Question 4 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is A **Why Option A is Correct:** Testing for an "open" stator winding coil requires checking the electrical continuity (resistance) of the windings. In a wye-connected motor, there are three phases (U, V, W). When the motor leads are disconnected, testing the resistance across each pair of leads (U to V, V to W, and U to W) measures the resistance of two phase windings in series. * In a healthy wye-connected motor, the resistance measurement for all three pairs of leads should be approximately equal (and very low, typically less than an ohm). * If one coil (phase winding) is "open" (a complete break in the wire), the resistance measurement for any pair of leads that includes the open coil will be infinitely high (or "OL" – open line – on the ohmmeter). * By comparing the three readings, an open circuit in one phase is immediately identifiable because the two pairs of leads involving that phase will show infinite resistance, while the third pair (the two good phases in series) will show a measurable resistance. **Why the Other Options are Incorrect:** * **B) Use a growler, listening for noise and vibration levels to increase when the growler blade is positioned over an open stator coil.** A growler is primarily used to test for **shorted** windings in the stator (or rotor). When a growler is placed over a shorted winding, the magnetic field induces a large circulating current in the shorted coil, causing increased vibration and a distinct "growling" noise. An open coil will simply interrupt the current path and will not typically cause the characteristic noise associated with a short circuit. * **C) Use an ohmmeter, placing one test lead on the shaft and the other to each of the disconnected motor leads in succession and compare resistances.** This procedure is used to test the **insulation resistance** between the windings and the motor frame (ground). An open winding test checks the continuity *within* the winding itself, not its isolation from the frame. For this test, the resistance should be extremely high (ideally infinite). If a reading is obtained, it indicates a ground fault, not necessarily an open winding. * **D) Use a growler, listening for noise and vibration levels to diminish when the growler blade is positioned over an open stator coil.** As explained in B, a growler is not the correct tool for finding an open circuit. While an open coil prevents current flow, the growler test relies on induced current to detect short circuits. Its use and interpretation are dedicated to detecting shorts, not opens.