Locked Rotor Current Ratio is defined as the value of locked rotor current divided by motor rated current. It is one of the core technical indicators for an electric motor, especially critical for 3 phase asynchronous AC motor selection, system design and protection configuration.
When the rotor is locked and speed equals zero, the motor generates locked rotor current. This condition simulates the instant of motor energization before rotation, or mechanical stall during operation. The ratio directly reflects the current surge impact on power grid and mechanical stress on the motor itself. For standard IE2 and IE3 three-phase asynchronous motors, the typical locked rotor current ratio ranges from 5 to 7 times rated current. High-efficiency IE4 motors usually show a slightly lower ratio of 4.5 to 6 times.
A higher locked rotor current ratio normally delivers stronger starting capability, which suits heavy-duty startup scenarios requiring large static friction torque. However, engineers should avoid a common misunderstanding: starting torque is not linearly proportional to locked rotor current. Rotor bar resistance and motor slot-pole design greatly affect starting torque. Some high slip motors feature high locked rotor torque but relatively low current multiples. Excessively high locked rotor current will cause sharp voltage drop on the busbar and rapid winding overheating once rotor stalls, shortening motor service life.
This parameter lays the foundation for electrical protection and power system matching. Designers select circuit breakers, thermal relays and motor protectors according to this ratio. If protection settings are too low, nuisance tripping occurs at startup; if set too high, the stalled motor cannot be cut off timely and may burn out. When the locked rotor current ratio is high, direct-on-line starting may not be acceptable, and soft starter, VFD or reduced-voltage starting should be adopted to limit inrush current.
In industrial procurement and overseas project bidding, both locked rotor current ratio and locked rotor torque ratio need to be reviewed together. Correct evaluation of this parameter guarantees reliable operation of the whole drive system.
