Fan drive motors can be either constant speed or variable speed. When the fan speed is to be variable there will be a maximum efficiency duty point at each operating speed. At this point the volume will vary directly as the speed change while the pressure will vary with the square of the speed change. Variable speed is useful when the system demand follows such a square law relationship, or remains close to a square law relationship. It is, however, of no value when the system requires reduction in volume while the pressure is it remain high or even required to increase.

It is traditional for a variable speed fan to be driven by a squirrel cage induction motor suitable for converter operation. Such a motor will have a "standard" (synchronous) speed dictated by the number of pairs of poles in the motor windings and by the supply frequency of the AC feed.

The voltage source for such a squirrel cage induction motor will normally consist of a DC link converter using electrical power supply coming via a transformer from a high voltage mains and converted to AC of the desired frequency to drive the squirrel cage induction motor at the new required speed. It is quite possible for the driven speed to be way in excess of the standard speed which would correspond the number of pairs of holes in the motor windings and the frequency of the high voltage mains supply.

In order to vary the fan performance to correspond to the varying fan duty requirements, the most common method of achieving this is to use adjustable inlet vanes positioned upstream of the fan, and to alter the angle of the vanes so as to alter the direction of the air entering the fan. By this means it is possible to satisfy a particular fan operating duty while maintaining the fan rotational speed constant.

Inlet vane control of a fan has one unique area where the maximum efficiency

Inlet vane control of a fan has one unique area where the maximum efficiency will be achieved, and this is usually when the inlet vanes are fully open. The adjustable inlet vanes are operated by a mechanical linkage which is powered from an actuator which may be an electric actuator or a pneumatic or hydraulic ram. As the position of the actuator is varied to open or close the inlet vanes between either fully open or fully closed, or into any intermediate position, the movement of the actuator is initiated electrically in response to a change of fan duty. Such a system is able to provide the desired performance where the system demand does not necessarily follow a square law relationship.

It is the object of the present invention to provide a fan operating system which is able to respond. equally well to changing system demands which do or do not follow a square law relationship.

In accordance with the first aspect of the present invention there is provided a fan operating system including a fan drive motor able to achieve an asynchronous speed higher than a predetermined synchronous speed of the motor, variable angle inlet vanes to the fan to control the angle of introduction of gas to the fan, and means to vary the inlet vane angle, wherein said means to vary the inlet vane angle comprises an actuator operated in response to the motor speed.

A further aspect of the present invention provides a method of operating a fan having a motor with a predetermined synchronous speed and in which adjustable inlet vanes are provided in the fan inlet, comprising operating in a first regime in which the inlet vanes are fully open and the motor speed is controlled to be equal to or less than to said predetermined synchronous speed, and operating in a second regime in which the motor operates at a speed higher than predetermined asynchronous speed and the inlet vanes are controlled to have a restricted degree of opening for achieving high pressure/lower volume operation as compared with said first mode.

In order that the present invention may be more readily understood the following description is given, merely by way of example, with reference to the accompanying drawings in which:- Figures I and IA illustrates a fan operating system comprising a fan drive motor and an actuator adjusting the degree of opening the inlet vanes, the system being schematically illustrated in a configuration where the inlet vanes have a restricted opening ; (as shown in Figure lA)

Figures 2 and 2a correspond to Figures 1 and la and show the situation where the inlet vanes are fully open; Figure. 3 is a plot, corresponding to the Figure 1 configuration, of pressure against volume with the restricted opening of the inlet vanes; Figure 4 is a plot of pressure against volume with variable speed when the inlet vanes are fully open, as represented by Figures 2 and 2a; Figure 5 is a plot of power against volume for a system in accordance with the present invention; and Figure 6 is a plot of pressure against volume corresponding to the range of volume units plotted for Figure 5, again illustrating the situation with the present invention.

As indicated above, there are normally quite different modes for which fans are designed, namely (i)"square law relationship"operating mode in which mere alteration of the fan speed will give the required duty point from a given range, and (ii) the alternative low volume/high pressure mode for which the fan has inlet vanes of variable opening to restrict the volume flow rate but the fan motor is run at high speed to provide the desired high pressure. The degree of opening of the vanes then selects the appropriate duty point in this non square law envelope.

Figures 1, la 2 and 2a illustrate a fan operating system in accordance with the present invention in which a drive motor of the fan can be driven at a high asynchronous speed while the inlet vanes are controlled to have a restricted opening as shown in Figures 1 and la. This is for the non-square law situation and corresponds to the pressure/volume curve of Figure 3 showing typical high pressure low volume duties which can be achieved.

Figures 2 and 2a show the same system as Figures 1 and la but with the actuator (operating cylinder) set differently so as to leave the inlet vanes fully open while the motor is operated a variable speed which will usually be less than 100% of synchronous speed. (In Figure 2a the inlet vanes one clearly shown as having a configuration different from that of in Figure la Figure 4 shows the typical curve which will be achieved with the variable speed operation of the fan when the inlet vanes are fully open as shown in Figure 2a.

The effect of this hybrid system is illustrated in Figure 6 in terms of three separate curved a, b, c, showing the effect of changing the speed of operation of the fan motor for maintained fully open position of the inlet vanes. Curve a illustrates ans"over 100% synchronous"speed, curve b illustrates"100% synchronous speed" and curve c represents and"under 100% synchronous"speed.

Superimposed on the"fully open inlet vane"curves a, b, and c is a set of three curves d, e, and f which illustrate, respectively, operation with a 40° closure of the inlet vane (curve d) a 50° closure of the inlet vanes (curve e) and a 60° closure of the inlet vanes (curve f).

In accordance with the present invention the fan operating envelope is defined under the 100% synchronous speed curve b for fully open vanes but is extended upwardly to embrace that area which lies both below the 114% asynchronous speed curve a and below the 40° inlet vane closure curve d.

Figure 5 illustrates the power curves such that b'corresponds to the 100% synchronous speed (fully open inlet vane) curve b of Figure 6, curve d'illustrates the power curve corresponding to curve d (40° inlet vane closure at an"over 100% synchronous"speed) of Figure 6, curve e'represents the 50° inlet vane closure curve e at"over 100% of synchronous"speed, and curve c'corresponds to the fully opened inlet vane"under 100% synchronous"speed curve c of Figure 6.

Figure 5 also includes a boomerang shaped operated envelope A which corresponds to the desired operating duty envelope of the fan operating system, and clearly illustrates the versatility of the system in accordance with the present invention in that limb A'of the operating envelope is all within. (or near) the square law region of operation below the 100% synchronous speed curve b with the vanes fully opened whereas limb A"of the operating envelope is above the maximum obtainable pressure region operating in accordance with curve b but is nevertheless substantially entirely within the extension of the envelope below the 40° closure curve d and below the over synchronous speed curve a with the fully opened vanes.

It is thus for the first time possible to satisfy a wide range of operating duty requirements which may or may not follow the square law relationship, and using a single installation which can be adjusted to accommodate these requirements.

In use of the system in accordance with Figures la, 2 and 2a, when operation

in the configuration of Figures 1 and la is required (on limb A"of the operating envelope A of Figure 6 the inlet vane control will be limited to a given angle (in this case 40° of closure) and the motor speed will be increased to"over synchronous" speed. The precise choice of duty point within the operating zone limb A"is selected by varying the setting of the inlet vanes to a greater degree of closure than 40°.

Clearly, to achieve the operating point P, shown near the left hand end of the limb A" in Figure 6 requires the inlet vanes to be closed to substantially 60° closure.

Reducing the degree of closure will allow the operation at points inboard of the free end of the limb A". Likewise operation in the limb A'of the operating zone requires reduction of the motor speed to some value between 100% (corresponding to the operating duty point P, at the elbow of the operating envelope and some reduced motor speed bringing it further down into the low pressure low volume regime of the limb A') When switching between the two limbs A"and A', and starting from the limb A", the motor will initially be operating in the high speed mode the inlet vane angle limited to its maximum (this example 40° of closure). Simply closing the inlet vanes still further will allow choice of a different duty point within the operating envelope limb A').

To move onto the other limb A'requires the simultaneous reduction of the motor speed and the opening of the inlet vanes so as to move towards the duty point P, in the elbow region of the operating envelope, after which (once the inlet vanes are fully open) the motor speed can be reduced still further to achieve the desired duty point. This initial operation of simultaneously reducing the motor speed while opening the inlet vanes is carried out in order to ensure that the absorbed power demand of the fan never exceeds the installed motor power.

Example A fan was operated in accordance with the curves shown in Figure 5 (power) and Figure 6 (pressure/volume) where the"over synchronous"speed curves d'and e' in Figure 5 represents a speed which is 114% of synchronous speed whereas the "under synchronous"speed curve'represents a speed which is 88% of synchronous speed. The same percentage values apply to the"over synchronous"speed curves a,

d, e, and f of Figure 6 and the"under synchronous"speed curve c of Figure 6.

It was found that operating in accordance with these over synchronous and under synchronous speeds allowed the duty point of the fan to be shifted from P, (high pressure, low volume) to the second duty point P, where the pressure is reduced in comparison to point P, but the volume is increased. It will be appreciated that at point P, the fan motor will be operating at approximately 114% synchronous speed and with the inlet vanes set at 60° closed, whereas at operating point P2 the motor speed was operating exactly synchronous with the inlet vanes fully open.

It was also found that it was possible to achieve the duty point P3 in the leg A' of the boomerang-shaped operating envelope by reducing the fan speed below the synchronous speed with the inlet vanes in the fully opened position.