When an electric motor fails to start after power is applied, the cause typically lies in one of three areas: mechanical binding of the driven equipment, malfunction of the starting apparatus, or defects within the motor itself. Accurate diagnosis requires systematic inspection to determine the source of the fault.

Initial Voltage Assessment

The first step in diagnosis is to measure the three-phase voltage at the motor terminals using a multimeter or a test pencil after energizing the circuit.

• Unbalanced or Missing Phase Voltage
  If the measured voltages are unbalanced or one phase is absent, the problem likely originates in the starting equipment. Conversely, if the voltages are balanced but the motor rotates sluggishly and emits abnormal noise, the issue may be excessive load or mechanical seizure. Disconnect power immediately and manually rotate the motor shaft. Smooth and uniform rotation indicates an overload condition; resistance or uneven movement suggests mechanical binding within the driven equipment.

• Normal Voltage but No Rotation
  When all three-phase voltages are correct and the motor does not turn, the cause is either a severe mechanical lock or an internal motor fault. Isolate the motor from the driven machinery, manually rotate both shafts, and attempt a standalone motor start. This procedure distinguishes between mechanical and electrical causes, allowing focused troubleshooting.

Mechanical Binding of Driven Equipment

When the mechanical load is jammed, it is essential to collaborate with maintenance personnel. Disassemble the affected machinery, inspect for deformation, foreign objects, or lubrication failure, and restore free movement before re-engaging the motor. Smooth shaft rotation is imperative to prevent recurrent startup failures.

Starting Equipment Malfunctions

Faults in the control and protection devices are a common source of startup issues. Key areas to inspect include:

• Switchgear and Contactors
  Verify the integrity of all contacts and terminal connections. Clean or replace pitted or carbonized contacts to ensure proper conduction.

• Thermal Overload Relays
  Confirm that the overload relay contacts are closed and the current setting is properly calibrated to the motor's rated values. Incorrect adjustment can falsely trigger protection circuits.

• Fuses and Circuit Protection
  Inspect the continuity of fuse elements. If a fuse is blown, analyze the underlying cause before replacing it. Select fuse ratings based on the motor's inrush and operating characteristics to prevent nuisance failures.

• Internal Wiring Errors
  Cross-check the starter’s wiring against the schematic. Any deviations must be corrected meticulously to ensure accurate phase sequencing and protection function.

Motor Internal Faults

If the starting apparatus and mechanical load are in good condition, the motor itself may be compromised. Common defects include:

• Stator Winding Failures
  Check for open circuits using a multimeter or megohmmeter. Locate and reconnect any broken points in the winding.

• Ground Faults
  Measure insulation resistance to ground with a megohmmeter. A zero reading indicates a ground fault, necessitating precise identification and repair of the affected section.

• Inter-Turn Short Circuits
  Use a short-circuit detector to examine stator windings. Significant shorting typically requires rewinding of the motor to restore proper operation.

• Bearing Damage
  Worn or damaged bearings can cause shaft misalignment, rotor-stator friction, or complete seizure. Replace defective bearings promptly to avoid rotor core damage.

• Incorrect Winding Connections
  After a rewind, misconnection of winding start and end leads can prevent proper rotation. Verify phase winding polarity and correct any erroneous terminations.

Silent Motor with No Rotation

A motor that shows no sound and no movement usually indicates a lack of supply voltage. Measure the voltage at the motor output terminals. Absence of voltage or presence on only one phase signals a power supply interruption. Perform a step-by-step backtrace from the motor toward the source to locate the break.
If the motor receives balanced voltage at its terminals but remains silent, an internal disconnection is likely. The neutral point or lead wire may have fractured. Use open-circuit testing to pinpoint and repair the discontinuity.

Motor Hums Without Rotation

If the motor emits a “humming” sound but fails to rotate, a blown fuse may have caused phase loss. Disconnect power immediately, inspect all protective devices, determine the root cause, and replace the fuse with an appropriate element.

Low Supply Voltage and Wiring Errors

Undervoltage conditions or incorrect wiring can also impede startup. A delta (Δ) configured motor mistakenly wired in a star (Y) connection, especially under load, can fail to start due to insufficient torque. Verify both the supply voltage and the motor’s terminal connections to ensure correct configuration for the application.

By methodically isolating mechanical, electrical, and control-related variables, most motor startup failures can be accurately diagnosed and rectified, ensuring safe and reliable operation.