Apparatus comprising an electric motor with variable motor power

An apparatus, for example a vacuum cleaner, is provided which comprises an electric motor with variable motor power, a control circuit for controlling the variable motor power, and a vacuum chamber for generating a partial vacuum by means of the electric motor and, the control circuit being effective to temporarily increase the variable motor power to a power level above a nominal motor power in dependence upon the partial vacuum generated in the vacuum chamber.

FIELD OF THE INVENTION 
The invention relates to an apparatus comprising an electric motor with a 
variable motor power, a control circuit for controlling the variable motor 
power and a vacuum chamber for generating a partial vacuum by means of the 
electric motor, the control circuit comprising means for temporarily 
increasing the variable motor power to a power level above a nominal power 
level. The nominal motor power is to be understood to mean a maximum motor 
power, which maximum motor power can be endured by the electric motor and 
by the apparatus for a long period. 
BACKGROUND OF THE INVENTION 
Such an apparatus may be constructed, for example, as a vacuum cleaner, the 
vacuum chamber being provided with a dust bag. In the case of a vacuum 
cleaner, the temporary increase of the variable motor power to a power 
level above the nominal motor power is desirable, for example, for 
removing a thread from a carpet or for cleaning a hard-to-reach crevice. 
Switching on the more than nominal power will be hereafter referred to as 
the "boost" function. 
Such an apparatus is known inter alia from patent application WO 
87/01921,which corresponds substantially to U.S. Pat. No. 4,920,607, the 
disclosure of which is hereby incorporated by reference and which 
describes an apparatus constructed as a vacuum cleaner. In order to 
increase the variable motor power to a power level above the nominal motor 
power, the vacuum cleaner described in said Patent has a boost function 
provided by said means, which boost function is operative for a first 
fixed time interval (10 seconds ) and a second fixed time interval after 
its initiation. During the first fixed time interval the boost function 
causes the variable motor power to be increased, the boost function being 
attended with a production of heat by the electric motor. Since the 
electric motor as well as the vacuum cleaner may be damaged as a result of 
the increased heat production, the first fixed time interval covers a 
period in which the electric motor as well as the vacuum cleaner cannot be 
damaged as a consequence of a continuous use of the boost function. During 
the second fixed time interval (20 seconds ) the boost function prevents 
the variable motor power from being increased, the boost function being 
attended with a diminished heat production. The second fixed time interval 
covers a period after which the electric motor as well as the vacuum 
cleaner cannot be damaged as a result of a subsequent use of the boost 
function. 
A disadvantage of the apparatus described in said patent application is 
that the increase of the variable motor power is usually discontinued 
prematurely as a result of the first fixed interval and is usually 
inhibited too long as a result of the second fixed interval. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide an apparatus in which means are 
provided which effect the temporary increase of the variable motor power 
beyond the nominal motor power or the boost function with an improved 
timing. 
According to the invention an apparatus comprising an electric motor with 
variable motor power is characterized in that means are provided and are 
constructed to temporarily increase the variable motor power in dependence 
upon a partial vacuum generated in a vacuum chamber. The invention is 
based on the recognition of a first fact that the electric motor as well 
as the apparatus cannot be damaged as a result of the temporary increase 
of the variable motor power if the speed of the electric motor remains 
substantially below a maximum speed value and on the recognition of a 
second fact that the partial vacuum is related to the speed of the 
electric motor. In accordance with the recognition of these two facts the 
temporary increase of the variable motor power in an apparatus in 
accordance with the invention is effected for a time interval related to 
the partial vacuum, the time interval being dictated by the partial vacuum 
before the increase or the partial vacuum during the increase, and the 
temporary increase of the variable motor power being inhibited or stopped 
if the partial vacuum exceeds a reference partial vacuum related to the 
maximum speed value. 
It is to be noted that the continuous control of the variable motor power 
in dependence upon the partial vacuum is known, for example from U.S. Pat. 
No. 2,789,660 but the temporary increase of the variable motor power to a 
power level above the nominal motor power in dependence upon the partial 
vacuum is not known. 
An apparatus in accordance with the invention may be characterized further 
in that the means comprise a pressure sensor for measuring the partial 
vacuum. The pressure sensor is an electronic device generating an electric 
signal in dependence upon the partial vacuum. By means of the signal 
generated by the pressure sensor the temporary increase of the variable 
motor power can be controlled in a simple way. 
A first embodiment of an apparatus in accordance with the invention may be 
characterized in that the pressure sensor is constructed by means of a 
pressure-sensitive switch for generating a digital signal in dependence 
upon the partial vacuum relative to a reference partial vacuum. The 
temporary increase of the variable motor power can be inhibited or stopped 
on the basis of the generated digital signal, the reference partial vacuum 
being related to the maximum speed value. 
The first embodiment of an apparatus in accordance with the invention may 
be characterized further in that the means are adapted to temporarily 
increase the variable motor power for a first time interval when the 
partial vacuum is below the reference partial vacuum, and for a second 
time interval when the partial vacuum is above the reference partial 
vacuum. In the present embodiment the temporary increase of the variable 
motor power can be effected for a comparatively short interval if upon 
initiation of the temporary increase the partial vacuum lies comparatively 
closely below a partial vacuum corresponding to the maximum speed value, 
and for a comparatively long time interval if the partial vacuum lies 
comparatively far below the partial vacuum corresponding to the maximum 
speed value. A preferred choice of the two time intervals and the 
reference partial vacuum precludes damage to the electric motor as well as 
the apparatus. In the most preferred case, the first time interval may be 
selected to be zero seconds. 
A second embodiment of an apparatus in accordance with the invention may be 
characterized in that the pressure sensor is constructed by means of an 
analog pressure transducer for generating an analog signal in dependence 
upon the partial vacuum. The temporary increase of the variable motor 
power can be inhibited or stopped on the basis of the analog signal. 
The second embodiment of an apparatus in accordance with the invention may 
be characterized further in that the means are adapted to temporarily 
increase the nominal motor power for a variable time interval, the 
variable time interval being dependent upon the analog signal. In the 
present embodiment the temporary increase of the variable motor power can 
be effected for a time interval in which the analog signal remains below a 
signal value corresponding to the maximum speed value. Since the analog 
signal is a variable representation of the speed the temporary increase of 
the variable motor power can be sustained for a variable time interval. 
The second embodiment of an apparatus in accordance with the invention may 
also be characterized in that the pressure sensor is capable of measuring 
the partial vacuum both for controlling the variable motor power and for 
temporarily increasing the variable motor power. As already stated, it is 
known to control the variable motor power by means of a pressure sensor. 
For example, in a vacuum cleaner the variable motor power is controlled so 
as to maintain an air stream produced by means of the electric motor 
constant, yielding a constant suction of the vacuum cleaner. In an 
apparatus in accordance with the invention the pressure sensor can be 
given two functions, which saves at least one pressure sensor. The 
pressure sensor plays a part in the known regulation of the variable motor 
power and, in addition, the pressure sensor plays a part in temporarily 
increasing the variable motor power, the relevant time interval for this 
temporary increase being determined on the basis of the partial vacuum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As an example, the apparatus is constructed as a vacuum cleaner. The 
construction which is generally known from WO-87/01921 (U.S. Pat. No. 
4,920,607), shows a casing 10, which is provided with an air inlet 11 and 
air outlet 12, a dust container 13 being connected to the air inlet 11. In 
order to generate an air displacement from the air inlet to the air 
outlet, a suction fan 14, which is driven by an electronic 15, is placed 
near the dust container 13, the electric motor 15 being controlled by an 
electronic circuit 16 and the electronic circuit 16 being adjusted by a 
control knob 18 and a push button 19. The control knob 18 and the push 
button 19 are mechanically fixed to the casing 10 and electrically coupled 
to the control circuit 16. As the vacuum cleaner can be controlled by the 
control knob 18 when being used at a power level below the nominal power 
level, the push button 19 has been provided to start a temporary increase 
of the power level to a level above the nominal power level. The apparatus 
in accordance with the invention comprises means for controlling the 
temporary increase in motor power in dependence on a partial vacuum and 
the construction is provided with a pressure sensor 20 which is fixed in 
the vacuum chamber, which vacuum chamber is constituted by a space between 
the air inlet 11 and the suction fan 14, the dust container 13 also being 
included in this space. The pressure sensor 20 is effective to convert the 
partial vacuum into an electric signal, which partial vacuum results from 
the air displacement being generated by the suction fan 14. The control 
circuit 16 can switch the electric motor 15 in case of an overload. The 
control circuit may comprise a microcomputer, which is adjusted by the 
control knob 18 via a first input, which operates the boost function by 
the push button 19 via a second input and by the pressure sensor 20 via a 
third input, and which is prevented from operating the boost function by 
the pressure sensor 20 via the third input, and which controls the 
electric motor 15 via a single output. The operation of the control 
circuit is further described with reference to FIGS. 2A and 2B. 
FIG. 2A shows a graph representing a speed in an apparatus in accordance 
with the invention as a function of an air displacement produced by the 
apparatus, the speed and the air displacement being symbolized by the 
letters R and Q respectively. The air displacement Q is obtained in that 
the apparatus has an electric motor of variable motor power, a control 
circuit for controlling the variable motor power, and a vacuum chamber for 
generating a partial vacuum by means of the electric motor, the control 
circuit setting the power of the electric motor to a value below a nominal 
motor power, Pnom. The nominal motor power, Pnom, is to be understood to 
mean a maximum motor power, which maximum motor power can be endured by 
the electric motor and by the apparatus for a long period. Since a higher 
motor power may be desired from time to time the control circuit comprises 
means for temporarily increasing the variable motor power to a power 
level, Pbst, above the nominal motor power Pnom, which temporary increase 
of the variable motor power is attended with an increased production of 
heat by the electric motor, which may lead to damaging of the electric 
motor and of the apparatus. Such damaging of the electric motor and 
apparatus is precluded by briefly inhibiting or duly stopping the boost in 
variable motor power. In accordance with the invention briefly inhibiting 
or duly stopping is achieved in that the means are constructed to 
temporarily increase the variable motor power in dependence upon the 
partial vacuum generated in the vacuum chamber. The invention is based on 
the recognition of a first fact that the electric motor as well as the 
apparatus cannot be damaged as a result of the temporary increase of the 
variable motor power if the speed of the electric motor remains 
substantially below a maximum speed value and on the recognition of a 
second fact that the partial vacuum is related to the speed of the 
electric motor. In FIG. 2A a letter r represents a reference speed related 
to the maximum speed, which reference speed r, when related to the power 
level Pbst, corresponds to a reference air displacement represented by a 
letter q. 
FIG. 2B is a graph representing a partial vacuum generated by an apparatus 
in accordance with the invention as a function of the air displacement 
produced by the apparatus, the partial vacuum and the air displacement 
being symbolized by a letter H and the letter Q respectively. The letter q 
represents the reference air displacement, in the same way as in FIG. 2A, 
which reference air displacement q on the basis of the power level Pbst 
corresponds to a reference partial vacuum represented by a letter h. A 
first and a second embodiment of an apparatus in accordance with the 
invention will now be described hereinafter by way of illustration with 
reference to FIGS. 2A and 2B, the means in both embodiments comprising a 
pressure sensor for measuring the partial vacuum. 
In the first embodiment of an apparatus in accordance with the invention 
the pressure sensor is constructed by means of a pressure-sensitive switch 
for generating a digital signal in dependence upon the partial vacuum 
relative to the reference partial vacuum h, a partial vacuum above the 
reference partial vacuum yielding a digital signal of a first signal level 
and a partial vacuum below the reference partial vacuum yielding a digital 
signal of a second signal level. On the basis of the first signal level, 
which corresponds to a speed above the reference speed r, the means can 
effect a temporary increase of the variable motor power for a first time 
interval. If the reference speed corresponds to the maximum speed value 
the first time interval may be selected to be zero seconds, although the 
electric motor as well as the apparatus can briefly withstand the 
temporary increase of the variable motor power. On the basis of the second 
signal level, which corresponds to a speed below the reference speed r, 
the means can effect the temporary increase of the variable motor power 
for a second time interval. If the reference speed corresponds to the 
maximum speed value a longer time may be taken for the second time 
interval because the electric motor as well as the apparatus cannot be 
damaged in the case of a speed below the maximum speed value. 
In the second embodiment of an apparatus in accordance with the invention 
the pressure sensor is constructed by means of an analog pressure 
transducer for the generation of an analog signal in dependence upon the 
partial vacuum, a partial vacuum above the reference partial vacuum h 
yielding for example an analog signal of comparatively high signal level 
and a partial vacuum below the reference partial vacuum h yielding for 
example an analog signal of comparatively low signal level. On the basis 
of the signals level, which corresponds to a speed above or below the 
reference speed r, the means can allow the temporary increase of the 
variable motor power for a variable time interval, the variable time 
interval being for example inversely proportional to the analog signal. 
The temporary increase will proceed for a longer time interval as the 
speed of the electric motor lies further below the maximum speed because 
the electric motor as well as the apparatus cannot be damaged at such a 
speed. In the second embodiment of an apparatus in accordance with the 
invention the pressure sensor can be given a double function, for which 
double function the pressure sensor plays a part in the continuous control 
of the variable motor power and in the temporary increase of the variable 
motor power. The continuous control is utilized for example for 
maintaining the air displacement produced by the electric motor constant, 
the resulting air displacement yielding a constant suction in the case of 
a vacuum cleaner. 
The invention is not limited to the embodiments described herein. Within 
the scope of the invention several modifications are conceivable to those 
skilled in the art. For example, the signal generated by the pressure 
sensor for controlling the temporary boost in the variable motor power can 
be processed in various manners, both prior to the boost and during the 
boost. Finally, it is to be noted that an apparatus in accordance with the 
invention can be constructed not only as a vacuum cleaner but also as a 
drill, a lawn-mower or another appliance.