Locked rotor current is the electrical current drawn by a motor when its shaft is completely unable to turn while full voltage is applied to its terminals. It represents the highest sustained current a motor will draw, typically 3 to 8 times the motor’s normal full-load current. This matters because that surge of current generates extreme heat in the motor’s windings and can cause serious damage within seconds if the motor isn’t shut down.
Why Current Spikes When the Rotor Stops
An electric motor works by creating a rotating magnetic field in its stationary outer shell (the stator), which pulls the inner spinning component (the rotor) along with it. As the rotor spins, it generates its own opposing voltage that naturally limits how much current flows through the motor. Think of it like a governor on an engine.
When the rotor can’t move, that opposing voltage never develops. The motor’s windings behave almost like a short circuit, and current floods in with very little resistance. A motor rated for 10 amps at full speed might pull 60 to 80 amps in a locked condition. The only thing limiting the current at that point is the raw electrical resistance of the copper wire in the windings, which isn’t much.
Common Causes of a Locked Rotor
A locked rotor condition happens when something physically prevents the motor shaft from turning. The most common causes are mechanical obstruction or jamming in the driven equipment, such as a seized conveyor belt, a frozen pump impeller, or debris caught in a fan blade. Bearing failure inside the motor itself can also lock the rotor in place. Overloading a motor beyond its capacity doesn’t technically lock the rotor, but it can slow the motor enough that current rises to near-locked-rotor levels, creating the same risk of damage.
Locked Rotor Current vs. Inrush Current
These two terms get confused constantly, but they describe different events. Inrush current is a very brief spike that happens the instant power is first applied to a motor, lasting roughly 20 to 100 milliseconds. It’s caused by the initial magnetization of the motor’s iron core, and it can peak at up to 12 times rated current. It’s a transient event that passes quickly.
Locked rotor current, by contrast, is a sustained condition. During a normal startup, the motor draws current at locked-rotor levels for the first few seconds while the rotor accelerates from a standstill. As the rotor picks up speed, current drops. For a direct-on-line start, this high-current phase typically lasts 1 to 10 seconds depending on the motor and load. If the rotor never accelerates (because it’s physically stuck), that high current simply continues indefinitely until something trips the circuit or the windings burn out.
So every motor startup briefly involves locked rotor current. The danger comes when the rotor never gets moving and that current doesn’t taper off.
How Much Current to Expect
The exact locked rotor current depends on the motor’s design. NEMA assigns code letters (printed on the motor’s nameplate) that indicate the locked rotor current in terms of kVA per horsepower. The range spans from code letter A (0 to 3.14 kVA/hp) at the low end to code letter V (22.4 kVA/hp and above) at the high end. Most general-purpose motors fall somewhere in the middle, around code letters F through H, which corresponds to roughly 5 to 7 times the full-load current.
To calculate the actual locked rotor amps for a specific motor, you multiply the code letter’s kVA/hp value by the motor’s horsepower, then divide by the supply voltage (adjusted for single-phase or three-phase power). This number is sometimes listed directly on the nameplate as LRA (locked rotor amps), making the math unnecessary.
Here are the NEMA code letter ranges for reference:
- A through D: 0 to 4.49 kVA/hp (low starting current designs)
- E through H: 4.5 to 7.09 kVA/hp (most common range)
- J through N: 7.1 to 12.49 kVA/hp (higher starting current)
- P through V: 12.5 kVA/hp and above (very high starting current)
What Happens to the Motor
The immediate threat is heat. Current flowing through the motor’s windings generates heat proportional to the square of the current. At six times normal current, the heating effect is 36 times greater than during normal operation. This heat builds fast. The insulation around the copper windings begins to break down, and if the condition persists, the windings can melt. The term “safe locked time” refers to the maximum number of seconds a motor can endure locked rotor current without significant loss of motor life. For smaller motors this might be as little as 5 to 10 seconds. Larger motors with more thermal mass may tolerate slightly longer, but not by much.
Beyond the motor itself, locked rotor current affects the broader electrical system. Starting a large motor (or having one stall) can drag voltage down across an entire facility. The motor’s own terminal voltage needs to stay above about 80% of rated voltage to have any chance of starting. If voltage drops below 71%, a running motor can stall. Other equipment on the same electrical bus feels the effects too: contactors may drop out below 60 to 70% voltage, sensitive electronic controls can malfunction below 90%, and lights will visibly flicker from voltage changes as small as 3%.
How Protection Systems Respond
Overload relays are the primary defense against a locked rotor condition. These devices monitor the current flowing to the motor and trip the circuit when current stays too high for too long. They’re designed with a time delay so they don’t trip during normal startup (when locked-rotor-level current is expected for a few seconds) but do trip if that current persists.
The trip speed is classified by “class” ratings. A Class 10 relay trips within approximately 10 seconds when current reaches 600% of the motor’s rated current. Class 20 trips within about 20 seconds at the same current level, and Class 30 within about 30 seconds. The right class depends on how long the motor normally takes to accelerate to full speed. A motor driving a high-inertia load like a large fan might need a Class 20 or 30 relay because its normal startup takes longer. A pump motor that reaches full speed in a couple of seconds can use a Class 10 relay for tighter protection.
Choosing the wrong trip class creates problems in both directions. A relay that’s too fast will trip during normal startups, shutting down equipment unnecessarily. A relay that’s too slow may allow a genuinely locked motor to cook its windings before protection kicks in. Matching the relay class to the motor’s actual acceleration time is one of the most important decisions in motor circuit design.
Why It Matters for System Sizing
Locked rotor current isn’t just a fault condition to protect against. It’s a number that drives the sizing of nearly every component in a motor circuit. Circuit breakers, contactors, cables, fuses, and even the transformer feeding the motor all need to handle the locked rotor current for at least the duration of a normal startup. If a 50-hp motor has a locked rotor current of 300 amps, every component between the power source and the motor needs to carry 300 amps for several seconds without tripping, melting, or dropping voltage below usable levels.
This is also why starting large motors on weak power systems requires special methods like soft starters or variable frequency drives, which ramp voltage up gradually instead of hitting the motor with full voltage all at once. These reduce the effective locked rotor current to something the system can handle, at the cost of slower acceleration and reduced starting torque.