<P> Universal motors are series wound . Shunt winding was used experimentally, in the late 19th century, but was impractical owing to problems with commutation . Various schemes of embedded resistance, inductance and antiphase cross-coupling were attempted to reduce this . Universal motors, including shunt wound, were favoured as AC motors at this time as they were self - starting . When self - starting induction motors and automatic starters became available, these replaced the larger universal motors (above 1 hp) and the shunt wound . </P> <P> In the past, repulsion - start wound - rotor motors provided high starting torque, but with added complexity . Their rotors were similar to those of universal motors, but their brushes were connected only to each other . Transformer action induced current into the rotor . Brush position relative to field poles meant that starting torque was developed by rotor repulsion from the field poles . A centrifugal mechanism, when close to running speed, connected all commutator bars together to create the equivalent of a squirrel - cage rotor . As well, when close to operating speed, better motors lifted the brushes out of contact . </P> <P> Operating at normal power line frequencies, universal motors are often found in a range less than 1000 watts . Their high speed makes them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high speed and light weight are desirable . They are also commonly used in portable power tools, such as drills, sanders, circular and jig saws, where the motor's characteristics work well . Many vacuum cleaner and weed trimmer motors exceed 10,000 RPM, while many Dremel and similar miniature grinders exceed 30,000 RPM . </P> <P> Universal motors also lend themselves to electronic speed control and, as such, were an ideal choice for domestic washing machines . The motor can be used to agitate the drum (both forwards and in reverse) by switching the field winding with respect to the armature . The motor can also be run up to the high speeds required for the spin cycle . Nowadays, variable - frequency drive motors are more commonly used instead . </P>

What type of motor used in vacuum cleaner