AC motor drives interface controllers to AC motors. They match the control signals (voltage and power levels) as well as the signal type (analog or digital). They also provide power conversion, amplification, and the sequencing of waveform signals. AC motor drives are used with many types of AC motors. Induction motors induce current into the rotor windings without any physical connection to the stator windings. They are suitable for many different environments and are capable of providing considerable power as well as variable speed control. Synchronous motors are no-slip devices that operate at constant speed up to full load. Subcategories include reluctance motors and permanent magnet devices such as AC servomotors. Many AC servomotors use brushless commutation with feedback provided by Hall effect sensors. Pole changing motors use pole number control, a method for changing the number of poles on the primary winding. Vector drive motors provide independent control of both the voltage and frequency, resulting in low-speed torque outputs that approach those of DC motors. In some designs, encoders or resolvers are used to provide feedback about position and speed. Linear motors generate force in only the direction of travel. Common technologies include moving coil, moving magnet, and switched reluctance designs.