TL;DR

If the motor runs below the commanded speed, check the VFD's frequency reference, slip compensation, and supply voltage. Mechanical load increase can also drag speed down.

What you might see

motor running below commanded speed
VFD output frequency below setpoint
downstream process throughput dropped
intermittent slow-down events

Likely causes

Tap to mark as ruled out

VFD frequency reference parameter changed by accident or maintenance
Bus undervoltage from a sagging supply or weak feeder
Increased mechanical load on the driven equipment
Slip compensation set higher than the motor manufacturer's recommendation

Required tools

VFD programming manual (Allen-Bradley, ABB, Yaskawa, or whoever made the drive)
Multimeter rated for the supply voltage
Clamp ammeter

Safety first

VFDs hold a lethal DC bus voltage even after disconnect. Wait at least 5 minutes for capacitors to discharge before opening the drive enclosure.
Work on energized drives requires PPE and an Energized Electrical Work Permit.

Procedure

7 steps · ~30 min

1
Read the VFD operator panel and confirm the actual output frequency vs. the commanded reference. Note both values.
2
Check the VFD parameter change log if available, most modern drives log the last few parameter edits with timestamp.
3
Measure the input line voltage to the VFD with a multimeter. It should be within ±10% of the drive nameplate.
Warning: VFD measurement requires opening an energized enclosure. Use category-rated PPE.
4
Read the drive's DC bus voltage parameter. Bus voltage well below the nominal value indicates a supply problem upstream.
5
Inspect the driven equipment, bearings, scaling, blockages, anything that increases the load torque.
6
If slip compensation is set higher than 1.5% (or whatever Baldor recommends for this motor frame), reduce it to the recommended value.[1]
7
If the line voltage sags consistently, address the supply side, check upstream transformers, feeder connections, and whether other large loads are starting at the same time.

Sources

Baldor Reliance Industrial Motor Maintenance Manual, Baldor Electric Company (ABB)
Baldor Industrial Motor Maintenance Manual, VFD application guidelines (general)
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