A 380 V laminator motor causes the protective device to trip when connected. Disconnecting one phase conductor prevents the trip, but the motor does not rotate. This pattern localizes the fault to the motor circuit, but it does not prove whether the defect is in a winding, motor cable, terminal box, or connection.
Interpret the observed behavior
| Test condition | Observed result | Engineering conclusion |
|---|---|---|
| Motor disconnected | Protection does not trip | The fault appears only when the motor branch is connected. |
| Motor connected | Protection trips | Suspect a short circuit or insulation fault in the motor, its cable, terminal box, or connections. |
| One phase conductor removed | No trip; motor does not rotate | The removed conductor changes the fault path. This supports—but does not prove—a fault associated with that phase or winding. |
Isolate the fault safely
Do not continue energizing the motor with a phase conductor removed. Isolate and lock out the supply before opening the terminal box or disconnecting conductors. Record every conductor position before disturbing the connections.
- Inspect the motor cable, terminals, links, and insulation for loose strands, damaged insulation, contamination, or contact with the enclosure.
- With the motor isolated, have a qualified technician test continuity between winding terminals and insulation between each terminal and the motor frame.
- If possible, separate the motor from its outgoing cable and test the cable and motor independently. This distinguishes a cable or terminal fault from an internal winding fault.
- If testing confirms an internal short circuit or insulation failure, send the motor to a specialized repair shop or replace it.
Evaluate the suspected winding fault
A winding associated with one phase may be nearly short-circuited, which matches the reported behavior. Treat this only as a working hypothesis until isolated electrical tests confirm it. Also verify that no terminal link or conductor strand is creating a short inside the terminal box.
Do not select a replacement motor or alter protection settings from the available evidence. First capture the motor nameplate data, connection arrangement, and test results; the source provides only the 380 V marking and trip symptoms.
Verify before returning to service
After repairing the cable, terminals, or motor, reconnect every phase conductor in its documented position. Verify acceptable phase-to-phase continuity and phase-to-frame insulation before energizing. On the first powered test, confirm that the protective device remains set as originally engineered and that the motor starts and rotates without another trip.
FAQ
Why does my 380 V motor trip the protection when connected?
The motor branch may contain a short circuit or insulation fault. Test the cable, terminal box, winding continuity, and insulation to the motor frame separately before deciding that the winding has failed.
Why does the trip stop when I disconnect one motor phase?
Removing that conductor interrupts or changes the suspected fault path. It points toward that phase circuit but does not distinguish between a damaged cable, terminal fault, and internal winding fault.
Can I run the motor with one phase disconnected?
No. In the reported test the motor did not rotate, and the missing conductor is masking the original fault rather than repairing it. Isolate the supply and test the complete motor branch.