PLC Simulator
F-25 · vendor-neutral function

VFD Flying Start Explained

Flying start—also called speed search or catch on the fly—estimates the speed and direction of an already rotating motor before the drive reapplies controlled torque.

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Why it matters

The behavior behind the parameter.

Fans, pumps and high-inertia machines can still be spinning after a coast stop or supply interruption. Starting from zero frequency against that rotating field can create a current surge, torque shock or trip.

  • It is a restart function, not an acceleration shortcut.
  • Direction and residual speed must be detected before torque is established.
  • The permissive and machine risk assessment still decide whether automatic restart is allowed.

Interactive proof

Test vfd flying start explained in three passes.

Keep the motor, load and starting condition constant. Change one function at a time, then use the electrical and mechanical evidence to explain the result.

  1. 01

    Run the motor above 25 Hz and select coast stop.

  2. 02

    Reissue run with flying start disabled and inspect the overcurrent evidence.

  3. 03

    Enable flying start, repeat the coast/restart sequence and compare the captured speed.

Load F-code F-25 in the function lab

Evidence to watch

The function is proved by changed behavior, not by a parameter value alone.

  • Residual motor frequency
  • Search/capture state
  • Output current during torque pickup

Common commissioning mistakes

These errors can make a healthy drive appear faulty or create a misleading test.

  • Enabling automatic restart without reviewing the hazard.
  • Expecting flying start to fix wrong motor data.
  • Testing only from a stationary shaft.

Primary technical references

Transfer the concept; verify the exact drive.

This guide uses vendor-neutral terminology. For real equipment, use the manual for the installed model, option cards and firmware, plus the machine risk assessment and approved commissioning procedure.

Rockwell PowerFlex 520-series user manualSiemens SINAMICS G120 function manualDanfoss VLT AutomationDrive programming guideABB ACS580 manual directory

Related VFD functions

F-22

VFD Stop Modes: Ramp, Coast, Quick Stop and DC Brake

A VFD can ramp the frequency down, remove torque and coast, use a faster quick-stop ramp, or apply DC injection braking. Each produces a different current, torque, stopping time and DC-bus response.

Read and test F-41

VFD Current Limit and Stall Protection

Current limiting caps drive output current and therefore available motor torque. The drive may slow or extend acceleration instead of immediately tripping when the load demand reaches that limit.

Read and test F-20

VFD Run Command Sources

A VFD run command can come from the local keypad, hardwired terminals or a fieldbus control word. The selected source must match the signal the PLC or operator is actually sending.

Read and test

From explanation to evidence

Change the function and watch the motor respond.

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