Start With the Right Question, Not the Right Part
"Motor won't start" is a symptom with a long list of possible causes, and the biggest time-waster in the field is jumping straight to replacing the motor before ruling out everything upstream of it. The overwhelming majority of "dead motor" calls are not actually motor failures — they're a missing supply, a control circuit that never energizes the starter coil, or a mechanical bind that the motor can't overcome. A disciplined diagnostic sequence gets you to the real cause in minutes instead of hours.
Before touching a meter, gather three pieces of information: what exactly happens when someone tries to start it (nothing at all, a hum, a trip, a start followed by an immediate stop), whether it ever worked and just stopped or has never run, and whether anything changed recently — a breaker replaced, other equipment added to the same panel, maintenance performed on the motor or driven load.
Step 1: Confirm Power Is Actually Present at the Motor
Lock out and tag out per your facility's procedure, then verify with a meter that the circuit is de-energized before doing any hands-on work — never assume a breaker position tells you the truth about what's live downstream.
With the disconnect open and verified dead, check the obvious upstream culprits: a tripped breaker or blown fuse at the panel, a tripped disconnect switch at the motor, and any loose lugs or backed-out terminal screws in the path — thermal cycling loosens screw terminals over months of vibration and load cycling, and a loose connection can look fine visually while still causing high resistance or an open circuit under load.
Once you've confirmed the disconnect is closed and the breaker is not tripped, re-energize per your LOTO procedure and check line voltage at the motor terminals (or at the starter output, if the motor is remote) with the motor commanded to run. Three readings tell three different stories:
No voltage on any phase — the fault is upstream of this point: open breaker, blown fuse, open disconnect, or a control circuit that never actually closes the starter contacts (see Step 2).
Voltage present on all three phases but the motor won't turn or just hums — this points toward a mechanical bind, a seized bearing, or a locked rotor condition, not a supply problem.
Voltage present on two phases but not the third — this is single-phasing, one of the most common and most damaging motor fault patterns, covered below.
Step 2: Check the Control Circuit Before Blaming the Motor
On any motor started through a magnetic contactor or motor starter — which is the vast majority of three-phase motors above fractional horsepower — the motor doesn't start unless the starter coil is energized, and the coil doesn't energize unless every device in the control circuit permits it. This is a series circuit by design: any single open device blocks the whole thing.
Work through the control circuit devices in order: is the control transformer fuse intact and is control voltage present at the transformer secondary; is the start/stop station wired correctly and not physically damaged; are all e-stops released (a tripped or unreset e-stop, sometimes located on a completely different part of the line, will block starting with no visible indication at the motor itself); are safety interlocks satisfied (guard doors closed, safety relays reset, permissive signals from an upstream or downstream process present); and has the overload relay tripped.
An tripped overload relay is one of the most common "motor won't start" causes and one of the easiest to miss, because a tripped overload often looks identical to any other reason the starter won't pull in — the difference is a small reset button or lever on the overload block that needs to be manually reset (or that resets automatically on some electronic overloads after a cooldown period). If the overload has tripped, don't just reset it and move on — find out why it tripped in the first place, because resetting a genuine overload condition without addressing the cause just delays a repeat trip, and possibly motor damage in between.
Step 3: Single-Phasing — Why It's Especially Damaging
Single-phasing occurs when a three-phase motor loses one of its three supply phases while still connected to the other two — from a blown fuse, an open connection, or a failed contactor pole on just one leg. A motor that is already running when it loses a phase will often keep turning (drawing much higher current on the remaining two phases in a desperate attempt to develop the same torque) and can overheat and burn out within minutes if not protected. A motor that is stopped and then commanded to start while single-phased typically won't start at all, or will hum loudly and draw locked-rotor current without turning.
To check for single-phasing: with the motor de-energized and locked out, measure line-to-line voltage on all three phase pairs at the disconnect or starter line side. All three should read the same nominal voltage within a few percent. A phase reading zero or significantly low identifies the open leg. Trace that phase back toward the source — check the fuse for that phase specifically (a common mistake is checking only for a tripped breaker and missing a blown single fuse in a fused disconnect), the corresponding contactor pole, and every terminal connection along that phase's path.
Electronic overload relays with phase-loss protection will trip immediately on single-phasing and are strongly preferable to basic thermal overloads for exactly this reason — a basic thermal overload only responds to the resulting current imbalance and may not trip fast enough to prevent damage on a motor that was already lightly loaded.
Step 4: Mechanical Binding and Locked Rotor
If line voltage is confirmed present at all three motor terminals and the control circuit is confirmed to be calling for the motor to run, but the motor still won't turn (or hums and trips on overload almost immediately), suspect a mechanical problem rather than an electrical one. Common causes: a seized bearing, a driven load that has jammed (a pump impeller bound by debris, a conveyor with a foreign object, a fan wheel that's struck something), corrosion or rust locking a shaft that's been idle for a long period, or a coupling or belt that's failed in a way that's increasing rather than decreasing the load.
Before re-energizing, de-energize and lock out again, then try to rotate the motor shaft by hand (through the coupling, or directly on the shaft if accessible). A shaft that won't turn by hand, or turns with obvious roughness, grinding, or excessive resistance, confirms a mechanical bind — decouple the motor from its load if possible and test the motor alone to isolate whether the problem is the motor's own bearings or the driven equipment.
A motor that hums loudly, draws very high current, and trips its overload or breaker within a second or two of being commanded to start — but that turns freely by hand when de-energized — is showing classic locked-rotor symptoms, which most often points to the driven load (not the motor) binding under load conditions that don't show up when spinning the shaft with no load attached.
Step 5: The Starter and Contactor Itself
If control voltage is confirmed present at the starter coil terminals but the contactor still doesn't pull in, or pulls in but the motor still doesn't get power, suspect the starter/contactor hardware directly: a burned-out coil (measure coil resistance — an open or shorted coil reads far outside the manufacturer's spec), welded or pitted contacts that no longer close properly, a mechanically stuck armature, or — on multi-pole contactors — one pole that has failed to close while the others operate normally (a subtle cause of single-phasing that originates in the starter itself rather than upstream).
Visually inspect contacts for pitting, burning, or one pole visibly not making contact with the others closed. A contactor that chatters (buzzes and doesn't fully seat) usually indicates low control voltage, a damaged shading coil on an AC contactor, or mechanical binding in the contactor body itself — not a motor problem.
Putting the Sequence Together
In order: verify power is present and balanced across all three phases at the motor terminals; verify the control circuit is actually calling for the motor to run (start station, e-stops, interlocks, overload reset status); check for single-phasing specifically if voltage is present but unbalanced or missing on one leg; check for mechanical binding by hand-rotating a de-energized shaft; and inspect the contactor/starter itself for coil, contact, and pole-balance problems. Following this order — power, then control, then phase balance, then mechanics, then the starter hardware — prevents the common mistake of replacing a perfectly good motor when the real fault was a tripped overload relay or a single blown fuse.