VFD Acceleration & Deceleration: Why Faster Is Often Worse

One of the most common causes of spindle faults and broken tools is aggressive acceleration and deceleration settings.

It’s tempting to make the spindle ramp up and down as fast as possible. Faster feels better. In practice, it often causes faults, tool breakage, and unnecessary stress on both the spindle and the VFD.

This article explains what acceleration and deceleration actually do, why faster is rarely better, and how to set safe, reliable values.

What Acceleration Time Really Does

Acceleration time controls how long the VFD takes to bring the spindle from 0 RPM to its commanded speed.

For example:

6 seconds means the spindle ramps smoothly to speed over 6 seconds
3 seconds means the same speed change happens twice as fast

Shorter acceleration times demand more current from the VFD and more torque from the spindle. That extra demand happens instantly, even before the bit touches material.

If the VFD detects excessive current or instability during this ramp, it will fault to protect the system.

This is where ERR02 and ERR34 commonly come from.

What Deceleration Time Really Does

Deceleration time controls how long the spindle takes to slow down and stop.

When a spindle decelerates, it becomes a generator. Energy flows back into the VFD’s DC bus. If that energy cannot be dissipated safely, voltage rises rapidly.

If deceleration is too aggressive, the VFD will fault to protect itself.

This is where ERR03, ERR05, ERR06, and braking-related faults come from.

Why Controller and VFD Settings Must Match

This is critical and often overlooked.

Your CNC controller assumes the spindle reaches commanded speed within a certain time. If the controller thinks the spindle is ready before it actually is, it will begin feeding into material too early.

If you change acceleration time in the VFD but not in the controller:

The controller may plunge before full RPM
The bit sees full cutting load at low RPM
Bits snap, even during light cuts
The VFD may fault due to sudden load

Any time you change acceleration or deceleration in the VFD, you must update the spindle acceleration settings in your controller to match.

This applies to Masso, GRBL, and other controllers.

“But It Was a Light Cut” Isn’t the Whole Story

Many customers are surprised when a small bit breaks during a shallow cut.

Common reasons:

The spindle was not at full RPM yet
Acceleration was too aggressive
The controller plunged early
Current spiked during ramp-up

Light cuts still require stable RPM. Torque and current spikes happen during acceleration, not just during heavy cutting.

Recommended Safe Ranges

These are real-world values we use and recommend.

Acceleration Time

Safe starting point: 6 seconds
More forgiving setups: 8 to 10 seconds
Avoid going below 5 seconds unless you fully understand the tradeoffs

Deceleration Time

Safe starting point: 6 seconds
Faster stops require a braking resistor
Without a braking resistor, do not push deceleration aggressively

Faster ramps do not improve cutting quality. They only reduce idle time by a few seconds while increasing the risk of faults.

Common Symptoms of Aggressive Ramps

If you experience any of the following, acceleration or deceleration is often the cause:

ERR02 during startup
ERR03 or ERR06 during stop
Bits breaking on the first plunge
Spindle faults that disappear when slowing things down
Problems that get worse after “tuning for speed”

Slowing down ramps often resolves these instantly.

Why Slower Is Better

Slower ramps:

Reduce current spikes
Protect bearings and windings
Prevent tool shock
Reduce nuisance faults
Improve long-term reliability

In CNC, stability beats speed every time.

Final Recommendation

If you are chasing spindle faults, broken tools, or unexplained shutdowns, check acceleration and deceleration settings before anything else.

Start conservative. Let the spindle reach full speed. Let the VFD do its job calmly.

You will get better cuts, fewer faults, and a much more reliable machine.