The electric motor has a substantially cylindrical casing, a stator unit and a rotor unit, housed inside the casing and coupling means designed to couple the rotor unit to the impeller so as to rotate it.
The above-mentioned coupling means are normally defined by a shaft protruding from the casing, rotated by the rotor unit.
In this description, for sake of simplicity, reference will always be made to the fact that the above-mentioned coupling means comprises a shaft protruding from the casing of the electric motor and rotated with the rotor unit, but without limiting the scope of the invention.
The impeller has a connecting hub coaxial with the shaft of the motor and a plurality of blades extending radially from the hub.
Usually, the hub of the impeller is cup shaped, that is to say, it has a bottom wall facing the wall of the motor from which the shaft projects, for connecting to the shaft of the motor, and a substantially cylindrical lateral wall from which the blades extend.
In order to limit the axial dimensions of the “axial electric fan” unit, the motor is at least partly housed inside the hub, surrounded by the lateral wall of the hub itself which, starting from the bottom wall, extends towards the motor.
Again with the aim of reducing as much as possible the size of the “axial electric fan” unit, electric motors of the “brushless” type are used, which have axial dimensions (thickness) which are relatively limited.
Moreover, during the design stage the distance between the bottom wall of the hub and the front wall of the electric motor facing the bottom wall of the hub is limited as much as possible.
Lastly, a tubular gap is defined between the motor casing and the hub of the impeller, that is, between the casing and the lateral wall of the hub, to allow the impeller to rotate freely.
The use of so-called “flat motor fans”, that is to say motors with limited axial thickness characteristics, is a beneficial factor of the axial electric fan unit since the space available in the engine compartment of modern cars is increasingly limited. In this regard, it should also be noted that, although “brushless” electric motors are used, the majority of the space of an axial electric fan is occupied by the electric motor itself, so, even if a large part of the motor is inside the hub, in order to contain the axial dimensions of the electric fan unit it is necessary to reduce the axial dimensions of the impeller.
However, since the axial dimensions of the impeller (its thickness) cannot be reduced below a certain structural limit, especially for high outputs wherein the impellers have very large diameters, in order to attempt to limit as much as possible the axial dimensions of the electric fan it is necessary to reduce as much as possible the distance between the bottom wall of the hub and the surface of the motor facing the bottom wall of the hub itself.
It should be noted that the distance becomes a critical point of the design and tends to become increasingly reduced.
It should also be noted that the shaft must protrude from the motor for a sufficient stretch in such a way that it can couple to the fan with mechanical safety.
In this regard, at the central point of keying the shaft to the hub, the bottom wall of the latter is equipped with a sintered steel bushing co-moulded with the bottom wall. This technology also makes it possible to reduce the distance between the bottom wall of the hub and the wall of the motor facing the bottom wall of the hub.
In addition to drawbacks mentioned above relative to the axial dimensions, which will be discussed further below, the electric fan unit and, more specifically, the rotor and impeller, have vibration problems.
It is known that the impellers of axial fans driven by electric motors (of any type: brushless, DC etc.) generally have a problem of transmission by the motor to the impeller of a torque ripple having a frequency which is generally a multiple of the number of revolutions of the motor, which is superposed on the desired continuous torque.
In other words, no type of electric motor generates a constant torque, but always has a variable “parasite” component which is superposed on the constant component. The“parasite” component is precisely the above-mentioned torque ripple. These torque ripples have a frequency which is generally a multiple of the speed of rotation of the motor. It follows that these frequencies change with the speed of the motor. If the rotor and impeller unit has a relative resonance frequency it means that there will be a certain predetermined speed of the motor at which the above-mentioned torque ripple has a frequency which is exactly the resonance frequency.
It therefore follows that the torque ripple generates its maximum damage when its frequency generates resonance of the elastic/inertial system consisting usually of the drive shaft (the so-called torsional spring) and the moments of inertia of the rotor of the motor and of the impeller.
In conclusion the so-called torque ripple induces vibration phenomena amplified at the resonance frequency of the impeller unit, shaft, motor rotor which in turn generate unwanted and unacceptable acoustic noise effects.
Use is known, in the prior art, of traditional dampers made of rubber interposed in various shapes and sizes between the motor and the impeller.
Reference is made in this regard to the patent publications GB 1464559; U.S. Pat. No. 4,193,740; EP1375923.
With reference to the above description regarding the need to reduce the axial dimensions of the electric fan unit, one must conclude that the use of the latter for cooling heat exchangers in the automotive sector results in a series of limitations which means that the use of the traditional damping structures described above is not practical to resolve the above-mentioned noise problem.
As mentioned above, the market request for minimum axial length of the electric fan unit provides only a few millimetres of motor shaft to couple the impeller to the motor and in particular the reduced distance between the bottom wall of the hub and the wall of the motor facing the bottom wall of the hub does not allow the use of traditional rubber dampers.
In addition, the impellers are made of a plastic material and must comply with specifications and withstand vibration tests and gyroscopic effects which require significant rigidity in an axial and radial direction and bending which is generated on the plane in which the impeller itself lies.
For this purpose, the above-mentioned impellers also contain a significant percentage of glass fibres (typically 35%) which tends to increase their rigidity.
The fact of reducing the distance between the bottom wall of the hub and the wall of the motor and the possible use of rubber “dampers” would reduce the rigidity of the impeller to the above-mentioned axial and bending forces and would introduce movements of the impeller during its operation which would cause the impeller to slide against the other parts present in the motor compartment of the motor vehicles or even against the supporting structure (shroud) of the impeller itself.
It should also be noted that the gyroscopic effect is very strong. The impeller is subjected to a torque force on its plane which if it were not rigid would have all the problems indicated above.
In other words, the impeller must absolutely not move or bend relative to its position adopted on the plane in which it lies because the spaces for bending are small and it would tend to touch other parts present in the motor compartment and break.
Moreover, the specifications due to environmental requirements and the reliability/life of the product are very stringent. For example, the impellers must be able to operate with operating temperatures ranging from −40° to +150° (ambient degrees) and must withstand all external agents such as petrol, oil, water, salt water, and other chemical components.
Also for these reasons, rubber is absolutely unsuitable for being used to make damping devices or structures.
In this context, the main aim of this invention is to overcome the above-mentioned drawbacks.