The temperature of the engine of an automobile is variable under different conditions. Thus, on hilly roads and in hot weather, the engine tends to give off large quantities of heat. For cooling it, a heat exchange fluid is used which flows through the engine and which removes the heat thereby recovered to the outside, but more particularly after having passed through a heat exchanger through which there also flows a stream of cooling air, which is usually atmospheric air taken from outside. That air flow can be accelerated so as to produce a high rate of heat transfer, by the rotor of a ventilating fan which is driven by an electric motor of the direct current kind, the assembly of fan and electric motor being referred to as a motorised ventilating fan unit.
The supply of electric current to the electric motor is provided by means of the battery and the alternator of the vehicle, and is controlled by means of an electrical control circuit which enables the temperature of the engine to be regulated. In particular, a relatively small cooling air flow occurs during normal operation. In the majority of cases, it is already known that quiet operation of such an installation is perfectly feasible.
However, in operation under extreme conditions, it is necessary to increase the flow of useful air for cooling the hot parts of the engine. The motorised ventilating fan unit, to this end, must then receive a command such as to increase the cooling air flow. However, the noise produced by the motorised ventilating fan unit is then considerable, which is a nuisance to the occupants of the vehicle.
Electric motors of motorised ventilating fan units are constant flux motors in which the speed of rotation of the motor is not dependent on reaction torque over a wide range of values. Such motors occupy little space, and they consume only a small quantity of current, while their selling price is relatively low. For these very reasons, however, it is not easy to viably apply the electronic control means of the prior art, because the slightly increased cost of the motor also means that the control of its speed of rotation needs to be made less costly.
One object of the present invention is to provide a control means for the speed of the constant flux electric motor which will be reduced in price.
In the main application in respect of which the invention is provided, automobile engines achieve high temperatures only comparatively rarely. In the prior art, it has already been proposed to provide control devices which enable two speeds of rotation of the electric motor of the motorised ventilating fan unit to be obtained.
Such speed changing devices in general include an electrical resistor, which is brought into circuit at low speed and which is arranged to reduce the voltage at the terminals of at least one of the supply brushes for the armature of the electric motor. This voltage reduction results in a reduction in the speed of rotation of the motor from a nominal (or high) speed to a lower speed.
These arrangements in the prior art have two principal drawbacks. The first of these is that the electrical resistor is a relatively expensive component, the price of which accounts for approximately 30% of the cost of the speed changing device. The second main drawback is that in moderate operation of the vehicle, in order not to upset the thermal balance due to the resistance, there is a general requirement that this resistor should be disposed in an air flow of a predetermined direction: however, this is made difficult because of the size then necessary for the housings in which the motorised ventilating fan units are installed.
The present invention provides a remedy for the drawbacks of the prior art.