanical load. Centrifuges, drums and other high-inertia machines can also retain significant speed after the drive has stopped producing output.
At the higher-performance end, the same subject starts to overlap with encoder feedback, vector control and servo motion control.
What can make a Flying Start fail?
Incorrect motor data, the wrong search direction, insufficient search time, a rapidly changing shaft speed or unsuitable control settings can make it difficult for the drive to find the motor. Some drives even have dedicated retry counters or faults for unsuccessful speed-search attempts.
If current rises sharply after the motor has been captured, the problem may no longer be the search itself. The drive's current limit, motor model, mechanical load and selected motor-control method should then be considered together.
In short: Flying start lets a VFD find a motor that is already moving and regain control without forcing the shaft to come to zero speed first. Continue through the OtomasyonAVM knowledge base with Speed Search, Auto-Tuning, Vector Control, Encoders, PLC & HMI and the wider VFD technical guide.