What electrical effect is used in wound-rotor motors to adjust their speed?

Wound-rotor motors use the principle of resistance variation to adjust their speed. In these motors, the rotor winding is connected to external resistors through slip rings. By varying the resistance in the rotor circuit, the slip (the difference between the synchronous speed of the magnetic field and the actual speed of the rotor) can be controlled.

Increasing the resistance results in a greater voltage drop across the rotor winding, which reduces the current flowing through the rotor. This leads to a decrease in the torque generated, allowing the rotor to slow down. Conversely, reducing the resistance increases the current and torque, allowing the rotor to speed up. This method provides a flexible and effective way to manage the speed of wound-rotor motors under varying load conditions without risking overheating or damage.

Other options, such as induction heating, variable capacitance, and direct current control, do not relate directly to the operational principle of wound-rotor motors and their speed adjustment. Induction heating concerns thermal effects in materials under high-frequency currents, variable capacitance pertains to capacitive circuits, and direct current control typically refers to DC motors rather than AC motors like wound-rotor types. Thus, the focus on resistance variation in wound-rotor motors is the key to