Locked rotor current (LRA) is the current an electric motor draws the instant it's energized while the rotor is stationary, before it starts turning. It is the highest current the motor will ever see in normal operation, typically five to eight times full-load amps (FLA), and if that current lingers too long the winding insulation can cook in seconds, not minutes.
Every time a motor starts from standstill, it briefly passes through a "locked rotor" condition, electrically speaking, even if the shaft is free to spin. LRA is the current draw during that instant: rotor stationary, full voltage applied, no rotation yet. It is sometimes called starting current or inrush current. A true locked-rotor fault is different: the rotor is mechanically jammed (seized bearing, jammed pump, blocked conveyor) and stays at that current level indefinitely instead of accelerating through it in a second or two.
An induction motor only limits its own current one way: with back electromotive force (back EMF), the voltage the spinning rotor induces back into the stator windings, which opposes the supply voltage and throttles current as speed rises. At standstill there is no rotation, so there is no back EMF. The motor behaves electrically like a transformer with a short-circuited secondary: the only things limiting current are stator and rotor winding resistance and leakage reactance, both low by design for running efficiency. That leaves stator impedance as the sole restraint on inrush, which is why LRA commonly lands at 5 to 8 times FLA on direct-on-line starts. As the rotor accelerates, back EMF builds, and current falls off exponentially until it settles at the FLA rating near synchronous speed. For a deeper look at the mechanism itself, see how back EMF governs motor current.
Motor nameplates rated 1/2 HP and larger carry a "Code" letter under NEMA MG-1 that tells you locked-rotor kVA per horsepower, without requiring you to dig up a full starting-current curve. It's a quick way to estimate inrush and size upstream protection and transformers.
| Code letter | Locked-rotor kVA/HP |
|---|---|
| A | 0 to 3.15 |
| F | 5.0 to 5.6 |
| G | 5.6 to 6.3 |
| H | 6.3 to 7.1 |
| J | 7.1 to 8.0 |
| K | 8.0 to 9.0 |
| L | 9.0 to 10.0 |
| V | 22.4 and up |
Many general-purpose NEMA induction motors fall somewhere in the F through L range, though the exact letter shifts with motor size, smaller motors often run toward the higher end of that band, larger ones toward the lower end. A higher letter means more locked-rotor kVA per horsepower, which means a bigger voltage dip and a bigger thermal hit on every start.
Winding heating follows I²R: power dissipated as heat scales with the square of current, and rotor resistance at standstill is also elevated by skin effect in the rotor bars, current crowds into the outer part of the bar at line frequency and only spreads evenly once slip frequency drops near running speed, compared to running conditions. Combine roughly 6 times normal current with several times normal effective resistance and the instantaneous heating in the rotor can run close to two orders of magnitude above the heating at rated load. A normal start burns off that heat in a second or two as the motor accelerates. A stalled or slow-accelerating start does not, and winding and rotor bar temperatures can climb past insulation limits within single-digit to low double-digit seconds. This is the same insulation life curve at stake in motor insulation class ratings: each class defines a maximum temperature, and locked-rotor heat is the fastest way to exceed it.
Because ordinary time-delay fuses and thermal-magnetic breakers have to tolerate normal LRA inrush without nuisance tripping, dedicated overload protection is what actually catches a stalled motor before it cooks.
A repeated pattern of high inrush duration, rising motor temperature, or vibration at start-up is usually the early signature of a developing mechanical problem, a tightening bearing, misalignment, or a load that's slowly seizing, long before it becomes a full locked-rotor trip. That is easier to catch with the same techniques used elsewhere in motor health monitoring, including insulation resistance testing and vibration screening against ISO 10816-3 vibration severity limits. Fabrico reads machine condition and OEE straight from the line, computer vision catches slow-building mechanical issues that current sensors alone can miss, and when a loss pattern like repeated hard starts or a stall is detected, it auto-routes a work order before the motor gets to a locked-rotor failure. It's EU-built with EU data residency and certified to ISO 27001, ISO 20000-1, and ISO 9001. Book a Fabrico demo.
They're measured the same way (current at zero speed, full voltage applied) but used differently. "Starting current" or "inrush" describes the normal, brief current spike every motor draws for the second or two it takes to accelerate. "Locked rotor" describes that same current level persisting because the rotor is not actually turning, whether that's a fault condition or the nameplate rating itself.
Most general-purpose induction motors on direct-on-line starts draw roughly 5 to 8 times FLA at the instant of starting. The exact multiple depends on motor design (reflected in the NEMA code letter), supply voltage, and starting method.
Because back EMF only exists once the rotor is turning. As speed rises toward synchronous speed, induced back EMF opposes the supply voltage and current falls off, settling at the FLA nameplate rating near rated speed. Standstill is the one condition where that self-limiting mechanism doesn't exist yet.
Compare the actual start time and current against the motor's rated locked-rotor withstand time (often listed as "hot" and "cold" stall times on the datasheet) and make sure the overload relay's trip class is faster than that withstand time. If a motor is tripping intermittently on start or taking longer to accelerate than it used to, that's usually a mechanical or supply-voltage problem developing, not a nuisance trip to bypass.
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