What does locked rotor refer to in electrical systems?

Locked rotor refers to the condition of an electric motor when its rotor is stationary at startup and unable to turn due to mechanical restraint or load on the output shaft. Understanding locked rotor currents is crucial because these currents represent the maximum current the motor will draw during this condition, which can be several times greater than the motor's full-load current.

The correct answer relates to how we quantify the potential power output of a motor under these circumstances. Specifically, locked rotor conditions are typically expressed in terms of kilovolt-amperes (kVA) per horsepower (HP). This expression helps in determining the transformer's size and circuit breakers needed for protecting the motor during start-up, as locked rotor currents can influence the electrical system's behavior significantly.

The other choices revolve around different aspects of motor operation but do not correctly capture the concept of locked rotor. For example, full-load amps per horsepower deals with the normal operating current at full capacity, while amps per pole or volts per pole are related to motor electrical characteristics but do not specifically measure the conditions when a rotor is locked. Understanding this distinction is critical when designing and analyzing motor control systems.