TL;DR

If the motor trips on start, megger the windings to ground. Above 100 MΩ is healthy; below 1 MΩ means a winding-to-ground fault and the motor needs to be rewound or replaced.

What you might see

breaker trips immediately on start
motor hums but doesn't rotate
ground fault indicator lit
overload relay trips at zero current

Likely causes

Tap to mark as ruled out

Winding-to-ground insulation breakdown from heat aging or moisture ingress
Phase-to-phase winding short
Open winding, broken conductor or burned termination
Faulty contactor or starter (not the motor itself)

Required tools

Megohmmeter (500V or 1000V test voltage)
Multimeter
Grounding stick or shorting jumper
LOTO kit

Safety first

A megger applies hundreds of volts DC. Always discharge windings to ground after a test before touching the leads.
NEVER megger a motor while it is connected to a VFD. Disconnect the VFD output cables first.
Lock out the motor disconnect before starting any winding test.

Procedure

10 steps · ~45 min

1
Lock out and tag out the upstream disconnect.
2
Disconnect the motor leads from the contactor or VFD output terminals.
Warning: If the motor is connected to a VFD, you MUST disconnect the VFD output before applying a megger. Megger DC voltage will destroy the VFD output stage.
3
Set the megohmmeter to 500V test voltage.
4
Connect one lead to motor lead T1, the other to the motor frame or ground. Apply test for 60 seconds and read the resistance.[1]
5
Healthy reading is greater than 100 MΩ. Between 10 and 100 MΩ is degraded but operable. Below 1 MΩ indicates a winding-to-ground fault.[1]
6
Repeat the test on T2 and T3 motor leads.
7
Discharge each winding to ground with a grounding stick before disconnecting the megger.
8
Test winding-to-winding resistance with a multimeter on the lowest ohms range. The three phase readings should be within 5% of each other.
9
If any test fails, the motor cannot be repaired in the field. Tag it Out of Service and send it to a qualified motor rewind shop, or order a replacement.
10
If the windings test healthy, look upstream, check the contactor coil and contacts, the overload relay, and the incoming voltage.

Sources

Baldor Reliance Industrial Motor Maintenance Manual, Baldor Electric Company (ABB)
Baldor Industrial Motor Maintenance Manual, insulation resistance test procedure (general)
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