Which statement is true regarding speed and torque control in permanent magnet motors?

The statement that speed and torque control in permanent magnet motors is accomplished by changing field strength is accurate because it reflects a fundamental principle of how these motors operate. In permanent magnet motors, the torque produced is directly proportional to the strength of the magnetic field and the amount of current flowing through the windings.

By altering the field strength—either through physical modifications or through the use of variable field-generating techniques—one can effectively manage the torque output. Increased field strength generally results in higher torque, which is essential for applications requiring variable speed and load handling. This ability to control torque is crucial for applications ranging from precise positioning to high-performance drives.

Other options may suggest methods that are not directly related to the core principles of speed and torque control in permanent magnet motors. For instance, changing the motor's size may impact the overall performance and characteristics but does not directly control speed and torque in the way that altering field strength does. Additionally, while increasing the voltage can influence speed, it does not specifically address the torque control dynamics intrinsic to permanent magnet motors as accurately as adjusting field strength does.