Motor Will Not Start: A Troubleshooting Guide

When a motor will not start, work from the control circuit toward the load. Most problems are found by asking whether the contactor pulls in and whether the motor gets power.

Safety. Motor circuits can kill. Lock out and tag out before touching the motor, its leads or the starter's power terminals. Some tests require the circuit to be live; those must be done by qualified persons following your electrical safety procedures, with appropriate protective equipment. When in doubt, de-energize.

Step 1: What does the starter do?

What you see	Go to
Nothing: contactor does not pull in	The control circuit, below
Contactor pulls in, motor does not turn	The power circuit and the load
Motor hums but does not turn	Single-phasing or a locked load
Motor starts, then trips	Overload and loading
Drive-fed motor	Check the drive status and fault first (see the VFD fault guide)

Contactor does not pull in

The contactor coil needs a complete control circuit. Work through it in order:

Control power. Is the control transformer or 24 V supply on, and are its fuses and breakers good?
Overload relay. Has it tripped? A tripped overload opens the control circuit. Find out why before resetting.
Stop and emergency stop devices and safety circuits. One open device stops the start. Check which are open and why.
Interlocks and permissives. Guards, limit switches, pressure or level switches and sequencing may block the start.
The start command. Is it coming from a pushbutton, a selector or a PLC output? If it is a PLC output, check whether the output is on and whether the program is calling for the run.
The coil itself. With the circuit de-energized, check the coil for continuity, and check for the correct coil voltage.

Contactor pulls in, motor does not turn

Confirm that all three phases are present on the line side of the contactor and that the fuses or breaker are good.
Confirm that all three phases reach the motor terminals when the contactor is closed. This is a live measurement and must be done safely by a qualified person.
Check for loose or burnt contacts, and for a motor lead that has come loose in the junction box.
De-energize, lock out and try to turn the shaft by hand. A seized motor or a jammed machine will not start. Check that a brake has released.

Motor hums but does not turn

This usually means the motor is not getting all three phases (single-phasing) or that the load is too heavy or locked. A single-phased motor can overheat quickly, so shut it down. Check for a blown fuse, a failed contact, a loose lead or an open winding. Low supply voltage during starting can also cause it.

Motor starts then trips

Overload set too low. Compare the overload setting to the motor nameplate's full-load current. The motor current calculator shows the NEC table value for comparison, but the nameplate takes precedence.
The load is heavier than expected. Measure the running current and compare it with the nameplate. Currents above nameplate mean mechanical trouble or a voltage problem.
Low voltage. Voltage that sags during starting raises the current and can trip protection. Check the voltage at the motor during the start, and check the voltage drop of long cable runs.
Frequent starts. Repeated starting heats the motor. Check the start frequency against the motor's limits.
Bearings or a dragging load. Noise, heat and rough rotation by hand point to a mechanical cause.

Before you restart

After a fix, think about why the failure happened. A tripped overload that is simply reset will probably trip again. Record what you found, what you changed and the readings you took, so the next person has a head start.

This is general guidance for training and reference. Always follow your site's electrical safety procedures and the equipment manufacturer's documentation.