Vinay Kumar yadav

Here is a clear, site-level engineering procedure to check a motor using a Megger (Insulation Resistance Tester):
🔧 Motor Insulation Test Using Megger
🔹 1. Isolation (Very Critical)
Before testing:
Switch OFF supply
Lockout/Tagout (LOTO if available)
Disconnect motor cables from panel
Ensure no backfeed (VFD, capacitor, etc.)

⚠️ Never megger a live or connected motor.

🔹 2. Select Correct Megger Voltage
Motor Rating
Test Voltage
Up to 440 V ---500 V DC
440 V – 3.3 kV----1000 V DC
Above 3.3 kV-----2.5 kV / 5 kV

🔹 3. Phase to Earth Test (Most Important)
Connection:
Megger + (Red) → Motor terminal (U / V / W)
Megger – (Black) → Motor body (Earth)
Test:
Press TEST button for ~1 minute
Note insulation resistance value
👉 Repeat for:
U → Earth
V → Earth
W → Earth
🔹 4. Phase to Phase Test
Test between windings:
U ↔ V
V ↔ W
U ↔ W
Same procedure (press test and record values)
🔹 5. Acceptable Insulation Values
IR Value
Condition
< 1 MΩ --Poor (Do not use)
1 – 5 MΩ ---Marginal
> 5 MΩ. ------Good
> 100 MΩ----- Excellent

👉 Practical rule (site engineers use):
Minimum IR = 1 MΩ per kV + 1 MΩ
🔹 6. Important Precautions
Motor should be dry and clean
Do not touch terminals during test ⚡
After test → Discharge winding to earth (very important)
For critical motors → take PI test (Polarization Index)
🔹 7. Advanced Check (Optional but Professional)
Polarization Index (PI Test)
PI < 1 → Bad insulation
PI 1–2 → Acceptable
PI > 2 → Good
🔍 Practical Example
If a 415V motor shows:
U-E = 12 MΩ
V-E = 10 MΩ
W-E = 11 MΩ
✅ Motor is healthy and safe to energy

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