What is an S-curve ramp in a VFD? Compare it with a linear ramp

In a variable frequency drive (VFD), an S-curve defines how a speed or output-frequency command changes over time. A linear ramp changes frequency at a constant rate, but its slope changes abruptly at the start and end. An S-curve lets that slope build and fall progressively. The difference is not merely what the line looks like: it changes how the machine is excited at the start and end of motion.

A linear ramp does not take the motor from zero to 50 Hz instantly. Both profiles spread the change over time. The S-curve aims to soften the transition in acceleration at the ramp boundaries, within the drive's current and torque limits and the load's ability to follow.

On this page: Interactive comparison · Field setup · FAQs.

Same target and duration: what changes?

In the model below, both profiles rise from 0 to 50 Hz in 4 seconds, hold 50 Hz from 4 to 6 seconds, and return to zero from 6 to 10 seconds. Move the time slider and watch both dots at the same instant. In the lower graph, df/dt is the rate of change of the frequency command, not measured acceleration of the physical motor shaft.

At 1 second, the linear command is 12.5 Hz while this S-curve is about 8.3 Hz. Both reach 25 Hz at 2 seconds and 50 Hz at 4 seconds. The difference lies in their slopes near the boundaries. Each moving dot is calculated from the same mathematical function that draws its curve, so it follows the line.

Why can a gentler S-curve be steeper in the middle?

For the linear profile to reach 50 Hz in four seconds, its frequency slope is 50 / 4 = 12.5 Hz/s. The S-curve reduces the slope at both ends yet must reach the same target within the same four seconds. In this example, a one-second smoothing region at each end produces a middle slope of 50 / (4 − 1) ≈ 16.7 Hz/s, roughly 33% higher than the linear case. Increase the smoothing share and watch the lower graph. Less initial jolt does not automatically mean less peak torque.

This is an educational S profile whose slope rises linearly, remains constant in the middle and then falls linearly. Real drives may use different mathematical profiles and settings. The “25%” control in this model is therefore not a parameter value to copy into a particular drive.

Are ACC/DEC time and S-curve shape the same setting?

No. Acceleration and deceleration time (ACC/DEC) specify the time allowed for a target change; S-curve settings describe how the rate of change behaves at the ramp's start and end. Manufacturers define them differently. Some drives provide separate beginning/end shape times, while on other models S-curve times are added to ACC/DEC. Thus, an ACC setting of four seconds does not guarantee an actual S-curve duration of exactly four seconds on every brand. Check the manual's definition of time.

Automatic ramp adaptation is another function: the drive may extend the effective ramp near a current or DC-bus limit. S-curve shaping sets the requested ramp profile. It cannot by itself prevent an actual ramp from being stretched by a current limit or make the motor shaft follow the command perfectly.

Does this S-curve show motor RPM or output frequency?

The graph shows an illustrative 0–50 Hz frequency command. A frequency reference is not an exact measurement of the shaft speed of an induction motor; pole count, slip, load and the control method matter. The S-curve here is a time profile. A V/f curve instead describes how voltage relates to frequency. Mixing up these curves can send an engineer to the wrong parameter.

What changes on a conveyor or crane?

On a product conveyor, easing into motion can reduce slipping or swaying when the profile and mechanics are matched to the load. On a crane, it can help limit suspended-load sway. Actual behaviour also depends on mass, backlash and the mechanical brake sequence. An S-curve does not replace the holding brake or a safety function.

A short ACC time combined with aggressive smoothing and high inertia can demand more torque or current in the middle of the ramp. Current limiting may then lengthen the real acceleration. With a demanding DEC setting, a high-inertia load can send energy back to the DC bus. The destination of regenerative energy depends on the braking hardware, not on the S shape; a suitable braking resistor can dissipate it as heat.

Editor's field tip: how to set it up

Start with the motor nameplate and real load cycle. Choose ACC/DEC from the machine's required timing, then increase S smoothing in small steps while checking starting current, tracking of the command, product movement and the actual stop time. Try loaded and unloaded starts. If sway improves but the drive hits a current limit in the middle, reconsider the overall ramp duration and the mechanical requirement. Turning up an S-curve parameter does not establish longer machine life or energy savings without measurement.

Frequently asked questions

Does a linear ramp take the motor instantly to full speed?
No. Here it reaches 50 Hz in four seconds; the abrupt change is in the slope at the ramp boundaries.

Does more S-curve smoothing always reduce current?
No. If total ramp time stays the same, longer smoothing regions may increase the middle slope. Actual current depends on the motor and load.

Can an S-curve replace a braking resistor?
No. It shapes the command; energy returning to the DC bus during braking still needs a suitable path.

Does “S = 25%” mean the same thing on every drive?
No. That percentage belongs to this illustration. Manufacturers use different shape and time parameters.

Related topics

Understand the ramp: ACC/DEC time · Automatic ramping · Frequency reference · V/f curve

Manage the load and stop: Current limiting · DC bus · Braking resistor · Regenerative energy

Technical sources: ABB, ACQ810 firmware manual, ramp shape parameters 22.06–22.09 · Schneider Electric, S-ramp timing.

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