Troubleshooting
Troubleshooting: Motor Won't Start
One question splits the whole problem: does the contactor pull in? That boundary separates the control circuit from the power circuit before you measure anything.
Safety. This is a reasoning aid, not a work instruction. Diagnosis and testing are performed by qualified personnel under your site’s LOTO procedures. Rotate a shaft by hand only under confirmed zero-power, zero-stored-energy conditions. Measure across live power terminals only when trained and equipped to do so. Measure, don’t guess.
Overview
A motor fed by a starter — across-the-line contactor, star-delta, or soft starter in bypass — that will not start is one of the most common field calls, and one of the most over-diagnosed. The trap is chasing the motor when the fault is in the control circuit, or chasing the control circuit when a power leg is open.
This page is for starter-driven and general no-starts. If the motor is fed by a variable frequency drive, the failure modes are different — start at Troubleshooting VFD Faults instead.
Start Here
The pivot question does the heavy lifting: does the contactor pull in when you command a start? Before touching anything, observe:
- Does the contactor energize? Listen/look for the pull-in. This alone splits control-side from power-side.
- Any overload trip indicator? A tripped overload explains both a no-pull-in and an open power leg.
- Is control power present? A dead CPT kills the whole control circuit.
- Any sound from the motor? A hum with no rotation is a specific power-side signature (single-phasing).
- What changed? New wiring, a recent trip, a jammed load, a brake that may not be releasing.
Decision Tree
flowchart TD
A[Does the contactor pull in?] -->|No| CTRL[Control circuit]
A -->|Yes, but no rotation| PWR[Power circuit]
A -->|Pulls in, motor hums| HUM[Lost phase / single-phasing]
A -->|Starts, then overload trips| OL[Load / sizing / transition]
CTRL --> C1[E-stop / interlock open]
CTRL --> C2[Overload tripped - aux contact open]
CTRL --> C3[No start command at the coil]
CTRL --> C4[Control power / CPT dead]
CTRL --> C5[Seal-in / holding circuit broken]
PWR --> P1[Blown fuse / lost phase]
PWR --> P2[Overload heater / element open]
PWR --> P3[Motor connection wrong or open]
PWR --> P4[Mechanical seize / brake still set]
HUM --> H1[One open leg: fuse, contact pole,<br/>conductor - measure all 3 phases]
OL --> O1[Load too high / jammed]
OL --> O2[Overload set below FLA]
OL --> O3[Star-delta transition fault]
Likely Causes
Contactor does not pull in — control circuit
- E-stop or interlock open — any series stop, guard interlock, or safety relay contact breaks the string. Check each is satisfied and its contact actually closed, not merely reset at the operator station.
- Overload tripped (aux contact open) — the overload’s auxiliary contact sits in the control string; a trip opens it. Looks like: ran, tripped, won’t restart until reset (and won’t reset until cooled). Check the trip indicator and aux-contact state.
- No start command — the pushbutton, PLC output, or run signal never reaches the coil. Check for the command at the control input, not at the HMI — an HMI “start” that never became a real output is a classic trap.
- Control power absent — a dead CPT, blown control fuse, or lost control supply. Nothing in the control circuit works. Route to control power wiring.
- Seal-in / holding circuit broken — a start that won’t hold: the contactor chatters or drops the instant the button is released, because the seal-in auxiliary contact or its wiring is open. See the seal-in topology in the motor-starter wiring guide.
Contactor pulls in but motor does not turn — power circuit
- Blown fuse / lost phase on one line — one of L1/L2/L3 missing downstream of the contactor. Motor hums, will not accelerate, trips overload. Check voltage on all three phases at the load side.
- Overload heater / element open — an open thermal element or burned heater breaks a power leg. Check continuity/voltage across each overload pole with the contactor closed.
- Motor connection wrong or open — a terminal-box wiring error (wrong star/delta arrangement) or an open winding lead. Check terminals against the nameplate diagram; check winding-resistance balance across phases.
- Mechanical seizure or brake still set — the motor is fine but the load or a failed-closed brake holds the shaft. Full current, overload trip. Try to rotate the shaft by hand under zero-power conditions; confirm the brake releases.
Motor hums / single-phases
- What it looks like — loud hum, no rotation from rest, rapid heating, overload trip in seconds. Will sometimes run if spun by hand — a giveaway.
- Cause — a lost phase: one open fuse, one failed contact pole, one broken conductor or terminal. A three-phase motor cannot start on two phases.
- Check — measure all three phases at the motor terminals with the contactor closed; the missing leg is the fault.
Trips overload on start
- What it looks like — begins to turn, accelerates slowly or partially, then trips; may restart and trip repeatedly.
- Causes — load too high or jammed; overload set below the motor’s actual FLA; excessive start frequency without cooling; or a star-delta transition fault — bad transition timer, open transition contact, or too-early switchover dropping the motor off during acceleration.
- Check — overload setting against nameplate FLA; test the load free/unjammed; on star-delta, verify the transition sequence and timing — wiring in the motor-starter guide.
What to Measure
- Voltage present at each stage — supply L1/L2/L3 at the disconnect, the line side of the contactor, the load side of the contactor, and the motor terminals. Walking the voltage down these stages localizes an open to one segment.
- Control voltage at the coil — is the coil actually seeing pull-in voltage? No coil voltage sends you into the control-circuit branch; coil voltage present with no pull-in means a failed coil or contactor. See control power wiring.
- Overload state — tripped or reset, and the aux-contact state. A tripped overload explains both a no-pull-in (aux open in the control string) and a power-leg open (element open) — check it early.
Common Root Causes
| Symptom | Likely cause | First check | Typical fix |
|---|---|---|---|
| Contactor won’t pull in | E-stop/interlock open in control string | Each interlock satisfied and closed? | Restore the open contact |
| Won’t pull in, ran fine earlier | Overload tripped, aux contact open | Overload trip indicator | Find load cause, reset |
| Won’t pull in, whole control dead | Control power / CPT / fuse | Voltage downstream of CPT | Restore control power |
| Starts but drops out on release | Seal-in / holding circuit broken | Seal-in aux contact and wiring | Repair seal-in path |
| Pulls in, no rotation | Lost phase / blown fuse | Voltage on all 3 phases at load side | Replace fuse / repair open leg |
| Pulls in, motor hums, heats | Single-phasing | Measure 3 phases at motor terminals | Restore the missing leg |
| Pulls in, full current, no turn | Mechanical seize / brake set | Rotate shaft by hand (zero power) | Free the load / release brake |
| Starts then trips overload | Load, undersized OL, or transition fault | OL setting vs FLA; star-delta timing | Correct sizing/load/transition |
When It’s Not What It Looks Like
- A “dead motor” that is a tripped overload. The motor and power circuit are fine; the overload’s aux contact has opened the control string. Reads like a no-start; it’s a control-side symptom of a power-side event — find why it tripped before resetting.
- A “control fault” that is single-phasing. The contactor pulls in perfectly, so the control side looks proven — but a lost phase downstream means no torque. Always confirm all three phases at the motor when the contactor closes but nothing turns.
- A “motor failure” that is a brake that never released. Full current and an overload trip look like a shorted or seized motor; the shaft is simply held by a failed-closed holding brake. Confirm brake release first.
- A “start button fault” that is a broken seal-in. The motor runs only while the button is held. The start command works; the holding contact or its wiring is open — a motor-starter seal-in problem, not a bad pushbutton.
- An overload trip that is really a transition fault. On star-delta, a mistimed or open transition looks like an oversized load tripping the overload — but the motor was accelerating fine until the switchover.
Related Pages
- How to Wire a Motor Starter — seal-in, overload, and star-delta transition wiring
- Control Power Wiring — CPT, control fusing, and the coil circuit
- Troubleshooting VFD Faults — for drive-fed motors instead of starters
- Motor Fundamentals — nameplate, FLA, slip, and torque behaviour
- Motor Troubleshooting Decision Tree — the broader motor/drive routing page
Trust Boundary — Engineering Judgment Required
This site is an independent educational reference for industrial automation standards. It paraphrases and organizes guidance in original language; it is not a substitute for the official published standards, professional engineering judgment, or legal review, and content has not been independently verified against current published editions.
Items marked Review pending await technical review against the identified edition. Items marked Planned have no reviewed source material behind them. Do not rely on this site for compliance determinations, safety-critical design decisions, or legal interpretation.