TL;DR

Stop the motor immediately. A burning smell almost always means stator insulation breakdown. The motor needs to be rewound or replaced, do not attempt to restart it.

What you might see

acrid burning odor
visible smoke from the motor housing or conduit box
ground fault indicator lit
discolored windings visible through the conduit box

Likely causes

Tap to mark as ruled out

Stator winding insulation breakdown from sustained overheating
Moisture ingress causing winding-to-ground arcing
Severe phase imbalance burning one phase winding
Contaminated cooling air carrying flammable vapors

Required tools

LOTO kit
Megohmmeter (for the no-visible-damage case)
Insulated screwdrivers
Camera or phone for documentation

Safety first

A burning motor is a fire hazard. If smoke is heavy or flames are visible, evacuate and call the fire response team.
Do not open a hot conduit box. Wait for the motor to cool.
Do not attempt to restart a motor with a burning smell, you risk a fire and irreversible insulation damage.

Procedure

9 steps · ~30 min

1
STOP the motor immediately. Hit the local E-stop or open the upstream disconnect.
Warning: If smoke is significant or flames are visible, evacuate the area and call your fire response team. Do not approach a burning motor.
2
Lock out and tag out the disconnect.
3
Wait at least 15 minutes for the windings and conduit box to cool before opening anything.
4
Open the conduit box and inspect for visible damage, discolored or charred windings, melted insulation, burned lead terminations.[1]
5
Photograph the damage for the maintenance record and the failure analysis.
6
If damage is visible, the motor cannot be repaired in the field. Tag it Out of Service.
7
If no visible damage, perform a megohmmeter test (see the won't-start procedure on this machine) to check insulation resistance.
8
Send the motor to a qualified motor rewind shop or order a replacement.
9
While waiting for repair, investigate the root cause, overload history, supply imbalance, ventilation problems, contamination. Do not return a replacement motor to service without addressing the root cause.

Sources

Baldor Reliance Industrial Motor Maintenance Manual, Baldor Electric Company (ABB)
Baldor Industrial Motor Maintenance Manual, winding inspection and insulation breakdown indicators (general)
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