Nema Mg1-32 Amp- 33 Page

Section 32 is the “how-to” for motor testing. It includes:

NEMA MG1-33 identifies three primary failure mechanisms:

If you are experiencing motor failures and suspect ignorance of MG1-32 or MG1-33, perform these checks:

For Insulation Failure (MG1-32):

For Bearing Failure (MG1-33):

Scenario: A water treatment plant needs a 200 HP, 480V pump motor. The available transformer is 300 kVA.

Step 1 – MG1-32 Calculation: Motor code letter = G (LRC = 5.6 kVA/HP) Starting kVA = 200 HP × 5.6 = 1120 kVA (full voltage) nema mg1-32 amp- 33

Transformer 300 kVA cannot supply 1120 kVA. Voltage drop would exceed 30%.

Step 2 – Apply MG1-32 reduced voltage: Use wye-delta starter: Starting kVA = 1120 × 0.33 = 370 kVA

Still exceeds 300 kVA transformer capacity. Section 32 is the “how-to” for motor testing

Step 3 – Apply autotransformer (50% tap): Starting kVA = 1120 × 0.25 = 280 kVA (acceptable for 300 kVA transformer)

Step 4 – Verify MG1-33 (AMP-33): Acceleration torque at 50% voltage = 25% of full torque. If pump torque > motor accelerating torque, motor will not accelerate. This violates MG1-33 because the motor will stay at locked rotor current for >20 seconds, tripping overload.

Solution: Increase autotransformer to 65% tap (42% torque, 42% kVA) or use a solid-state soft starter. For Bearing Failure (MG1-33):