What is the impact of using a primary-resistance starter on a motor's potential difference during high-speed operation?

Using a primary-resistance starter has a significant impact on the potential difference applied to a motor during high-speed operation. When a motor is initially started, primary resistance is used to limit the inrush current flowing to the motor. This resistance is present to prevent excessive current that could damage the windings or cause other issues during the starting phase.

As the motor reaches high speeds, the need for this primary resistance diminishes because the back electromotive force (back EMF) generated by the motor increases. Back EMF opposes the applied voltage, effectively reducing the voltage across the motor's windings. With the primary resistance still in place, the overall potential difference across the motor decreases, as a portion of the voltage is dropped across the resistance instead of being applied to the motor itself.

Therefore, during high-speed operation, the use of a primary-resistance starter leads to a reduction in the potential difference available to the motor, causing it to operate with lower voltage than the supply voltage would indicate. This concept is integral to understanding motor control and the effects of resistance in electrical circuits.