How to Set Up VFD Flying Start for a Spinning Load
When a drive tries to start into a load that is already spinning, a coasting fan, a windmilling pump, or a motor still turning after a brief power loss, a normal start from zero fights the rotation and trips on overcurrent. Flying start, also called speed search, lets the drive find the motor's actual speed and pick it up smoothly instead. This page is for the technician setting up a restart into a spinning load. It explains when flying start is needed, how the drive locates the running speed, and how to configure and verify a smooth catch without a jolt.
VFD Flying Start Setup in one line: To set up a VFD flying start, enable the flying-start or speed-search function so that on a start command the drive detects the motor's current speed and direction rather than assuming zero, then matches its output to that speed and takes control smoothly. Use it wherever the load can still be spinning at a start, such as fans, high-inertia pumps, and restarts after a brief power dip. Configure the search direction and any current limit the drive offers, then verify the drive catches the spinning load without an overcurrent trip or a mechanical jolt.
Know When You Need Flying Start
Flying start earns its place wherever a load keeps turning after the drive stops commanding it. A large fan coasts for a long time on its own inertia, a pump can windmill backward under a returning column of fluid, and after a momentary power interruption a motor is still spinning when power returns. Trying to start from zero into any of these forces the drive to yank the rotor to a standstill and back up, which draws a large current and usually trips. The behavior the feature addresses is described in the page on VFD flying start.
Recognize the symptom that says you need it. A drive that trips on overcurrent only when restarting soon after a stop, but starts fine from a fully stopped machine, is telling you the load is still spinning at the restart. This overlaps with the general startup-overcurrent diagnosis in the guide on diagnosing VFD overcurrent at start, where a spinning load is one of the causes; flying start is the specific fix when rotation at restart is the reason.
Weigh flying start against simply waiting or braking. If the process can tolerate letting the load coast to a stop before restarting, a normal start works and flying start is unnecessary. If the load takes too long to stop, or the process needs a fast restart, flying start is the clean answer. On some machines a controlled stop using DC injection or a brake, as in the page on VFD DC injection braking, is used to bring the load to rest quickly instead, so choose between catching the spin and stopping it based on what the process allows.
Configure How the Drive Finds the Speed
Enable the flying-start function and set how the drive searches. On a start command with flying start active, the drive does not ramp from zero; instead it probes the motor to estimate its present speed, then brings its output frequency to match before taking full control. Enabling the feature is the first step, and on many drives you then set parameters that govern how aggressively or gently the search runs, which trades catch speed against smoothness.
Set the search direction to match the load. Some loads can only spin forward, some can spin backward, and a windmilling pump or a fan in a duct with reverse airflow can be turning the opposite way to normal. Tell the drive which directions to search so it does not waste time or, worse, try to grab the motor assuming the wrong direction. A pump that can backspin needs the drive to recognize reverse rotation and handle it, either by catching it or by braking it to a stop first depending on the design.
Mind the interaction with the motor model and control mode. Flying start works differently under simple volts-per-hertz than under vector control, which relies on the motor model to estimate speed, so a good autotune, covered in the guide on running a VFD motor autotune, helps the speed search be accurate. Confirm the flying-start settings suit the control mode you chose, because a search tuned for one mode may catch poorly in the other.
Verify a Smooth Catch Without a Jolt
Test the catch under the real condition, a genuinely spinning load. Get the load turning, by letting it coast after a stop or by the process itself, then command a start with flying start enabled and watch what happens. A good catch shows the drive detecting the speed, matching it, and taking control with little current spike and no mechanical jolt; the machine simply continues turning under drive control. A poor catch shows a current surge, a lurch, or a trip, which means the search settings or direction need adjustment.
Confirm it handles the worst case, not just a gentle one. Test catching the load at high coasting speed and, if the load can backspin, in the reverse direction, because those are the conditions that break a marginal flying-start setup. Also test the real trigger you care about, such as a restart immediately after a brief power interruption, since that is often why flying start was configured in the first place. A setup that only works when the load is barely moving has not been proven for the case that matters.
Tie the verified behavior to how the machine rides through disturbances. On sites with brief supply dips, a drive configured for flying start plus an appropriate power-loss ride-through can resume cleanly where a naive drive would trip and stay down, keeping an unattended pump or fan in service through a momentary event. Because both the trip events and the running frequency are values a monitoring system trends, a machine that used to trip on every brief outage and now rides through them shows the flying-start configuration working in the recorded history.
Frequently Asked Questions
When do I need flying start on a VFD?
You need it whenever the drive might start into a load that is still spinning, such as a coasting fan, a windmilling pump, or a motor still turning after a brief power interruption. Starting from zero into a spinning load forces the drive to drag the rotor to a stop and back up, drawing a large current that usually trips on overcurrent. Flying start lets the drive detect the running speed and pick it up smoothly instead, which is the fix when rotation at restart is the cause of the trip.
How does a VFD know the speed of a spinning motor?
With flying start enabled, the drive probes the motor on a start command to estimate its present speed and direction rather than assuming zero, then ramps its output to match that speed before taking full control. The exact method depends on the drive and the control mode, and under vector control an accurate motor model from a good autotune helps the estimate. Once the drive has matched the motor's speed, it assumes control smoothly without the current surge a from-zero start would cause.
Can flying start catch a pump that is spinning backward?
It can if the drive is configured to search in the reverse direction, which is important for pumps that windmill backward under a returning fluid column. Tell the drive which directions to search so it recognizes reverse rotation instead of assuming forward. Depending on the design, the drive either catches the backspinning load directly or first brakes it toward a stop, so confirm which behavior your drive uses and test it with the pump actually spinning backward before relying on it.
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