Most CNC routers have a three motor drive system utilizing either servo or stepper motors. Linear motors are most commonly induction motors or stepper motors. Computer controlled stepper motors are a type of motion-control positioning system. Most drives in computers use stepper motors. Control for the solenoids and stepper motors can be either closed loop or open loop. Stepper motors have a natural frequency of operation. A stepper, or stepping, motor is a synchronous, brushless, high pole count, polyphase motor. Camshafts of automobiles, miniature timing systems, and stepper motors often utilize these belts. These include dc brush, dc brushless, stepper, or in some cases, even induction motors. Typical systems include stepper motor or fan control. The US National Electrical Manufacturers Association (NEMA) standardises various aspects of stepper motors. Stepper motor performance is strongly dependent on the driver circuit. However, it is winding current, not voltage that applies torque to the stepper motor shaft. There has been increasing popularity in closed loop stepper motors in recent years. The switched reluctance motor (SRM) is a form of stepper motor that uses fewer poles.