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The locked-rotor current is crucial to understand because it represents the current drawn by a motor when power is applied but the rotor is not yet moving. Essentially, this is the initial surge of current required to start the motor. For a squirrel-cage induction motor that's designed to run at a higher voltage, connecting it to a lower voltage supply, like 520 V instead of the rated 575 V, leads to a decrease in locked-rotor current. Because the relationship between voltage and current is direct, operating the motor at a reduced voltage results in a lower starting current. This might seem beneficial because it reduces the stress on the electrical system, but it also means that the torque available to start the motor is also decreased, which might affect the motor's ability to start under load.

Locked-rotor torque, essentially the torque when the motor starts, depends greatly on the applied voltage. When a motor rated for a higher voltage operates on a decreased supply, like in our squirrel-cage induction motor scenario, the locked-rotor torque diminishes significantly as it's proportional to the square of the voltage. This succinct relationship ensures that any reduction in voltage causes a hefty decrease in the torque available to overcome the inertia of the motor at startup. The motor might face difficulty getting up to speed, especially if it's under a heavy load or part of a high inertia system. It's important for students to grasp that while a lower locked-rotor current can be easier on the electrical infrastructure, the accompanying lower torque can inhibit the motor's startup performance.

No-load current is the amount of current drawn by the motor when it operates without a load—essentially when it's running freely. This current is primarily affected by the motor's need for magnetizing the core. If a squirrel-cage induction motor is supplied by a voltage that's less than its rated voltage, the no-load current will also reduce. However, in practice, this change isn't typically large as the no-load current is not a prime factor for motor performance under operational loads. It's essential to be aware that this decrease does not imply improved efficiency, as the motor requires a certain amount of current for efficient operation even under no-load conditions.

One parameter that might puzzle students is the no-load speed. Despite changes in voltage, the no-load speed of a squirrel-cage induction motor remains unchanged, primarily because it is dictated by the frequency of the power supply and the motor's inherent characteristics, like the number of poles. Since these factors are not altered when the voltage changes, the no-load speed does not significantly shift. This knowledge is fundamental as it reassures students that not all performance metrics are adversely impacted by a reduction in operating voltage.

Full-load current refers to the current the motor draws when it is operating at its rated capacity or full load. Typically, we would expect that lowering the voltage causes a proportionate drop in current. However, when it comes to squirrel-cage induction motors, the full-load current actually increases slightly when voltage decreases. This counter-intuitive behavior is because the motor tries to maintain its torque output by drawing more current. Students should note that this increase is generally within safe operating limits, but it's a fine balance as motors are not designed to run continuously with high currents, which can result in overheating and reduced lifespan.

Power factor is the measure of how effectively electrical power is converted into useful work output. The full-load power factor can increase when operating a motor at a reduced voltage. This happens because the motor's full-load current doesn't increase proportionally to the decrease in voltage, slightly enhancing the power factor as the ratio of real power to apparent power improves. An increased power factor suggests that the motor is using the electrical power more efficiently. This concept is rather significant as students often learn that power factor is key to the energy efficiency of motors in practical applications.

Efficiency at full load is the ratio of the mechanical power output of the motor to the electrical power input. Reducing the operating voltage of a squirrel-cage induction motor can lead to a minor decrease in full-load efficiency. Since the motor draws more current to generate the same output power, input power marginally increases, which in turn pushes down efficiency. Students must understand that efficiency is critical for motor operation — it impacts energy costs and heat generation. Therefore, running a motor at a voltage lower than its rated voltage may not be economically or mechanically beneficial in the long run.