Locked rotor current, often abbreviated LRA for locked rotor amps, is the current a motor draws when voltage is applied but the shaft is not turning, either at the very first instant of starting or when a running motor stalls. With the rotor stationary the motor offers very little opposition to current, so it pulls several times its normal running current, commonly in the range of six to eight times rated. This large, foundational number is what protection devices and cable sizing have to accommodate, and how long a motor can survive at that current is what separates a normal start from a damaging stall. Understanding LRA is understanding the worst-case current a motor circuit must be designed around.
Locked Rotor Current in one line: Locked rotor current (LRA) is the steady current a motor draws when energized with the rotor held stationary, at startup before it accelerates or during a stall, and it is typically about six to eight times the motor's rated full-load current. Because the stationary rotor generates no counter-voltage to limit current, the motor draws this large current until it either accelerates away or the protection removes it. NEMA code letters classify a motor's locked rotor current, and the value drives overload and short-circuit protection settings and conductor sizing.
A running induction motor limits its own current through a mechanism called counter-electromotive force. As the rotor spins, it induces a voltage that opposes the applied voltage, and this back-voltage rises with speed, holding the current down to the modest level a healthy motor draws when running. The faster the motor turns relative to the rotating magnetic field, the more this self-limiting effect works. At normal running speed the motor draws only its rated full-load current because the counter-voltage is doing most of the work of limiting it.
When the rotor is not turning, that counter-voltage does not exist. The full applied voltage drives current through the low impedance of the motor windings with nothing opposing it, so the current shoots up to the locked rotor value, several times rated. This is exactly the condition at the instant of starting, before the motor has begun to accelerate, which is why every direct start begins with a burst of locked rotor current that then falls off as the motor speeds up and its counter-voltage builds. It is also the condition during a stall, when a running motor is stopped by its load but still energized.
The two scenarios differ in an important way. During a normal start the locked rotor current is transient: the motor accelerates within a second or two, its counter-voltage rises, and the current drops to running level, so the windings experience the high current only briefly. During a stall the rotor never accelerates, so the locked rotor current persists, and since that current heats the windings intensely, a sustained stall becomes a thermal emergency. The same LRA value is benign for the short duration of a start but destructive if it continues, which is the whole reason motor protection exists.
Because locked rotor current varies between motor designs, there is a standardized way to classify it: the NEMA code letter, a single letter stamped on the motor nameplate. Each code letter corresponds to a range of locked rotor apparent power per unit of motor rating, which in turn tells you roughly how much locked rotor current to expect for that motor. Engineers use the code letter to estimate the inrush a particular motor will impose so they can select protection and supply components that will tolerate the start without nuisance tripping while still protecting against a genuine fault.
Beyond how much current flows, the critical question is how long the motor can endure it. Motor manufacturers characterize this with a thermal damage limit, often shown as a curve of allowable time versus current: the higher the current, the shorter the time the motor can survive before its windings or rotor overheat and are damaged. At locked rotor current, that safe time is short, a matter of seconds to some tens of seconds depending on the motor, because the intense current heats the stationary rotor and windings rapidly with none of the cooling that motion normally provides.
Protection design lives in the gap between a normal start and a damaging stall. The overload protection must be set to ignore the brief, expected locked rotor current of a healthy start, so it does not trip the motor every time it starts, yet act quickly enough to disconnect the motor if the locked rotor current persists, as in a stall, before the thermal damage limit is reached. Getting this right means coordinating the protection's tripping behavior with the motor's own start time and thermal damage curve, so the motor starts reliably but is never left drawing locked rotor current long enough to cook itself.
Locked rotor current ripples through the whole motor circuit design. Conductors and contactors must carry the LRA surge at every start without overheating, so cable sizing considers not just the running current but the repeated starting bursts. Short-circuit protection must be set high enough to let the legitimate locked rotor current pass without tripping yet low enough to catch an actual fault. The upstream supply, whether a transformer or a generator, must be stiff enough that the LRA draw does not collapse the voltage so far that the motor cannot start or that other equipment is disturbed. LRA is, in short, the number that sizes much of what surrounds the motor.
Reduced-voltage starting methods exist largely to tame this locked rotor current. A soft starter, a wye-delta starter, or a variable frequency drive each reduces the current drawn during startup below the full locked rotor value, easing the stress on the supply and the motor. Choosing among them depends partly on how large the untamed LRA would be and how much the supply and the process can tolerate it. Even with these methods, the underlying locked rotor characteristic of the motor is what they are working against, so it remains the reference point for the whole starting scheme.
For remote and unmanned oilfield sites, seeing motor current behavior at start and during operation is where cloud SCADA earns its place. A motor drawing locked rotor current far longer than a normal start should is a stalled or struggling motor heading toward thermal damage, and surfacing that current profile lets an operator or an automated response act before the motor is destroyed. Across a fleet, comparing starting current signatures can reveal a motor whose starts are drifting longer, an early sign of a mechanical problem loading it down, or confirm that a reduced-voltage starter is actually limiting the inrush as intended, turning the abstract locked rotor value into an observable, actionable field measurement.
It varies by motor design but is commonly around six to eight times the motor's rated full-load current. Some motors fall outside that range, which is why the NEMA code letter on the nameplate is used to classify a specific motor's locked rotor current more precisely. The value matters because it is the large current the motor draws at every start and during any stall, and the whole circuit must be designed to accommodate it.
The NEMA code letter classifies the motor's locked rotor current by indicating a range of locked rotor apparent power per unit of motor rating. From it, an engineer can estimate how much inrush the motor will draw at start, which is needed to size protection and supply components so the motor starts without nuisance tripping while still being protected against genuine faults. It is a compact, standardized way to communicate a motor's starting current characteristic.
During a normal start the motor draws locked rotor current only briefly, a second or two, before it accelerates and the current drops, so the windings barely heat. In a stall the rotor never turns, so the locked rotor current persists, and that intense current heats the windings and rotor rapidly with none of the cooling that motion provides. The motor's thermal damage limit at locked rotor current is short, so protection must remove a stalled motor within seconds to tens of seconds to prevent damage.
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