A motor that will not start is one of the most common complaints, and it has a structured diagnosis that follows from the symptoms. Whether the motor is completely dead or hums without turning tells you a great deal, and from there a decision tree leads efficiently to the cause — power, control, single-phasing, mechanical, or the motor itself. Understanding this diagnosis, built on the fundamentals and the method, resolves the won’t-start complaint systematically.
Figure 13.1 — Decision tree for a motor that won’t start. Whether it hums, whether all three phases are present, and whether the shaft turns by hand lead systematically to the cause — always working safely with proper procedure.
Dead: no hum, nothing
If the motor is completely dead — no hum, no attempt to turn, nothing — the problem is that the motor is not getting power, and the diagnosis follows the power and control path. Something is preventing power from reaching the motor: no supply, a tripped or open disconnect or breaker, blown fuses, a contactor not closing, an open control circuit, or tripped protection. The diagnosis works back through this path: is there supply, is the disconnect on, are the fuses good, is the contactor closing, is the control circuit operating, has the protection tripped? Each is checked (safely) until the break in the power or control path is found. A completely dead motor almost always has an interruption in getting power to it, and finding that interruption — in the supply, the switching, the protection, or the control — is the diagnosis. Understanding that a dead motor with no hum points to a power or control interruption directs the diagnosis to that path, working back from the motor through the overload, contactor, fuses, disconnect, control circuit, and supply to find where power is being stopped. This is a systematic search of the power and control path for the interruption that leaves the motor dead.
Hums but won’t turn
If the motor hums but will not turn, the situation is different and more specific: the motor is getting some power but cannot start turning, which points to a limited set of causes. The most important is single-phasing — the loss of one of the three phases — which leaves the motor with power on two phases, able to hum but not to start, because two phases alone cannot create the rotating field needed to start. Other causes are a locked or jammed rotor (mechanical obstruction preventing turning), low voltage (insufficient to start under load), or a wrong connection. The humming indicates power is present but the motor cannot start, so the diagnosis checks these: are all three phases present at the motor (checking for single-phasing), does the shaft turn by hand (checking for mechanical lock), is the voltage adequate? Single-phasing is a prime suspect and is checked by verifying all three phases reach the motor. Understanding that a humming, non-starting motor points to single-phasing, a mechanical lock, low voltage, or a connection problem directs the diagnosis to these specific causes, with single-phasing foremost, and the checks — three phases present, shaft turns freely, voltage adequate — distinguish among them to find why the powered motor cannot start turning.
Working the decision tree
The won’t-start diagnosis comes together as a decision tree that uses a few key observations to reach the cause efficiently. First, does the motor hum? No hum leads to the power and control path; a hum leads to the single-phasing and mechanical checks. If it hums, are all three phases present at the motor? Missing a phase means single-phasing, and the open phase must be found. If all three phases are present, does the shaft turn by hand? A shaft that will not turn means a mechanical lock; a shaft that turns freely with all phases present but the motor still not starting points to the motor itself, calling for motor testing (voltage at terminals, insulation, winding resistance, overload setting). Following this tree — hum or not, three phases or not, shaft turns or not — leads systematically from the symptom to the cause, with each branch narrowing the possibilities. Understanding and working this decision tree, always safely, resolves the won’t-start complaint methodically: the observations at each branch point direct you down the correct path to the cause, whether it is a power or control interruption, single-phasing, a mechanical lock, or a fault in the motor itself, reached by a logical sequence of simple checks rather than by guesswork.
The two big branches
The won’t-start diagnosis hinges on one early observation that splits it into two big branches: does the motor hum? This single question divides the problem cleanly, and understanding why makes the diagnosis efficient. If the motor is completely dead with no hum, it is getting no power at all, so the problem is an interruption in the power or control path — the motor is not even energized. If the motor hums, it is getting some power (it is energized) but cannot turn, so the problem is different: it can develop a field or partial field but cannot start, pointing to single-phasing, a mechanical lock, or low voltage. These two branches — no hum (no power) versus hum (power but can’t turn) — lead to entirely different investigations: the power and control path for the dead motor, the phase and mechanical checks for the humming one. So the hum question is the key early split, and understanding why it divides the problem so cleanly — no hum means not energized, hum means energized but unable to start — makes it the natural first branch of the won’t-start diagnosis. Asking this one question first, and following the appropriate branch, structures the whole won’t-start diagnosis efficiently, directing you down the correct path from the single, easily made observation of whether the motor hums.
Scenario: humming points to single-phasing
A scenario shows the humming branch leading to single-phasing. A motor would not start but hummed loudly when the start was attempted. Following the won’t-start diagnosis, the hum indicated the motor was getting power but could not start turning, directing the diagnosis to single-phasing, a mechanical lock, or low voltage. Checking all three phases at the motor revealed one phase missing — single-phasing — so the motor, powered on only two phases, could hum but not develop the rotating field needed to start. Tracing the missing phase found a blown fuse in that phase, which had left the motor single-phased. Replacing the fuse (after checking why it blew) restored all three phases, and the motor started normally. This scenario shows the humming symptom leading correctly to single-phasing via the diagnosis: the hum directed the check to the phases, which revealed the missing phase, traced to a blown fuse. It reinforces that a humming, non-starting motor points to single-phasing among a few causes, checked by verifying all three phases at the motor, and that finding the missing phase (here a blown fuse) resolves it. The scenario illustrates the humming branch of the won’t-start diagnosis working as intended, leading from the symptom to single-phasing to the specific open — a blown fuse — that caused it.
When all checks point to the motor
If the won’t-start decision tree runs all the way through — the motor hums, all three phases are present, the shaft turns freely — yet the motor still will not start, the problem points to the motor itself, and understanding this endpoint directs the final diagnosis. Having eliminated the power and control path (the motor hums, so it is powered), single-phasing (all three phases present), and a mechanical lock (the shaft turns freely), what remains is a fault within the motor: a winding problem, an internal connection fault, or a rotor problem, preventing it from developing the torque to start despite being properly powered and mechanically free. At this point, the diagnosis turns to testing the motor — measuring the voltage actually at its terminals (confirming proper supply reaches it), and testing the windings (insulation and winding resistance) to find an internal electrical fault. Understanding that reaching this endpoint of the tree — powered, all phases present, mechanically free, but still not starting — points to the motor itself directs the final diagnosis to motor testing. It reinforces that the decision tree systematically eliminates the external causes (power, control, phases, mechanical) so that if none is found, the fault is localized to the motor, calling for the motor tests that examine its internal electrical condition to find the winding or connection fault preventing it from starting despite everything external being correct.
A model for other diagnoses
The won’t-start diagnosis, with its decision tree, is a model for approaching other motor complaints, and understanding it as a model shows how to structure any diagnosis. The won’t-start diagnosis works by using key observations (hums or not, all phases or not, shaft turns or not) to branch systematically toward the cause, each observation narrowing the possibilities. This structure — key observations branching to localize the fault — applies to other complaints too: an overheating diagnosis branches on the current, balance, voltage, and cooling; a noise diagnosis branches on whether uncoupling and shaft-turning reveal a mechanical or electrical cause. So the won’t-start decision tree models a general approach: identify the key observations that distinguish the possible causes, and use them to branch systematically toward the fault. Understanding the won’t-start diagnosis as a model — a structured branching on key observations — shows how to structure other diagnoses similarly. It reinforces that systematic troubleshooting is about using discriminating observations to narrow down the cause, which the won’t-start tree exemplifies clearly, so that the approach it models — branch on key observations to localize the fault — can be applied to any motor complaint, structuring the diagnosis around the observations that best distinguish the possible causes. The won’t-start diagnosis is thus not just one procedure but a model for the systematic, observation-driven branching that characterizes good troubleshooting of any motor problem.
The most common causes first
A practical efficiency for the won’t-start diagnosis: within the structured approach, check the most common and easily checked causes first. While the decision tree provides the structure, efficiency within it comes from checking the likely, easy things before the unlikely, hard ones. For a dead motor, check the obvious power and control things first — is the disconnect on, has an obvious protective device tripped, are the fuses good — before deeper investigation. For a humming motor, check for single-phasing (a common cause, easily checked by measuring the phases) early. This ordering within the structured diagnosis — common and easy causes first — resolves many faults quickly while retaining the structure for the harder cases. Understanding that the structure can be traversed efficiently by checking likely, easy causes first makes the won’t-start diagnosis both structured and efficient. It reinforces applying the decision tree with practical efficiency: follow its structure, but within it check the common, easily checked causes first, resolving many faults quickly. This combines the reliability of the structured approach with the efficiency of checking the likely things first, so that the won’t-start diagnosis is both systematic (the tree) and efficient (common causes first), quickly resolving the many faults with common causes while retaining the structure to handle the less common ones methodically, which is how experienced technicians apply the diagnosis — structured but efficient, likely causes first within the reliable framework.