Fuel pumps are used in motor vehicles to transfer liquid fuel, typically gasoline or diesel from a fuel tank to an internal combustion engine. The pump is driven by a small DC motor and to minimize fuel leakage through bearing seals etc, the fuel passes through the interior of the motor. This works very well even with motors having commutators, with the fuel cooling the motor and eliminating sparking between the brushes and the commutator. However, with the advent of high alcohol fuels, chemical reactions between the commutator and the fuel has become problematic leading to the use of graphite commutators and renewed interest in brushless motors to drive the fuel pumps. There are many advantages of brushless motors, especially in automobile applications, such as longer life by eliminating the use of brushes and a commutator. Typically, the brushless motor may be a single phase motor or a three phase motor. Traditionally, these motor have been termed as brushless direct current motors or BLDC motors for short as often the input power to the motor controller is DC power, typically from a battery or rectified AC supply. However, recently the term brushless AC motor or BLAC motor has been coined. This motor is a special type of BLDC motor in which the controller sends power to the motor in the form of a sinusoidal wave instead of a pulse or square wave. However, many people still use the term BLDC to include both types of motors as the difference is in the type of controller. That said, certain modifications are usually made to make the brushless motor more efficient with one or the other type of controller. For the sake of simplicity, we will refer to both types of brushless motors by the generic term BLDC motor or simply as a brushless motor.
For existing three phase, 4-pole BLDC motors available in the marketplace, including outer rotor and inner rotor models, 6 slots is its most popular and simplest stator lamination structure in the low power applications. FIG. 1 illustrates a prior art schematic winding diagram for a 3-phase BLDC motor. The stator 12 has a stator core 13 with six stator poles 14, referred to as slots. The rotor 16 has four magnetic poles 18 formed by four permanent magnets fixed to the outer surface of a rotor core 17 (in known manner). The winding 20 forms a coil 22 about each stator pole 14. The winding 20 is a 3-phase star winding, meaning that the winding 20 has three legs or phases, with one end (A,B,C) of each leg being connected to the stator terminals (one for each phase) and the other end (X,Y,Z) of each leg being connected together at point 24 to form a star connection. Hence the motor is referred to as a three phase, four pole, six slot BLDC motor. In this geometry there are six coils, two coils for each phase. Thus each leg has two coils 22 electrically connected in series. This is difficult to wind in small diameter motors.
The main problem with the existing stator geometry is that the winding configuration is complicated for small diameter, lower power applications, such as the automotive fuel pump, water pump and air pump, etc. In these applications, the stator inner diameter is very small, just around 20 mm, therefore it is difficult to assemble more coils, especially for mass production.
Another problem with existing products is the high cogging torque, which creates noise and vibration. This has restricted the use of BLDC motors in many fields which need low noise and low vibration. In order to solve this problem, one of the most effective methods is the 4 pole, 9 slot configuration, as shown in FIG. 2. From this figure we can see that the winding becomes even more complicated with three coils per phase and a lower efficiency caused by longer winding end-turns.
Another problem with existing products is that it is not suitable for overmold technology in the fuel pump application. As diverse fuels will be used in the future, such as alcohol containing fuels, etc, to avoid oxidation of the magnetic wire, overmold is desired. However, this results in the stator becoming a solid body, i.e., there is no space for fuel to pass through, except through the air gap between the rotor and the stator. However, to maintain motor efficiency, the air gap is very small with the result that the fuel flow is insufficient.