Single-phasing — the loss of one of the three phases — deserves its own chapter because it is a common, serious, and characteristic fault. A motor that loses a phase behaves in distinctive ways and can quickly overheat and be damaged, so recognizing single-phasing and understanding its effects and causes is important. It is also a fault that reinforces why measuring all three phases matters, since single-phasing is invisible if you only check one.
Figure 14.1 — Single-phasing: one phase lost. A blown fuse, open contact, or broken conductor leaves the motor on two phases. It keeps running (if already spinning) but the two live phases carry much higher current, causing rapid overheating; from standstill it only hums.
What single-phasing does
When a three-phase motor loses one phase, its behavior depends on whether it was already running. If the motor was already running when the phase was lost, it continues to run on the remaining two phases — but the two live phases now carry much higher current to supply the power that three phases previously shared, causing rapid overheating that can quickly damage the windings. If the motor was stopped when a phase was lost, it cannot start on two phases — two phases cannot create the rotating field needed for starting — so it only hums without turning, drawing heavy current, which will overheat it if not disconnected. So single-phasing causes overheating and high current in both cases: a running motor overheats from the doubled current in two phases, and a stopped motor overheats from the heavy locked-rotor current while merely humming. Understanding these effects — a running motor continues but overheats, a stopped motor only hums and overheats — explains why single-phasing is serious and must be caught quickly: it rapidly overheats and damages the motor, whether the motor was running or stopped when the phase was lost, making prompt recognition and disconnection important to prevent winding damage.
Recognizing single-phasing
Recognizing single-phasing comes from its characteristic signs and from measuring all three phases. The signs include a motor that hums but will not start (if it lost a phase while stopped), a running motor that is overheating and drawing high current on two legs, a growling noise, and tripping protection. The definitive check is to measure all three phases: single-phasing shows as a missing or very low voltage on one phase, or heavily imbalanced currents with one phase low or zero. This is exactly why measuring all three phases matters — single-phasing is obvious when you check all three and see one missing, but completely invisible if you only measure one phase and happen to pick a live one. Recognizing single-phasing thus relies on the symptoms (hum, overheating, high current, growling, tripping) and confirming with a three-phase measurement showing the lost phase. Understanding how to recognize single-phasing — its symptoms and the three-phase measurement that confirms it — lets you catch this common, damaging fault, and it underscores the importance of always measuring all three phases, since single-phasing is a fault that a single-phase measurement would miss entirely while a three-phase measurement reveals immediately.
Finding the lost phase
Once single-phasing is recognized, finding where the phase is lost follows the power path to locate the open. The phase can be lost anywhere in the supply to the motor: a blown fuse, an open contact in the contactor, a tripped single-pole device, a broken or disconnected conductor, or a bad connection in one phase. Finding it means tracing that phase’s path and checking for the open — measuring along the phase to find where it goes from present to absent, or checking each element (fuse, contact, connection) in the affected phase. The split-half approach applies: measure at a midpoint of the affected phase to determine which half contains the open, then narrow down. Common culprits are a single blown fuse (leaving the other two phases intact), a contactor with one worn or non-closing contact, and a loose or broken connection in one phase. Understanding how to find the lost phase — tracing the affected phase’s path to locate the open, checking the common culprits — completes the single-phasing diagnosis, from recognizing the fault to locating and fixing the open that caused it. Finding the open in the one affected phase, whether a fuse, contact, or connection, and restoring it returns the motor to full three-phase operation.
Why single-phasing is so damaging
Single-phasing is particularly damaging, and understanding why explains the urgency of catching it. When a running motor loses a phase, it still must deliver its mechanical load, which requires a certain power, but now only two phases are available to supply that power instead of three. The two remaining phases must therefore carry much more current — substantially more than their normal share — to make up for the lost phase and deliver the same power. This greatly increased current in the two live phases causes rapid overheating, far faster than a normal overload, because the current is so much higher than normal in those windings. The motor may not stop — it keeps running on two phases — so the overheating continues unnoticed unless caught, potentially burning out the windings quickly. This is why single-phasing is so damaging: it forces excessive current through two phases while the motor keeps running, causing rapid, continuing overheating that can destroy the motor. Understanding this — the lost phase’s power must come from the other two, forcing excessive current and rapid overheating — explains the urgency of recognizing single-phasing quickly, because the damage accumulates fast while the motor continues to run, making prompt detection and disconnection important to save the motor from the rapid overheating that single-phasing causes.
Scenario: the overheating motor on two phases
A scenario shows single-phasing in a running motor. A motor that had been running fine began overheating, drawing high current, and making a growling noise, though it kept running. Recognizing these as possible single-phasing symptoms, the technician measured all three phases and found one phase carrying little or no current while the other two carried much more than normal — the signature of single-phasing in a running motor. A phase had been lost (traced to a failed connection), and the motor, continuing to run on two phases, was forcing excessive current through them, causing the overheating and growling. Because it was caught — by measuring all three phases when the overheating and noise appeared — the motor was stopped before the windings burned out. Restoring the lost phase returned the motor to balanced three-phase operation. This scenario shows single-phasing in a running motor recognized by its symptoms (overheating, high current, growling, still running) and confirmed by the three-phase measurement showing one phase low and the others high. It reinforces that single-phasing can strike a running motor, causing rapid overheating from the excessive current in the two remaining phases, and that measuring all three phases catches it — revealing the low phase and the high others — in time to stop the motor before the overheating destroys it, which is why prompt recognition of the single-phasing symptoms and the three-phase check are so important.
Preventing single-phasing damage
Beyond recognizing and finding single-phasing, understanding how it is prevented and its damage limited completes the picture. Modern motor protection often includes phase-loss or phase-imbalance protection specifically designed to detect single-phasing and disconnect the motor quickly, before the overheating damages it — a valuable protection given how rapidly single-phasing can burn out a motor. Proper overload protection also helps, as the high current of single-phasing may trip the overload, though phase-loss protection is more direct. Good maintenance — sound connections, good contactor contacts, quality fuses — reduces the causes of single-phasing (failed connections, worn contacts, blown fuses). Understanding these preventive measures — phase-loss protection to catch single-phasing quickly, and good maintenance to reduce its causes — shows how the damage from this serious fault is limited. It reinforces that single-phasing, being so damaging, is worth protecting against specifically (phase-loss protection) and preventing through good maintenance of the connections, contacts, and fuses whose failure causes it. When troubleshooting, the presence or absence of phase-loss protection is relevant, and recommending it where a motor lacks it is good practice given single-phasing’s destructiveness. Understanding the prevention and protection against single-phasing — not just its diagnosis — completes competent handling of this important fault, addressing not only how to find it but how to guard against its rapid, damaging effects.
Why measuring all three phases is a rule
Single-phasing makes vivid why measuring all three phases is a firm rule of motor work, and understanding this cements the practice. Single-phasing — a common, serious, damaging fault — is invisible if you measure only one phase and happen to pick a live one, which would show normal, missing the fault entirely, while the motor overheats toward destruction. Measuring all three phases reveals it immediately: the missing or low phase stands out against the others. So single-phasing is the clearest reason for the rule of always measuring all three phases: a fault this common and damaging must not be missed, and only measuring all three catches it reliably. This rule — measure all three phases, always — protects against missing single-phasing and the other imbalance faults that a single-phase measurement would overlook. Understanding why the rule exists — single-phasing and imbalance are invisible to a single-phase measurement but obvious across all three — cements the practice of always checking all three phases. It reinforces that measuring all three phases is not optional thoroughness but a firm rule, justified by faults like single-phasing that are common, serious, and detectable only by comparing all three phases. Adopting this rule — always measure all three — ensures you catch single-phasing and imbalance faults that would otherwise be missed, which is why measuring all three phases is emphasized throughout as a fundamental practice of competent, thorough motor troubleshooting.
The fuse that tells a story
A practical detail in single-phasing: when you find a blown fuse causing single-phasing, ask why it blew, because the fuse blowing is often itself a symptom of another problem. A fuse does not usually blow without cause — it blew because of an overcurrent, which might have been a fault, an overload, or an aging fuse, but the cause should be considered rather than just replacing the fuse. Simply replacing the blown fuse restores the phase, but if the fuse blew because of a fault or overload, the replacement may blow again, or the underlying problem may persist. Understanding that a blown fuse has a cause motivates asking why it blew when you find one causing single-phasing, addressing the underlying cause and not just the symptom. It reinforces that finding the blown fuse is not always the end of the diagnosis: the fuse blew for a reason, and that reason — a fault, overload, or simply the fuse’s age — should be considered, so that the single-phasing is not just temporarily fixed by a new fuse that might blow again. The blown fuse tells a story — it blew because of an overcurrent — and reading that story, asking why it blew, ensures the real cause is addressed, not just the blown fuse replaced, which is the difference between truly fixing the single-phasing and merely restoring the phase until the underlying cause blows the fuse again.