US2349559A

US2349559A - Centrifuge drive

Info

Publication number: US2349559A
Authority: US
Publication date: 1944-05-23
Anticipated expiration: 1961-05-23

Description

May 23, 1944.

C. A OLCOTT CENTRIFUGE DRIVE Filed Nov. 8, 1940 TIME SW/TCH unsrik SWITCH m con/mar UNZOAD/NG 5, 550

TIME SECONDS INVENTOR C. A. OLCOT T BY ATTORNEY Patented May 23, 1944 UNITED STATES PATENT OFFICE CENTRIFUGE DRIVE Charles A. Oloott, West Milford, N. J.

18 Claims.

This invention-relates to centrifuges such as Application November 8, 1940, Serial No. 364,777

employed in the manufacture and refining of sugar and more particularly to methods and means for the electric driving of such apparatus.

The refining of sugar involves at one step of the process the centrifugal separation of sugar crystals from a mass of magma throughout which the crystals are dispersed. The magma is loaded into the basket of the centrifuge and the latter is spun at high speed to expel the fluid material, leaving the crystals within to be unloaded. In a typical installation in accordance with present day practice the centrifuge basket is 40 inches in diameter and at top speed its rotational ve locity is approximately 1800 R. P. M. The economics of the process are such that the centrifuges are in almost continuous operation and special provisions are made to reduce to a minimum the time required to load the casket, accelerate it to top speed, decelerate it and no load it. The entire cycle may occupy only a few minutes. One cycle of operation may involve 500 lbs. oi magma.

The purging of the crystals is more effective the greater the centrifugal force developed dur ing the high speed operation of the centrifuge but this is limited at least in part by the limited strength of the basket. Centrifuges are driven most advantageously by electric motors of the squirrel cage induction type designed with reference to the available alternating current supply to operate at the desired speed. The speed of such motors is not continuously variable and, in fact, the operating speed cannot readily be changed except in large increments dependent on the number of poles and the operating power frequency, and even such changes require rewiring of the machine. For example, a motor having twelve poles and operated on 60 cycle current can be wired to operate at speeds approximating 600, 900, 1200, 1860 and 3600 R. P. M., these speeds corresponding respectively with 12-, 8-, 6-, 4- and 2-poie connection.

In many cases the centrifuge is capable of safe operation at a speed substantially higher than that for which'the driving motor is wired but not enough higher'to permit rewiring of the motor for operation at the next higher synchronous speed. Thus if the actual operating speed is i888 R. P. M.. the basket may, for example, safely withstand 2200 R. P. M. but not 3600, the next higher synchronous speed. Heretofore in practice where the centrifuge is directly connected to an individual driving motor it has been necessary to'forego the advantages that would be obpacity.

tained with operation at 2200 R. P. M. in such cases.

Qne of the principal objects of the present in-- vention is to improve the effectiveness of purging in the operation of a centrifuge. Another object is to provide for the efllcient operation of electrically driven centrifuges at speeds higher than those for which their motor drives were originally arranged or alternatively to reduce the time required to complete the centrifuging cycle.

In accordance with the present invention the foregoing objects and other objects that will appear hereinafter are attained by providing for operation of the centrifuges on power currents of different frequencies. Current of one fixed frequency is ordinarily supplied by the main power system associated with the refinery, whether it be a power plant supplying only the electrical requirements of the refinery or an electric power network system supplying power also throughout the area in which the refinery is located. The frequency of this main system is generally not subject to change.

The invention in its preferred form contemplates the provision of an "auidliary power system adapted to supply power currents of a different frequency for selective application to the centrifuge driving motors. The auxiliary system need have but a small fraction of the power generating capacity of the main system and it may take the form merely of a frequency converter set connected to and driven from the m i p w sy e o supply current of the desired irequency, or it may, for example, embrace a house power generating plant of moderate ca Assuming that the centrifuge driving alternating current motors, selected rence to the frequency of the main powe s tern to drive the centrifuge baskets at a predetermined speed, and that operation at a higher speed short of the next higher synchronous is desired, the invention further con= templates such choice of the frequency of the auxiliary system in relation to the design of the driving motors that the latter when driven from the auxiliary system operate at the desired higher speed. In the simplest and preferred case motors the frequency of the auxiliary system is higher the power capacity required of the auxiliarysystem is minimized by drawing on the main power system for the relatively high power required to accelerate the centrifuge from loadoperation at the higher ultimate or synchronous speed obtainable on connection to that system.

Bearing in mind that if the auxiliary system is of small capacity or if in any case it is energized by the conversion of power from the main system, its efiiciency of operation may be relatively low and the energy taken from it comparatively costly, it may be' noted that the feature last described tends to place the greater part of the energy load, as well as the power load, on the presumably highly efficient main system.

In accordance with another feature of the invention, the regenerative deceleration or braking of the centrifuge that ordinarily follows immediately the period of operation at maximum speed is effected by automatically reconnecting the centrifuge driving motor to the main system rather than by allowing the motor to regenerate into the auxiliary system to which it was last connected. One of the several advantages attendant on this feature is that the efficiency of energy reconversion is relatively high, for the dynamic power of the centrifuge is returned directly to the main power system rather than either indirectly through a frequency changer or into the auxiliary power system.

The nature of the present invention and its various features, objects and advantages will ap-- pear more fully from a consideration of the following description of the illustrative embodi-' ments shown in the accompanying drawing. in the drawing,

Fig. 1 illustrates schematically a system for the driving of centrifuges in accordance with the invention;

Fig. 2 shows a modification thereof;

Fig.3 comprises curve diagrams to which reference will be made in the description of Figs. 1 and 2.

Referring more particularly now to Fig. 1 there is shown in outline the power supply and control circuits for the operation of all of the centrifuges in a given refinery. The apparatus and circuits individual to one centrifuge are shown completely in schematic form and it will readily appear how the system may be extended to accommodate any number of centrifuges in the same plant.

The centrifuge is represented diagrammatically by the basket I which is driven by its individual centrifuge motor 2. The main power supply system, which as indicated hereinbefore may be local to the plant or part of a power supply network, is represented by the bus-bars 3 and is presumed to be 3-phase. The auxiliary power supply sys tern is represented by a frequency changer i the input terminals of which are connected to busbars 3 and the output terminals of which are connected to the 3-phase bus-bar system 5. It may be assumed for specific example that the main power system operates at 60 cycles, that the frequency changer produces power currents of 80 cycles, and that the driving motor 2 when connected to a cycle source hasa' synchronous speed of 1800 R. P. M. Throughout this specification it should be understood that operation at a given synchronous speed may not actually be -achieved inasmuch as there may be with certain types of motors commonly used a certain amount.

of slip. The latter may be, for example, about 5 per cent. Thus the specified synchronous speed of 1800 R. P. M. may correspond to an actual,

speed Of 1710 R. P. M.

The connection of the driving motor 2 to one or the other of the bus-

bar systems

3 and 5 may .be effected by means of a biased multi-element switch 8 that is operated by means of an electromagnet 8. In the normal position of switch 6 bus-bar system 3 is connected through contact elements I to the contactors in which are in the input leads to the motor 2. The contactors III are controlled by master switch II to make or break the power circuit leading to the motor. In the alternative position of switch 6 contact elements 8 are closed to connect bus-bar system 5 to the motor circuit contactcrs i0.

Switch magnet 9, which may be energized from any suitable source, is operated through the intermedlary of a time switch l2 at a predetermined time after, and in response to, closure of master switch II. There may be provided also a second time switch l8 so associated with magnet 9 and time switch l2 as to cause magnet 8 to be deenergized, therefore releasing switch 6, at a predetermined time after the operation of time switch If. In view of the fact that time switches and their arrangement for performing the functions indicated are well known in the art a detailed description of them is considered superfluous.

From the foregoing description of Fig. 1 it should now be understood that on manual closure of master switch ii, e0 cycle power is applied to the centrifuge driving motor 2, that after an interval fixed by timer I2 switch 6 operates to disconnect the motor from the 60 cycle system and to connect it to the auxiliary cycle system, and that after a further interval fixed by timer ii the motor is reconnected to the 60 cycle system.

The operationof the Fig. 1 system as above described may be stated briefly as follows. It may be assumed that initially the centrifuge is either at rest or rotating at the slow speed sometimes used in loading the basket. On closure of master switch il power is supplied to the motor from the 60 cycle system thereby causing the centrifuge to accelerate rapidly. This initial acceleration may be effected in two or more stages if desired, in accordance with the usual practice, with the motor poles being automatically changed to correspond with successively higher synchronou speeds. For example, the motor may be a twelve-pole two-speed motor having a low synchronous speedof 900 R. P. M. and a high synchronous speed of 1800 R. P. M. with automatic change over from the one to the other as the motor gains speed.

The time required to accelerate the centrifuge to a predetermined speed will be substantially the same from one centrifuging cycle to another, hence it may be said that the time switch If! operates to switch the driving motor to the 80 cycle system after a predetermined speed is reached. Preferably the timer i2 is so adjusted that the switching takes place only after the initial acceleration has brought the centrifuge to a speed at least half the top speed tliat'is to be attained on connectlonto the 80 cycle system so that substantial benefit is derived from the apportionment of the energy and power loads on the two power systems. For maximum benefit of this character the centrifuge may be allowed to accelerate, while connected to the 60 cycle system, until it reaches substantially top speed for that frequency and thereupon immediately tranferred to the 80 cycle system. It is preferred,

however, to obtain more nearly constant, continuous acceleration and for this objective the transfer to the 80 cycle system may advanta-' geously be made, in the illustrative example herein disclosed, at a speed of about 1600 R. P, M.

On switching the driving motor to the 80 cycle system the centrifuge resumes or continues acceleration to a still higher speed. Since the synchronous speeds are proportional to frequency the centrifuge in this case would tend to approach a synchronous speed of 2400 R. P. M. Excess windage losses, however, increase the slip and would hold the actual top speed to about 2200 R. P. M. After operation at this speed for any predetermined length of time (chosen with regard to the particular type of magma being treated),

the motor 2 is switched by operation of the timer ii to the 60 cycle power supply system. The motor 2 then operates as a generator and returns power to the 60 cycle system, thereby being decelerated. Preferably, and in accordance with the prevailing practice, this regenerative braking is effected with the motor poles automatically ,switched over to low speed connection, and the 'speed may be thereby reduced to 1000 R. P. M. for example. Further deceleration and complete stoppage may be obtained by the subsequent automatic application of a mechanical brake, and any of the variety of devices that are in current use may be employed for this purpose.

I find it desirable in any case to have the voltages of the two power supply systems at least approximately proportional to their respective frequencies. Thus if the main power system is a 440-volt 60 cycle system, the auxiliary system may be a 585-volt 80 cycle system. It will be understood, then, that when the regene ative bralring begins the electromotive force developed in the motor is considerably higher than the voltage of the power system to which it is then connected, which is a condition contributing to rapid deceleration.

The sequence of operations in the centrifuge cycle may be better understood by reference to Fig. 3. The solid line curve in Fig. 3 shows how the rotational velocity of the centrifuge in R. P. M. varies throughout the centrifuging cycle in a typical case. While connected to the 60 cycle system, the centrifuge is accelerated in 85 sec-= onds to a speed of about 1600 R. P. M., which is only slightly less than the actual maximum speed which might ultimately be obtained as indicated by the dotted branch curve. At this point the driving motor is transferred to the 80 cycle system whereupon the centrifuge is accelerated to its ultimate speed of approximately 2200 R. P. M. and maintained at that speed for a period of 32 seconds. At the end of this period of high speed operation regenerative braking on the 60 cycle power system begins and the speed is rapid- 1y reduced to about 1000 R. P. M. At this point a mechanical brake is applied to bring the centrifuge to rest or to a low speed su table for the unloading operation. The complete cycle in the example illustrated requires but 360 seconds and it is immediately followed by the next cycle.

Brief consideration of the dotted line curve in Fig. 3 will fac litate an understanding of 2. This curve shows how the input current to the driving motor varies while the motor is connected to receive power from the 60 cycle system. The important point to observe is that after an initial surge to a high value the input current drops off rapidly as the centrifuge gains speed and that there is then a fixed relation between the rotational velocity and the input current. In accordance with a modification of Fi 1, use is made of the fact that when the centrifuge has accelerated to the point where it is to be transferred to the cycle power supply, the input current to the driving motor is of a predetermined value such as indicated in Fig. 3. Means are provided, then, responsive when the input current drops to the predetermined value, to operate switch 6 in lieu of timer l2.

As shown in Fig. 2 the alternative means for operating switch 0 may comprise a current limit relay arrangement II in the power leads to motor 2. After closure of switch II the relay arrangement It operates when the input current drops to the predetermined value. If desired, a timer may be provided also to perform the function of timer II in Fig. i. The relay arrangement 15 can com prise a contact making ammeter such as that shown in Fig. l of Patent 2,189,846, issued Feb. 13, 1941), to R. L Z. Valtat, the needle of the meter being adapted to make contact with the contact member when the current decreases to 0 amperes. Obviously, the meter is not ener for the period of time required for the Inc v to accelerate, its energization being delayed, p3 any suitable means such as the relay timing circuit shown in Patent 2,182,637, issued Dec. 5, 1939, to N. E. Marbury, until the current is decreasing or, in other words, until the motor is oprating on the descending portion of the dotted curve shown in Fig. 3.

Although the present invention has been described in terms of specific embodiments particularly adapted for present day refinery practice, it will readily be understood by those skilled in the art that the invention is susceptible of embodiment in various other forms within the spirit and scope of the appended claims.

What is claimed is 1. In combination with a centrifuge, an alterhating current driving motor for said centrifuge, a source or low frequency power currents and a source of high frequency power currents, said high frequency being at least five percent greater than but less than twice the low frequency, means for connecting said motor to said low frequency source for acceleration of said centrifuge, and means automatically operative after a predeter mined period of acceleration for switching said motor from said low frequency source to said high frequency source to continue the acceleration of said centr 2. The comhinat n of elements as in claim 1 and being furti characterized in that the voltaid 'ces are at least approximately tion to their respective frequencies.

.ihination in a system for the operaa centrifuge, an alternating current driv-. irig motor for said centrifuge, a source of low "frequency power currents and a source of high iireauency currents, said low frequency being at least half said high frequency and said high freduency being at least five percent greater than said low frequency, means for connecting said motor to said low frequency source for acceleration of said centrifuge, and timing means for switching said motor from said low frequency source to said high frequency source after a predetermined period of acceleration.

4. In combination with a centrifuge, an alterhating current driving motor for said centrifuge, a source of low frequency power currents and a source of frequency power currents, said high frequency being at least five percent greater than but less than twice the low frequency, means for connecting said motor to said low frequency source for acceleration of said centrifuge, and means, including apparatus actuated when said motor has reached a speed which is at least one half of the running speed at said high frequency,

for switching said motor from said low frequency source to said high frequency source.

5. The method of operating a direct-connected, electrically-driven centrifugal machine which comprises accelerating said machine with alterhating current of a first frequency to a predetermined speed which is at least one half of the running speed of said machine at said first frequency and successively, continuously accelerating said machine to a higher speed with alternating current of a, higher frequency, said higher frequency being at least five percent greater than but less than twice the first frequency. a

6. The method of operating a direct-connected, electrically-driven machine which comprises continuously accelerating said machine, first for a predetermined time interval with alternating current of one frequency and then with alternating current of a higher frequency that is at least five percent greater than and less than twice the first frequency, said predetermined time interval being long enough to bring said machine up to a speed which is at least one half of the full speed of said machine at said first frequency.

7. In combination with a plurality of centrifugal machines each having an individual alternating current electric drive, a first system for supplying power currents of a certain frequency, a second system of comparatively low power capacity for supplying power currents of a higher frequency that is at least five percent greater than and less than twice said certain frequency, means for individually connecting said machines to said first system for preliminary acceleration to at least half of full speed, means for individu= ally connecting said machines to said second sys tem for further acceleration to and operation at full speed, and means for individually connecting said machines to said first system for regenerative braking.

8. The method of operating a direct-connected electrically driven centrifugal machine which comprises initially accelerating said machine with alternating driving current of a first fre quency and successively, continuously accelerating said machine with alternating driving cur-, rent of a second frequency which is at least five percent greater than and less than twice the first frequency, the synchronous speed approached during the initial acceleration being at least half the synchronous speed approached during the subsequent acceleration.

9. A method in accordance with claim 8 in which said initial acceleration is continued until a speed of the order of the first-mentioned synchronous speed is reached.

10. In combination with a sugar centrifuge or the like, an individual alternating current driving motor therefor, a main electric power system supplying current of a first fixed frequency, an auxiliary electric power system supplying cur- ,rent of a different fixed frequency which is at least five percent greater than and less than twice the first fixed frequency, the power capacity of said auxiliary system being small compared with that of said main system, means for connecting said motor to said main system for acceleration, switching means for transferring said motor to xii) asserts said a system for operation at full speed dependent on the frequency of said auxiliary system, and means for delaying the transfer to said auxiliary system until after said motor has accel= erated to at least half of said full speed, whereby the power required to accelerate to full speed is derived at least in large part from said main system and the comparatively small power required for full speed operation is derived from said auxiliary system.

ii. In combination with a direct-driven centrifugal machine, an alternating current driving motor therefor, a main electric power system supplying current of a first fixed frequency, an auxiliary electric power system of comparatively small power capacity supplying current of a different fixed frequency which is at least five percent greater than and less than twice the first fixed frequency, means for connecting said motor to said main power system for rapid acceleration toward a first synchronous speed, switching means for transferring said motor to said auxiliary power system for acceleration toward a higher synchronous speed that is less than twice said first synchronous speed, and means for delaying the transfer to said auxiliary system until after said machine has been accelerated to substantially more than half said higher synchronous speed.

12. In combination with a sugar centrifuge, a direct-connected individual alternating current driving motor therefor, a 21 electric power system supplying current of a first fixed frequency, an auxiliary electric power system supplying current of a higher fixed frequency that is at least five percent greater than and less than twice said first frequency, the power capacity of said auxiliary system being only a small fraction of that of said main power system, means connecting said motor to said main power system for acceleration toward a first synchronous speed, switching means operative to transfer said motor to said auxiliary power system for further acceleration toward a higher synchronous speed and operation at full speed, said synchronous speeds being proportioned to the frequencies of the currents supplied to the motor, and means for delaying the action of said switching means until after said motor has attained at least half of said full speed. I

13. In a system for the operation of a directdriven centrifuge, ,an electric driving motor for said centrifuge, a main electric power system and an auxiliary electric power system, said auxiliary system having a small fraction of the power capacity of said main system, means for accelcrating said motor with current supplied from said main power system to a speed not less than seven-tenths of full speed, and switching means for connecting said motor to said auxiliary systerm for operation at a full speed dependent on the frequency of the current being supplied.

14. In a system for the direct electric drive of a centrifuge, an alternating current driving motor, a main electric power system, an auxiliary electric power system the power capacity of which is small compared with that of said main system, and means for operating said motor through a cycle, comprising continuous acceleration to and operation at full speed, such that the electric energy required for acceleration is at least comparable in magnitude with the energy required for the remainder of the cycle, said operating means including means to connect said motor to said main power system for a first period of acceleration and switching means, automatieally operative after said motor has attained substantially more than half of full speed, to transfer said motor to said auxiliary system for a second period of acceleration and operation at full speed, the frequency of said auxiliary power system being at least five percent greater than and less than twice that of said main power system.

15. In a system comprising a battery of centrifugal machines with individual electric driving motors of a type such that the speed thereof tends to be proportional to the frequency of the driving currents supplied and two electric power supply sources of substantially different frequencies and widely diil'erent power capacities, the source of higher frequency having the smaller power capacity, the method of operating said machines which comprises accelerating each of said motors initially with current from the source of greater power capacity toward a first synchronous speed to an attained speed at least half the full speed reached during the operating cycle, successively accelerating said motor with current from the other source toward a second synchronous speed at least five percent greater than but not more than twice the first. to full speed, and continuing full speed operation with current from said other source, where- 'by full speed is determined by the frequency of said source of small power capacity and at least a large part of the energy required to accelerate to full speed is derived from said source of large power capacity.

16. In a system for the operation of sugar centrifuges with individual electric motor drives where two electric power sources of diiferent frequency are available and the higher frequency source is of relatively small power capacity compared with the source of lower frequency, the method of effecting rapid, continuous acceleration and full speed operation for a period com parable with the period of acceleration which comprises accelerating the motor with current from the power source of greater capacity to a speed substantially more than half of full speed, further and continuously accelerating the motor to full speed with current from the power source of lesser capacity, and maintaining full speed operation with current from the last-mentioned source.

17. In combination with a centrifuge, an alternating current'driving motor for said centrifuge, a source of low frequency power currents and a source of high frequency power currents, said high frequency being at least five percent greater than but less than twice the low frequency, means for connecting said motor to said low frequency source for acceleration of said centrifuge, and timing means initiated at the time of the connection of said low frequency source to said motor and operative a predetermined time thereafter to switch said motor from said low frequency source to said high frequency source to continue the acceleration thereof.

18. The combination of elements as in claim 17 in which said predetermined time interval is sufiiciently long to permit said motor to accelerate to a speed which is at least one half of the full speed of said motor when operated from said high frequency source.

CHARLES A. OLCO'I'I,