Specification GB 2250783 discloses a radial fan blade wheel comprising two coaxial guide rings separate from each other in the axial direction, the blades being fitted between the rings. The electric motor of the fan is placed in the hub of the blade wheel. Axial pockets are provided on the circumference of each guide ring. In these pockets, weights can be placed to balance the blade wheel.

The balancing method disclosed in the above-described specification is rela- tively complicated, especially because it involves placing weights on two sepa- rate guide rings having different diameters. Moreover, achieving both axial and radial balancing is very difficult, and the balancing in the method according to said specification has to be performed manually because of the complex struc- ture and pockets of the blade wheel.

The object of the invention is to eliminate the drawbacks of prior art and achieve a new type of method for balancing a fan blade wheel. In the method of the in- vention, the outer rim of the blade wheel is provided with holes going through the outer rim and weights are inserted/screwed into said holes to a desired depth according to the axial and radial balancing need.

The features characteristic of the method of the invention are presented in detail in the claims below.

The method of the invention is simple. In addition, it produces a good blade wheel balance in both axial and radial directions. Moreover, the method of the invention can be applied for fully automated balancing using an appropriate ap- paratus.

In the following, the invention will be described in detail by the aid of an exam- ple with reference to the attached drawings, wherein Fig. 1 presents a blade wheel of an axial impeller, and

Fig. 2 illustrates blade wheel balancing according to the invention.

Fig. 1 presents an axial impeller according to the invention, designed for use e. g. in air conditioning systems and producing an axial air flow. The impeller comprises a housing and a plastic blade wheel 1 rotatably fitted on a shaft. At the center of the blade wheel 1 is a hub 11 with a hole 12 for a shaft, with blades 13 fitted around the hub, between the hub 11 and a cylindrical outer rim 14.

The blade wheel 1 rotated by an electric motor drive comprising a synchronous motor and a frequency converter controlling it, the frequency converter being connected to a 3-phase alternating-current network and producing a sinusoidal output voltage with controlled frequency variation. The stator of the motor may be fixed to the housing around the outer rim of the blade wheel 1, and the rotor may be integrated with the blade wheel so that it consists of permanent mag- nets (not shown) fitted on the circumference of the outer ring part.

For balancing of the blade wheel, the outer rim 14 is provided with holes 15 go- ing through the outer rim at certain distances between them, these holes being used to hold balancing weights 16 (Fig. 2). The weights 16 are shorter than the length of the holes, so it is possible to set weights 16 to desired depths in the holes as required for axial and radial balancing.

The balancing can carried out on a balancing machine having a rotary drive mechanism for rotating the blade wheel. The machine can be so controlled that it will stop the blade wheel at a position where an additional weight 16 is needed. An external cylinder 17 provided in the machine can push/screw the weight 16 to a correct depth. After this, the machine may carry out a test (rotat- ing the blade wheel) to check the balancing.

The holes can be made in connection with the manufacture, implemented using e. g. an injection moulding method, of the blade wheel.

It is obvious to the person skilled in the art that different embodiments of the invention are not limited to the example described above, but that they may be varied within the scope of the following claims.