Which of these situations would likely lead to locked rotor current?

• A. The motor is under normal operating conditions
• B. The motor is experiencing a mechanical jam
• C. The motor is overtime running
• D. The motor has a stable load

Answer: (B)

Locked rotor current refers to the high starting current that occurs when an electric motor's rotor is not able to turn (or is "locked") while being energized. Under normal circumstances, when a motor starts, it experiences an increase in current to overcome inertia and reach operational speed. However, if the rotor is jammed mechanically, it cannot turn, resulting in a significant surge in current, known as locked rotor current.

In the situation where the motor is experiencing a mechanical jam, power is still supplied to the motor, but since the rotor cannot move, it draws an excessive amount of current that is much higher than its normal operating current. This condition can potentially cause damage to the motor windings, increase heat, and trigger protective devices to prevent failure.

Normal operating conditions, overtime running, or having a stable load imply that the motor is functioning as intended and can rotate freely. In these instances, the motor would not draw a locked rotor current, as the rotor is able to turn and the electrical parameters remain within safe operating limits. Therefore, the presence of a mechanical jam is the definitive scenario that leads to locked rotor current.