What controls the speed of a motor?
The speed at which the motor runs depends on the load that is put on the shaft trying to slow it down, and also on the design and type of motor. There is no lower limit but as the speed increases some motors will not work because of mechanical problems, for example brushed motors are not much use over 15,000 RPM. At very high speeds over 100,000 RPM bearings start to be a problem and air resistance enters the picture. A vacuum is one way to get rid of the friction but then the motor has difficulty in getting rid of it’s heat. By using magnetic bearings and a vacuum speeds to 1 million RPM are possible. Each type of motor has a different method of controlling the speed. This can be just varying the voltage for a DC motor or universal AC/DC motor to complex electronic circuits often using micro-controllers for the brushless motor.
