A “universal” prior art motor, such as is shown in FIG. 1, is an electric series wound motor which can run on AC or DC power. Such motors are small, frequently connected directly to standard 110 volt AC power and run at high speed. They are generally employed in hand-held and counter top appliances such as mixers, hand drills, vacuum cleaners and in gardening tools such as edge trimmers and leaf blowers.
Such motors use two field coils wound around two poles for creating a magnetic field in the stator and a rotor with many coils (usually twelve) “wave” wound around the rotor in the rotor slots so that parts of the rotor coil conductor turns lie parallel to the rotor axis. The rotor coils are connected to the commutator pads (usually twelve). Two brushes, which are connected in series with the stator field coils, contact the commutator. The power is connected to the field coils and the speed of the motor is varied by a simple power supply which varies the voltage. When an electric current flows through the conductors of the rotor coils, which are in the magnetic field, the conductors feel a force which turns the rotor about its axis.
No other design has yet as effectively satisfied the special niche need for high speed, variable speed and low cost in small appliances as has the “universal” motor. Typically the speed required in such devices is above 10,000 rpm. Such high speed electric motors deliver high power in a relatively small lightweight package at a low price. The “universal” motor also meets the requirement for smaller motors in hair blowers. In these devices, the motor usually has permanent magnets replacing the stator field coils.
Regardless of whether a stator has coils or permanent magnets for generating magnetic flux, the coils in the rotor also produce a magnetic field which according to some literature “distorts and opposes” the main stator field. This is termed “rotor reaction” and is considered a detriment to efficient motor action.
In the currently used universal motors, with field coils, the longitudinal length of the conductors in the wound rotor is an important parameter because it is the interaction of the conductors with the stator's magnetic field which produces the torque. This torque is found to be approximately proportional to the longitudinal length of the rotor conductors. Therefore, the “bank” of coils in the wound rotor need only be a single “bank” and be whatever length produces the torque desired. The wound rotor purpose is to provide as many conductors, arranged longitudinally, as can be fitted in the space. If there were some other way to do this other than making coils it may still function. The coils are incidental, not essential.
Currently, available universal motors use brushes which are linearly guided and must be connected to electrical power via flexible pigtails. This requires an arrangement which is costly, fragile and subject to breakage and is difficult to service.
Two other types of electric motor have been built. They are the variable reluctance, and the switched reluctance motors. These motors have advantages of low cost through the elimination of unnecessary copper but have severe disadvantages of torque variation (ripple) and acoustic noise which, so far, is only partly mitigated by expensive tailored electronically controlled drives.