"Locked rotor current" refers to the current that flows through an AC motor's windings when the motor's rotor (the rotating part) is prevented from turning while the motor is still energized. In other words, it's the current that the motor draws when it's initially started and the rotor is held stationary, often due to some kind of mechanical obstruction or resistance.
When an AC motor is started, it experiences a higher current draw than its normal operating current. This is because, when the rotor is stationary, there is no back-EMF (electromotive force) being generated in the motor, which typically counteracts the incoming voltage during normal operation. As a result, the motor windings experience a lower effective impedance, causing a surge in current. This current can be many times higher than the motor's rated or full-load current.
The locked rotor current is important to consider for several reasons:
Motor Protection: The high current during locked rotor conditions can cause excessive heating and damage to the motor windings. Therefore, motor protection devices like fuses, circuit breakers, and overload relays are designed to prevent sustained operation under locked rotor conditions.
Power System Impact: The locked rotor current can cause voltage drops in the power system, affecting other connected devices and potentially causing a dip in the overall power quality of the system.
Sizing of Protective Devices: The locked rotor current helps determine the appropriate sizing of protective devices in the motor's circuit. These devices need to be rated to handle the surge current without tripping unnecessarily during motor startup.
Motor Starting Systems: Understanding the locked rotor current is essential when designing motor starting systems, such as soft starters or variable frequency drives (VFDs). These systems can be used to limit the current during motor startup, reducing the mechanical stress on the motor and the impact on the power system.
It's worth noting that the locked rotor current is a transient condition that only occurs for a brief period during motor startup. Once the motor starts to rotate and generate back-EMF, the current levels drop to their normal operating levels, also known as the full-load current.