How to Set VFD Accel and Decel Time to Avoid Trips
Acceleration and deceleration times tell the drive how fast to ramp the motor between speeds, and set carelessly they are a leading cause of nuisance trips: too-short accel spikes current, too-short decel pumps the DC bus into an overvoltage fault. This page is for the technician tuning ramps during commissioning who wants them fast enough to be useful but slow enough to be reliable. It explains what the two times do, how the load's inertia sets the practical floor, and how to catch overcurrent on accel and overvoltage on decel before they leave you with an unreliable machine.
VFD Accel and Decel Setting in one line: To set VFD accel and decel times that avoid trips, start longer than you think you need, then shorten each until the drive just stops tripping and add a margin. Accel time too short forces high current into a high-inertia load and trips on overcurrent, so lengthen it until the current stays within limits. Decel time too short makes the motor regenerate faster than the drive can absorb, pumping the DC bus into an overvoltage trip, so lengthen it or add braking hardware. Match both ramps to the load's inertia, verify across the real speed range, and leave headroom for the worst case.
Understand What Accel and Decel Actually Control
The accel time is the time the drive takes to ramp from zero to base or maximum frequency, and the decel time is the reverse. They are not just comfort settings; they directly determine how much torque, and therefore current, the drive must produce to change speed against the load's inertia. A short ramp demands high torque quickly, and high torque means high current on accel and high regenerated energy on decel. Every ramp is a negotiation between how fast the process wants the change and how much the drive and motor can safely deliver.
Inertia sets the floor on how short a ramp can be. A high-inertia load like a large fan, a flywheel, or a loaded conveyor stores a lot of rotational energy, so accelerating it quickly needs a lot of current and stopping it quickly dumps a lot of energy back into the drive. The larger the inertia, the longer the minimum practical ramp, and no parameter setting can beat physics. Understanding the inrush the motor draws helps, and the underlying behavior is described in the page on motor inrush current.
The two directions fail in opposite ways. Too-short accel trips on overcurrent because the drive is trying to add speed faster than the current limit allows, a symptom worked through in the guide on diagnosing VFD overcurrent at start. Too-short decel trips on overvoltage because a decelerating motor becomes a generator, feeding energy back into the DC bus faster than the drive can absorb or dissipate it. Knowing which fault points which way tells you which ramp to lengthen.
Tune Accel and Decel Against the Load
Start conservative and tighten. Set both ramps comfortably long for the first runs, confirm the motor accelerates and decelerates without tripping, then shorten each ramp step by step until the drive just begins to complain, and back off to leave a margin. This converges on the fastest reliable ramp far more safely than guessing a short time and chasing trips. The load, not a spec sheet, tells you where the limit is.
For accel, watch the current during the ramp. If the current climbs into the drive's limit and the drive folds back or trips, the ramp is too aggressive for the inertia and you lengthen it. Adding an S-curve or ramp smoothing where the drive offers it softens the start and end of the ramp, reducing the current and mechanical shock at the transitions, which helps a high-inertia or belt-driven load follow the ramp without a spike.
For decel, watch the DC bus. A decelerating motor regenerates, and that energy raises the bus voltage; if decel is too fast the bus climbs to the overvoltage trip point, a level you can read per the guide on interpreting VFD DC bus voltage readings. The simplest fix is a longer decel time so the motor gives energy back slowly enough for the drive to handle. Where the process needs a fast stop, the regenerated energy has to go somewhere, which points to braking hardware rather than a longer ramp.
Handle Fast Stops and Verify the Result
When the process genuinely needs a stop faster than the bus can absorb, add somewhere for the energy to go rather than forcing an impossible ramp. A braking resistor with a brake chopper burns the regenerated energy as heat, letting the drive decelerate hard without an overvoltage trip, a scheme described in the page on the VFD braking resistor and brake chopper. For low-speed stopping, DC injection braking can hold or stop the shaft, as covered in the page on VFD DC injection braking.
Verify the ramps across the whole operating range, not just one convenient speed. A ramp that behaves at mid-speed can still trip when starting from a stop against a full load or stopping from top speed, because torque demand and regenerated energy vary across the range. Run the worst cases the process will actually see: the heaviest start, the fastest commanded stop, and any repeated cycling that stacks heat into a braking resistor. A ramp is only proven once it survives the hardest thing the process does.
Leave headroom for the real world. Voltage sag, a colder and stiffer load on a winter start, a partially blocked fan, or a fuller tank all push the ramp harder than the bench test, so a ramp tuned right at the ragged edge will trip in service. Back both times off from the trip point enough that ordinary variation does not tip them over. A machine that never nuisance-trips because its ramps have margin is worth more than one that ramps a fraction faster and stops unpredictably.
Frequently Asked Questions
Why does my VFD trip on overvoltage when it decelerates?
A decelerating motor acts as a generator, feeding energy back into the drive's DC bus. If the decel time is too short, the bus voltage rises faster than the drive can absorb or dissipate the energy and hits the overvoltage trip point. Lengthen the decel time so the motor returns energy slowly enough for the drive to handle. If the process needs a fast stop, add a braking resistor and chopper so the regenerated energy has somewhere to go besides the bus.
How short can I make the acceleration time?
As short as the load's inertia and the drive's current limit allow, and no shorter. A high-inertia load stores a lot of rotational energy, so accelerating it quickly demands high current that eventually hits the drive's limit and trips. Start with a longer accel time, shorten it until the current just approaches the limit, then back off for margin. The load's inertia sets the physical floor; no parameter can beat it, so the fastest reliable ramp is found empirically.
Should accel and decel times be the same?
Not necessarily. They control different physics: accel is limited by how much current the drive can push to speed the load up, while decel is limited by how much regenerated energy the drive can absorb as the load slows down. A load may accept a short accel but need a longer decel, or the reverse, depending on inertia and any braking hardware. Tune each independently against its own failure mode rather than assuming one time serves both directions.
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