When testing a motor with an ohmmeter, the measurements C-R = 4, C-S = 6.2, R-ground = 20.4 indicate what condition?

When testing a motor with an ohmmeter, the measurements C-R = 4, C-S = 6.2, R-ground = 20.4 indicate what condition?

• A. Shorted windings
• B. Open circuit
• C. Insulation fault
• D. A grounded condition

Answer: (D)

The thing being tested is insulation integrity and whether a winding is connected to the frame through a fault. For a healthy motor, the windings show continuity between common and the windings, but the resistance from a winding to the frame (ground) should be effectively infinite because the insulation isolates the windings from the metal frame. In this case, C-R and C-S show normal winding readings, indicating the windings themselves are not shorted or open. However, the measurement from the run winding to ground is 20.4 ohms, a finite path to the frame. That shows there’s a leakage path through the winding insulation to the frame, i.e., a grounded condition. The other fault types don’t fit this pattern: a shorted winding would drive winding-to-winding resistances very low or abnormal, and an open circuit would give infinite resistance between windings. So the finite run-to-ground reading points to a grounded condition.

The thing being tested is insulation integrity and whether a winding is connected to the frame through a fault. For a healthy motor, the windings show continuity between common and the windings, but the resistance from a winding to the frame (ground) should be effectively infinite because the insulation isolates the windings from the metal frame.

In this case, C-R and C-S show normal winding readings, indicating the windings themselves are not shorted or open. However, the measurement from the run winding to ground is 20.4 ohms, a finite path to the frame. That shows there’s a leakage path through the winding insulation to the frame, i.e., a grounded condition. The other fault types don’t fit this pattern: a shorted winding would drive winding-to-winding resistances very low or abnormal, and an open circuit would give infinite resistance between windings. So the finite run-to-ground reading points to a grounded condition.