Pump speed loop hunting
Speed loops on variable frequency drives have a characteristic that trips people up: a significant part of the loop dynamics lives in the drive, not in the process, and it is configured somewhere most control engineers never look.
Acceleration and deceleration ramps limit how fast the drive will follow a speed command. When those ramps are longer than the process dynamics — which is common, because commissioning defaults are generous — the controller asks for a step and receives a slope.
Check the drive before the controller
Read the accel and decel times out of the drive and compare them against how fast the process responds. If the ramps are longer, they are the dominant lag in the loop and tuning is working around them.
The failure mode is predictable: the loop looks sluggish, somebody increases the gain to compensate, and now the loop overshoots badly when the ramp finally catches up. The symptom presents as hunting but the cause is a rate limit.
Ramps are sometimes there for a genuine mechanical or electrical reason — belt drives, high-inertia loads, supply limitations. Find out before changing them.
Minimum speed and the region below it
Most drives are configured with a minimum speed. A loop whose demand falls below that minimum will sit at the limit, exactly like a saturated output, and will wind up while it does.
If a loop hunts only at low load, check whether it is bouncing off the minimum speed setting.
What causes it, most likely first
Drive accel and decel ramps too long
most common tuning cannot fix this
The drive rate-limits the controller's output before it reaches the motor.
- How to confirm it
- The speed feedback rises as a straight line rather than a curve. Compare the ramp time against the loop's response time.
- What to do
- Establish why the ramps are set where they are. If they are defaults, shortening them helps more than re-tuning.
Hitting minimum speed at low load
common tuning cannot fix this
The loop saturates against the drive's minimum speed and winds up.
- How to confirm it
- Hunting appears only at low throughput. Speed feedback flat at the minimum while the controller asks for less.
- What to do
- Review the minimum speed setting against the actual turndown required.
Tuning too aggressive for a fast loop
common tuning can fix this
Speed loops respond quickly and need modest gain.
- How to confirm it
- Decaying oscillation with a short period.
- What to do
- Reduce proportional action.
Derivative action on a noisy speed feedback
less common tuning can fix this
Pulse-derived speed feedback is quantised and noisy; derivative amplifies that.
- How to confirm it
- Rate is non-zero and the output is busy at high frequency.
- What to do
- Set rate to zero.
Questions that come up
Should the drive ramps be as short as possible?
No. They exist to protect the drive, the motor and the driven equipment from mechanical and electrical stress. The point is that they should be chosen deliberately rather than left at a default, and the control engineer should know what they are.
Related
Tuning guides
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Glossary
Last reviewed 2026-08-01.