What locked rotor amps mean and why you should care
Locked rotor amps (LRA) refer to the maximum inrush current an AC motor draws when the rotor is held stationary and full voltage is applied at start. During this condition the motor behaves like a near short circuit because there is no mechanical motion to generate back EMF, so current rises to a high but predictable level. Understanding LRA is essential for selecting proper starters, overcurrent protection, and conductors, and for ensuring reliable motor installation in commercial and industrial settings.
How locked rotor amps occur and how they differ from running current
In normal operation an induction motor develops back electromotive force (back EMF) as it turns, which limits current to the running full-load amps (FLA). At start, if the rotor cannot move—due to a jam, locked shaft, or during initial energization—the back EMF is essentially zero. The stator windings then draw a much larger current in an attempt to establish magnetic field and overcome the mechanical load. This peak current is the locked rotor amps, which can be several times higher than the motor’s normal full-load current, even before protective devices react. The distinction between LRA and steady-state FLA is critical because temporary protection must tolerate the brief LRA spike while still protecting against sustained overloads.
Typical locked rotor amps values and variables that influence them
LRA is not a single fixed number for every motor; it depends on motor design, voltage, and system strength. Larger motors and those with lower synchronous speed tend to draw higher LRA. System source strength and transformer impedance also affect how high the measured LRA can be. The following table shows typical ranges for common polyphase induction motors under standard conditions.
Locked rotor amps by motor rating (typical ranges)
| Motor rating (hp) | Locked rotor amps (LRA) approximate range | Notes |
|---|---|---|
| 1/2 hp | 15–25 A | Single-phase value varies more with voltage |
| 1 hp | 25–40 A | Three-phase LRA roughly 6–8× FLA |
| 5 hp | 70–120 A | Design-dependent; low-voltage motors on weak systems can see higher values |
| 10 hp | 120–200 A | Higher inertia loads may prolong high-current duration |
| 25 hp | 250–400 A | System impedance notably affects measured LRA |
Locked rotor amps and electrical protection choices
Because LRA is transient, protection devices must allow the inrush current without nuisance tripping while still responding to dangerous overloads. Devices to consider include:
- Motor circuit protectors (MCP) with time-delay or adjustable trip characteristics
- Standard circuit breakers with long-time delay curves set above expected LRA
- Contactors and overload relays coordinated to the motor’s FLA and service capacity
- Soft starters or variable-frequency drives (VFDs) to reduce inrush current by ramping voltage
Conductors must also carry starting current without excessive voltage drop, so verify conductor ampacity and temperature rise per relevant electrical standards in your jurisdiction.
Practical steps and when to seek professional input
If you suspect high locked rotor amps in an installation, start with these checks: confirm the motor nameplate ratings, measure LRA safely using clamp-on ammeters during start, verify the voltage at terminals, inspect for mechanical binding or overloads, and ensure protection settings are correctly coordinated. When in doubt, involve a licensed electrician or motor specialist for testing and design, especially when upgrading control equipment or addressing repeated faults.
Key takeaways
- Locked rotor amps is the high current drawn during motor start when rotor speed is zero
- LRA can be several times higher than full-load amps and depends on motor size, design, and system strength
- Proper starter selection, device coordination, and conductor sizing are essential to handle LRA safely
- Use manufacturer nameplate data and measure inrush current in situ to validate assumptions
- When issues persist, consult a qualified electrical professional for diagnostics and protection adjustments