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Method for controlling the drive motor of a positive displacement vaccum pumpRelated Patent Categories: Pumps, Condition Responsive Control Of Pump Drive Motor, By Control Of Electric Or Magnetic Drive MotorMethod for controlling the drive motor of a positive displacement vaccum pump description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070071610, Method for controlling the drive motor of a positive displacement vaccum pump. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001] The invention relates to a method for controlling a drive motor of a positive displacement vacuum pump, and to a positive displacement vacuum pump comprising a drive motor control. [0002] Positive displacement vacuum pumps are, for example, membrane pumps, rotary vane pumps, piston pumps or Roots pumps, and are frequently used as fore vacuum pumps in combination with a high vacuum pump. A special feature of these positive displacement vacuum pumps is that the final pressure attainable by said pumps, i.e. the fore vacuum pressure, is speed-dependent to a high extent, wherein the speed must be high at high inlet pressures, and must be low at low inlet pressures for attaining an optimum suction capacity. This can be explained by the fact that at low inlet pressures filling of the suction space takes place at a relatively slow rate due to the small difference between inlet pressure and suction pressure in the working chamber. This results in a poor filling level of the positive displacement vacuum pump at low inlet pressures, which filling level can only be improved by extending the opening times of the inlet valve, i.e. a speed reduction. [0003] From DE 198 16 241 C1 a positive displacement vacuum pump is known which is operated, in dependence on an inlet pressure value, at two different speeds, namely at a high speed for evacuation purposes, and at a low speed for reaching the lowest final pressure possible. Relatively much time is required between the beginning of the pumping process and reaching of the final pressure. SUMMARY [0004] It is an object of the invention to provide a method and a positive displacement vacuum pump with the aid of which the final pressure can be more rapidly reached. [0005] According to one aspect, a method is provided for controlling a drive motor of a positive displacement vacuum pump comprises the method steps of storing a pressure-speed curve, determining the inlet pressure value, determining the speed value from the curve, and operating the drive motor at the determined speed value. [0006] First, a curve is stored in which a single constant upper speed value n.sub.1 is associated with inlet pressure values p larger than or equal to an upper limit pressure p.sub.1, and which comprises an alteration range for inlet pressure values p smaller than the upper limit pressure p.sub.1, wherein in the alteration range different speed values n.sub.v are associated with the inlet pressure values p. [0007] During operation of the drive motor the inlet pressure value p is permanently determined, the associated speed n is determined from the inlet pressure value p of the curve, and the drive motor is operated at the determined speed n. While at high inlet pressure values p above the upper limit value p.sub.1 the drive motor is operated at a maximum constant speed n.sub.1, a corresponding speed value n.sub.v is approximately continuously associated for speeds above the upper limit value p.sub.1 in dependence on the inlet pressure value p. In this manner, the effective suction capacity of the positive displacement pump can be kept at the highest level possible for each inlet pressure value. Thus, the time between beginning of evacuation and reaching of the final pressure is decreased. By adapting the speed to the inlet pressure value, the required drive energy and, due to the lower average speed level, the wear are reduced. Thereby, the maintenance and operating costs are reduced, and thus the efficiency of the positive displacement vacuum pump is improved. [0008] Preferably, the curve comprises a lower range for inlet pressure values p smaller than or equal to a lower limit pressure p.sub.2, wherein a single constant lower speed value n.sub.2 is associated with the lower range, and the alteration range is limited to inlet pressure values p larger than the lower limit pressure range p.sub.2. The curve thus comprises both an upper pressure range of constant speed and a lower pressure range of constant speed, as well as an alteration range of non-constant speed between said two ranges. Such a curve is, for example, necessary and useful for fore vacuum pumps which need a given minimum speed for pumping action since below the minimum speed no pumping capacity can be attained, in particular due to backflow losses. This applies, for example, to oil-sealed rotary vane pumps. Thus it is ensured that the positive displacement vacuum pump is always operated above a speed at which the pumping function is still guaranteed even at very low inlet pressures. [0009] According to a further aspect, the curve comprises, instead of an upper range, a lower range for inlet pressure values p smaller than or equal to a lower limit pressure p.sub.2, wherein a single constant lower speed n.sub.2 is associated with the lower range. [0010] Preferably, in the alteration range decreasing speeds n.sub.v are associated with decreasing inlet pressure values p, i.e. low speed values n.sub.v are associated with low inlet pressure values p. [0011] Preferably, the upper limit pressure p.sub.1 ranges between 20 mbar and 1 mbar, and the lower pressure p.sub.2 ranges between 1.0 mbar and 0.005 mbar, wherein the upper limit pressure p.sub.1 is larger than the lower limit pressure p.sub.2. [0012] According to a preferred aspect, the upper constant speed value n.sub.1 ranges between 2,200 and 1,000 rpm, and the lower constant speed value n.sub.2 ranges between 300 and 1,300 rpm, wherein the upper constant speed value n.sub.1 is larger than the lower constant speed value n.sub.2. [0013] Preferably, the positive displacement pump is a fore vacuum pump arranged upstream of a high vacuum pump, and the inlet pressure value p is the suction-side pressure of the high vacuum pump. The inlet pressure value p thus is the pressure in the recipient evacuated by the high vacuum pump. Alternatively, the inlet pressure value p may also be the fore vacuum pressure immediately before the inlet of the fore vacuum pump. [0014] According to a preferred aspect, the inlet pressure-speed curve is saved in a characteristic diagram storage. In the characteristic diagram storage, a corresponding speed n is associated with each inlet pressure value p. [0015] Preferably, the drive motor is an asynchronous motor driven by a correspondingly driven frequency converter. However, the drive motor may also be configured as a synchronous motor. [0016] The positive displacement vacuum pump comprises a drive motor, an inlet pressure sensor and a drive motor control which controls the speed n of the drive motor in dependence on the inlet pressure value p determined by the inlet pressure sensor. Further, the drive motor control comprises a storage for storing a curve that indicates a respective speed n of the drive motor for the inlet pressure values p of the inlet pressure sensor, wherein the curve comprises two ranges: the first range is an upper range for inlet pressure values p larger than or equal to an upper limit pressure p.sub.1, with a single constant upper speed value n.sub.1 being associated with said first range. The second range is an alteration range for inlet pressure values p smaller than the upper limit pressure p.sub.1, wherein in the alteration range different speed values n.sub.v are associated with the inlet pressure values p. [0017] Preferably, the drive motor control comprises a processor which has connected therewith the inlet pressure sensor and evaluates the signals from the inlet pressure sensor. The evaluated inlet pressure sensor signals can be supplied to a pressure indicator associated with the positive displacement vacuum pump. The inlet pressure sensor signals are thus not only evaluated by the drive motor control with regard to controlling the drive motor, but also converted into an indication format, and finally supplied to an indicator associated with the vacuum pump. Thus a separate evaluating and indicating device for indicating the inlet pressure is not required. BRIEF DESCRIPTION OF THE DRAWINGS [0018] An embodiment of the invention will now be described in greater detail with reference to the figures in which: [0019] FIG. 1 shows a schematic representation of a pump assembly comprising a positive displacement vacuum pump according to the invention configured as a fore vacuum pump, and a high vacuum pump, and [0020] FIG. 2 shows an inlet pressure-speed curve according to which the speed of the drive motor of the positive displacement vacuum pump is controlled. DETAILED DESCRIPTION Continue reading about Method for controlling the drive motor of a positive displacement vaccum pump... 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