WO1993020947A1

WO1993020947A1 - Centrifuge with electric control comprising a frequency converter

Info

Publication number: WO1993020947A1
Application number: PCT/DK1993/000124
Authority: WO
Inventors: Helmuth K. Nielsen; Mogens Christensen
Original assignee: Alfa Laval Separation A/S
Priority date: 1992-04-08
Filing date: 1993-04-05
Publication date: 1993-10-28
Also published as: DK47592D0; DK47592A

Abstract

The centrifuge has an electric motor (18) of the squirrel cage type for the rotation of the centrifuge drum (2) during centrifugation and an electric control (20) comprising a frequency converter (21) for the control of the RPM of the electric motor (18). When cleaning the drum (2) a frequency converter (21) is coupled into the normal current supply line (25) to the electric motor (18) by means of two change-over switches (22, 23). The nominal power of the frequency converter corresponds to the power consumption of the electric motor (18) when the centrifuge drum (2) rotates at a speed corresponding to the centrifugal force at the internal side of the drum (2) being 1.2 to 0.8 times the gravitational force.

Description

CENTRIFUGE WITH^'ELECTRIC CONTROL COMPRISING A FREQUENCY CONVERTER.

The invention relates to a centrifuge with an electric motor of the squirrel cage type for the rota¬ tion of the centrifuge drum during centrifugation and an electric control comprising a frequency converter for the control of the RPM of the electric motor.

In operation of centrifuges used for separation of solids from one or more liquids or for separation of various liquids from one another, it is frequently necessary to clean the centrifuge drum of deposits of solids collecting in the drum.

Such cleaning may be effected manually requiring the centrifuge to be disassembled or the cleaning may be effected in place as described in US patents Nos 4,036,426 and 4,978,3-31. During cleaning a cleaning liquid is supplied to the drum which is subsequently rotated at a RPM creating a centrifugal field in the drum of about 1 g.

In a centrifuge with an essentially horizontal axis of rotation, the liquid will be carried up along the inner side of the drum from where it falls down again. This creates a strong turbulence which tears off the deposits of solids. The cleaning effect may be enhanced by making the drum run alternately in one and the other rotational direction. Although this cleaning method is described in the above US patents with reference to a centrifuge with a horizontal axis of rotation, a so-called decanter centrifuge, the method may also be practised in centrifuges with a vertical axis, so-called centri- fugal separators.

Centrifuges of the type described in the above mentioned US patent No. 4,978,331 rotate typically with a speed of about 3000 RPM in the normal mode of opera¬ tion, and during cleaning in place the speed amounts to approximately 50 to 100 RPM depending on the size and type of the centrifuge. US patent 4,978,331 states that this low speed may be obtained in a centrifuge in which the drum conventionally is driven by one electric motor and the screw conveyor by a second electric motor, by permitting the electric motor of the screw conveyor to drive the drum through a special coupling. It is further known to make use of arrangements in which the centrifuge drum during cleaning is being driven by a separate, small electric motor, a so-called barring motor.

DK Patent Application No. 2423/87 deals with a centrifuge of the above mentioned type in which the frequency converter is used to control the RPM of the centrifuge so as to optimize the rotational velocity in consideration of the substances to be separated, while at the same time the start-up and slow-down of the electric motor takes place without switching-over and within reasonable time intervals for start-up and slow-down and at a minimum loss of energy.

For specific modes of operation such as for example cleaning operations it is stated that it is possible to run at any number of revolutions whatever between 0 and the maximum number of revolutions.

The centrifuge according to the invention dif¬ fers from the prior art in that the nominal power of the frequency converter corresponds to the power con- sumption of the motor when the centrifuge drum rotates at a velocity corresponding to the centrifugal force at the internal side of the drum being 1.2 to 0.8 times the gravitational force, preferably 1.0 to 0.8 times the gravitational force, and the electric control com- prising two change-over switches for connecting the frequency converter between a current supply and the electric motor when the centrifuge is to rotate at a low velocity and for disconnecting the frequency con¬ verter at normal centrifugation during which the electric motor is being fed directly from the current supply.

This results primarily in a considerable reduc¬ tion of the electric control of the centrifuge, because the frequency converter requires considerably less space and has a lower need for cooling than is the case with the prior art centrifuge. This has the advantage that the frequency converter may be installed in the cabinet commonly used to accommodate the electric control of the centrifuge, inclusive of the control for carrying out the cleaning operation. The result is that the space requirement for the centrifuge is reduced and that the centrifuge on the whole may be manufactured at a lower cost in comparison with the prior art centri¬ fuge.

The invention will now be explained by an embo- diment and with reference to the drawing illustrating strictly schematically and partly in a sectional view a centrifuge 1 with a drum 2 rotatably journalled in bearings 3 and 4. A screw conveyor 5 in the drum is rotatably journalled in bearings 6 and 7. The drum and the screw conveyor are connected with a planetary gear 8 whose input shaft is driven through a grooved pulley 10 and v-belts 11 that are connected with a double grooved pulley 12 rotatably journalled on a shaft 14. The double, grooved pulley is driven by V- belts 13 connected with a projecting part of the housing of the gear 8. At the drum end, opposite the planetary gear 8 the drum carries a grooved pulley 15 which by means of v-belts 16 is connected with a second grooved pulley 17 on an electric motor 18. The drawing further illustrates an electric con¬ trol 20 shown in dot-and-dash lines. The control 20 comprises a frequency converter 21 which by means of two interconnected change-over switches 22 and 23 may be connected in series with a current supply line 24 and a line 25 to the electric motor 18. The control also comprises in a known manner, the necessary change-over switches and means for driving the electric motor 18 during normal operation of the centrifuge and a unit 26 comprising program works or circuits used in the cleaning phase to reverse the direction of rota- tion of the electric motor 18 at regular intervals.

The centrifuge operates in the following manner. During normal centrifugation the centrifuge drum 2 is driven to rotate by means of the electric motor 18. Power is supplied from line 24 via the change-over switch 23 to line 25. It is evident that the fre¬ quency converter 21 in this situation is de-ener¬ gized.

When cleaning the drum a cleaning liquid is introduced to the drum in a known manner. The drum is then stopped, following which the change-over switches 22' and 23 are switched from the position illustrated in the drawing to their second position so that the fre¬ quency converter 21 is connected between the current supply line 24 and the line 25 to the electric motor 18. The control unit 26 then controls the frequency converter through conduits 27 so that a preprogrammed cleaning program may be completed by accelerating the electric motor 18 to the RPM necessary to create a centrifugal field at the internal side of the centrifu- ge drum, said field being equal to or slightly smaller than the gravitational force. The cleaning program may for instance consist in that the rotational direction of the electric motor is altered at predetermined in¬ tervals while simultaneously adjusting the frequency up and down for starting or stopping the motor respecti¬ vely. When the program has finished the frequency con- verter 21 is disconnected by means of the change-over switches 22 and 23, by switching to the position illu¬ strated in the drawing so that normal centrifugation may be resumed. Even though the invention is described in con¬ nection with a so-called decanter centrifuge the inven¬ tion may also be used in connection with a so-called separator of the type illustrated in the above mentio¬ ned DK patent application No. 2423/87.

Claims

P A T E N T C L A I M S 1. A centrifuge (1) with an electric motor (18) of the squirrel cage type for the rotation of the cen¬ trifuge drum (2) during centrifugation and an electric control (20) comprising a frequency converter (21) for the control of the RPM of the electric motor (18), characterized in that the nominal power of the fre¬ quency converter (21) corresponds to the power consump¬ tion of the electric motor (18) when the centrifuge drum (2) rotates at a velocity corresponding to the centrifugal force at the internal side of the drum (2) being 1.2 to 0.8 times the gravitational force, pre¬ ferably 1.0 to 0.8 times the gravitational force, and the electric control (20) comprising two change-over switches (22, 23) for connecting the frequency con- verter (21) between a current supply and the electric motor (18) when the centrifuge (1) is to rotate at a low velocity and for disconnecting the frequency con¬ verter (21) at normal centrifugation during which the electric motor is being fed directly from the current supply.