Fault inspection of the motor(2)

Fuse Blown or Thermal Relay Tripped During Startup

Fault Inspection Steps

Check if the fuse rating is appropriate. If it is too small, replace it with a suitable one and test again. If the fuse continues to blow, inspect whether the drive belt is too tight or the driven load is excessive, check for short circuits in the circuit, and verify if the motor itself has a short circuit or ground fault.

Ground Fault Inspection Method

Measure the insulation resistance of the motor windings to ground using a megohmmeter. If the insulation resistance is below 0.2 MΩ, the windings are severely damp and shall be dried. If the resistance is zero or a test lamp glows near normal brightness, the phase is grounded. Winding grounding usually occurs at the motor terminal box, power cable entry hole, or where windings extend out of the slot. For minor grounding faults at the slot outlet, insert bamboo strips or insulating paper between the stator core and windings. After confirming no grounding exists, wrap the windings, apply insulating varnish, dry them, and reuse only after passing inspection.

Winding Short Circuit Inspection Method

Disconnect connecting wires and measure the insulation resistance between any two phases using a megohmmeter or multimeter. A reading below 0.2 MΩ or close to zero indicates an inter-phase short circuit. Measure the current of the three windings separately; the phase with abnormally high current is the short-circuited phase. An inter-turn short circuit detector can also be used to check inter-phase and inter-turn short circuits.

Method for Identifying Winding Start and Terminals

During motor repair and inspection, it may be necessary to reidentify the start and terminal ends of stator windings if terminal markings are missing or forgotten. Common methods include residual magnetism test, induction test, diode indication test, and direct terminal swapping verification. The first few methods require instruments and practical experience, while the terminal swapping method is simpler, safer, more reliable and intuitive.

Use the multimeter resistance range to identify which two terminals belong to the same phase, then arbitrarily mark the start and terminal ends of each phase. Connect the three marked starts (or terminals) to the power supply separately, and connect the remaining three terminals (or starts) together. Start the motor at no-load. If startup is slow and accompanied by loud noise, one phase is reversed. Immediately cut off power, swap the two terminals of one phase, and restart. If the problem persists, the swapped phase is correct; restore its connection and swap the other two phases in sequence until the motor starts normally. This method is simple but only applicable to small and medium-sized motors permitted for direct-on-line starting. It shall not be used for large-capacity motors that prohibit direct starting.

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