Megger
A megger is a portable megohmmeter or insulation resistance tester that measures insulation resistance of motors, cables, windings, transformers, and similar equipment in megaohms or higher. In CNC and millwork shops, it verifies spindle motor, servo motor, or cable insulation integrity before energizing machinery after maintenance or suspected ground faults.
On the shop floor, a megger earns its keep when a spindle or servo drive trips, after a machine washdown, or after any repair that disturbs motor wiring. The technician disconnects the motor leads from the drive or contactor, then applies one test lead to a winding or conductor and the other to the motor frame or ground. A DC test voltage, commonly 500 V for motors rated under 1000 V, is applied, and the insulation resistance is recorded. Healthy windings generally read above 1 MΩ; lower or unstable readings point to moisture, coolant contamination, damaged cables, or insulation breakdown. This test also supports preventive maintenance by trending readings over time, revealing creeping degradation in a spindle cable before it becomes a burned winding or catastrophic motor failure. Because the test voltage can damage sensitive electronics, confirming full isolation before testing is non-negotiable.
- Testing without full isolation: If a motor remains connected to a VFD, servo drive, or contactor, the megger's test voltage can back-feed sensitive electronics and produce misleading readings or damage the drive. Disconnect all equipment before applying test leads.
- Dirty or wet connection points: Coolant film, cutting oil, dust, or condensation forms a leakage path that mimics winding failure. A wet cable gland or contaminated terminal box yields a bad megger reading even when the motor is sound.
- Wrong voltage or one-off judgement: Using too high a voltage overstresses insulation, while too low a voltage misses marginal weakness. A single acceptable reading is not enough because unstable or falling readings on repeated tests indicate developing problems.
What electrical property does a megger measure?
A megger measures insulation resistance, not continuity, current, or winding resistance, across de-energized motor windings, cables, and electrical equipment.
Why is the megger test taken between the winding and the motor frame?
Because the frame-to-winding path reveals whether insulation is leaking current to earth ground. This is the primary failure mode for motor ground faults.
Can a spindle motor run normally yet still fail a megger test?
Yes. A motor can remain functional while insulation has degraded enough to create future leakage or intermittent faults, especially under heat, coolant exposure, or vibration. Low readings point to moisture ingress, contamination, or insulation breakdown.