Centrifuge motor control

Information

  • Patent Grant
  • 6507161
  • Patent Number
    6,507,161
  • Date Filed
    Friday, March 30, 2001
    23 years ago
  • Date Issued
    Tuesday, January 14, 2003
    22 years ago
Abstract
The present invention overcomes the disadvantages of previously known motor controllers for centrifuge machines wherein a motor controller is provided for a centrifuge machine including a logic control module, one or more power cells, and one or more contactors. The logic control module is capable of interfacing with the main centrifuge controller and provides control over the power cells and contactors to provide a voltage ramp-up to accelerate the centrifuge basket. As such, the logic control module avoids the current draining problems associated with across the line starting of the centrifuge motor. The power cells receive a voltage from the main power supply, and output to the contactors variable power to control centrifuge motor speed. Further, the configuration of multiple contactors to reverse the power supplied to the centrifuge motor windings may eliminate the need for a second, reverse direction motor.
Description




BACKGROUND OF THE INVENTION




The present invention relates in general to heavy cyclical centrifugal machines and, more particularly, to an apparatus for controlling the speed and direction of a rotating centrifugal basket of the machine. While the present invention is generally applicable to heavy cyclical centrifugal machines, it will be described herein with reference to batch centrifugal machines used for manufacturing and refining sugar.




A centrifugal machine uses centrifugal force to separate substances, such as, for example a liquid component (the filtrate) from a solid component (the cake), in a slurry which has been introduced to the centrifugal machine. A filtering perforate wall traps the cake by a filter, whereas the filtrate passes through the filter.




A problem encountered when operating heavy cyclical centrifugal machines of the type used to manufacture and refine sugar is the inaccurate control of the speed of rotation of centrifugal baskets of the machines. These baskets should be fully loaded to their maximum capacities to maximize the productivity of the machines. Unfortunately, should the rotation of the centrifugal basket inadequately dispel the filtrate, the cake may be compromised. Variations in the loading properties of the charge material, massecuite for sugar manufacture and refining, can affect the efficiency of cycle to cycle centrifugal processing. Since these variations in loading properties are difficult or impossible to control, it has been an ongoing goal in the industry to control the motor operations of centrifugal machines such that the machines may be loaded with maximum charge in spite of the charge material variations.




The operational speeds of a heavy cyclical centrifugal machines are known to be established through the use of 2-speed motors, which utilize a dual set of internal windings such that the motor may operate at either a low or a high speed. However, a portion of a typical centrifugal machine cycle may require the rotational speed of the basket to be maintained at some intermediate value on the low speed windings. One known method of accomplishing this task is to repeatedly open and close a set of electrical contacts that energize and de-energize the low speed windings. This causes wear on the electrical components and may require frequent maintenance.




Further, it is a practice to reverse the direction of rotation of the centrifugal machine basket while discharging the charge material from the centrifugal machine basket. This is typically implemented by mechanically braking the rotation of the centrifugal machine basket until the centrifugal machine basket is at rest. The main 2-speed motor is electrically disengaged, and a second motor is engaged to rotate the centrifugal machine basket in the reverse direction. Upon completion of the discharge phase of the centrifugal machine cycle, the second motor disengages and the 2-speed motor re-engages to start a new cycle. Thus, the cost of the centrifugal machine is increased, and the motor control circuitry is complicated by the need to switch between multiple motors during each cycle.




Additionally, peak power demands, which occur typically during accelerating the centrifugal basket, can cause considerable power drain. This is because engaging the 2-speed motor low or high speed windings amounts to “across-the-line” starting of the motor. This has the effect of huge current demand on the electrical transformer during motor acceleration. The power drawn during operation affects the refiners ability to process sugar cost efficiently.




Accordingly, there is a need for an improved motor control for a centrifugal machine that eliminates the need for a second motor for operating the basket is a reverse direction, and reduces the peak power drawn by the centrifugal machine.




SUMMARY OF THE INVENTION




The present invention overcomes the disadvantages of previously known motor controllers for centrifuge machines wherein a motor controller is provided for a centrifuge machine including a logic control module, one or more power cells, and one or more contactors. The logic control module is capable of interfacing with the main centrifuge controller and provides control over the power cells and contactors to provide a voltage ramp-up to accelerate the centrifuge basket. As such, the logic control module avoids the current draining problems associated with across the line starting of the centrifuge motor. The power cells receive a voltage from the main power supply, and output to the contactors variable power to control centrifuge motor speed. Further, the configuration of multiple contactors to reverse the power supplied to the centrifuge motor windings may eliminate the need for a second, reverse direction motor.




In accordance with one embodiment of the present invention, a motor controller for a centrifuge machine comprises a first power cell having an input coupled to a main power supply, and an output. The first power cell is switchable between an on state where power is supplied to the output, and an off state where no power is supplied to the output. The motor controller also comprises a first contactor connected between the output of the first power cell and first windings of a motor. The first contactor is switchable between a first state, wherein an electrical connection is made between the first power cell and the motor, and a second state wherein an electrical connection is broken between the first power cell and the motor. Additionally, the motor controller comprises a logic control module coupled to the first power cell and the first contactor. The logic control module is arranged to interface with the controls of the centrifuge machine to selectively apply and vary power to the motor. Power is supplied to the motor when the logic control module switches the first contactor to the first state to establish an electrical connection between the first power cell and the motor. The logic control module further communicates with the first power cell to vary the power output by the first power cell, and accordingly adjusts the power to the motor thereby controlling the rotation of the centrifuge. For example, where the first power cell is implemented as a pair of silicon controlled rectifiers (SCRs), the logic control module controls the amount of power the first power cell supplies to the motor by varying the rate at which the logic control module turns the first power on and off.




When used with certain heavy duty cylindrical centrifugal machines, three phase AC power may be required to power the motor. Under such circumstances, the motor controller further comprises second and third power cells. The power supply comprises a three phase power supply and each of the first, second and third power cells couple a respective phase of the three phase power supply to the first contactor.




Further, more elaborate motor control schemes may be realized by incorporating into the motor controller a second contactor connected between the first power cell and second windings of the motor. The second contactor is switchable between a first state, wherein an electrical connection is made between the first power cell and the motor, and a second state wherein an electrical connection is broken between the first power cell and the motor. The second contactor is coupled to the logic control module. The logic control module is further arranged to control the first and second contactors for selectively supplying power to the first and second windings of the motor. For example, the motor may be a 2-speed motor having first windings, which are low speed windings connected to the first contactor. The second windings may be high speed windings connected to the second contactor. The logic control module is arranged to switch both the first and second contactors into their respective second states, thus the motor controller supplies no power to the motor. By maintaining the second contactor in the second state, and turning the first contactor to the first state, the power cell is coupled to the first (low speed) motor windings, and isolated from the high speed windings. The logic control module may control the speed of the motor by varying the power delivered to the low speed windings via the power cell. In contrast, where high speeds of centrifuge rotation are required, the logic control module switches the first contactor to the second state isolating the low speed windings from the power cell, and transitions the second contactor to the first state, thereby coupling the power cell to the high speed motor windings. The logic control module may optionally switch off the power cell prior to changing the state of either the first or second contactors to avoid switching the contactors while energized.




A third contactor may optionally be connected between the first power cell and the first windings of the motor, the third contactor is switchable between a first state, wherein an electrical connection is made between the first power cell and the motor, and a second state wherein an electrical connection is broken between the first power cell and the motor, the second contactor coupled to the logic control module. The third contactor is wired in parallel with the first contactor and arranged to supply power to the motor such that the motor rotates in a direction opposite of the direction the motor rotates when powered through the first contactor.




Further, the motor controller incorporates the first power cell to adjust the power delivered to the motor while accelerating the motor. The main power supply may supply power to the motor while the motor is rotating at full speed, or alternatively, the motor control may utilize the power cell to power the motor throughout the entire centrifuge cycle.




To more accurately control the motor, the motor controller may optionally include a speed determinative device connected to a first input of the logic control module. The speed determinative device may be a tachometer for example. When using a speed sensing device such as a tachometer, sophisticated programming of the motor controller may be realized. For example, a predetermined speed band may be programmed into the logic control module. During at least a portion of a cycle of operation, the motor speed may be adjusted so that the rotation of the centrifuge is maintained within the speed band. For example, during loading, it the rotation may be maintained at a speed suitable to centrifuge the material being processed.




Additionally, the motor controller may include a voltage suppression device arranged to prevent voltage spikes from reaching the first power cell. For example, a varistor may be used to absorb voltage spikes and transients. Likewise, a current sensing device such as a transformer may be connected to the logic control module to monitor current draw by the motor.




In accordance with another embodiment of the present invention, a centrifuge comprises a basket arranged to receive materials for processing. A motor interconnects to the basket to provide basket rotation in both a forward and reverse direction. A motor controller is coupled to the motor for providing control of the motor, including direction of rotation and rotational speed. The motor controller comprises at least one power cell coupled to a main power supply arranged to control a voltage applied to the motor. The voltage adjusts the rotational speed of the motor. A first contactor couples the power cell to the motor. The first contactor is switchable between a first state wherein an electrical connection is made between the power cell and the motor, and a second state wherein an electrical connection is broken between the at power cell and the motor. A logic control module is coupled to the power cell and the first contactor. The logic control module is arranged to selectively apply and vary power to the motor. For heavy duty cyclical centrifuges, the voltage is a three phase voltage. The motor controller further comprises three power cells, one power cell arranged to control an associated one phase of the three phase voltage.




The motor controller communicates with the power cell to produce a voltage ramp-up to accelerate the basket. The motor controller adjusts the speed of rotation of the basket by selectively turning on and off the power cell. To better adjust the speed of the basket, the motor controller may optionally include a speed determining device coupled to the logic control module. For example, the speed determining device may comprise a tachometer. The tachometer utilizes for example, a magnetic pickup positioned to sense the speed and direction of a toothed gear mounted on a shaft of the motor. The tachometer sends speed control data to a tachometer control unit, the tachometer control unit forwards the information to the logic control module.




The motor controller further comprises a second contactor coupling the power cell to the motor. The second contactor is switchable between a first state wherein an electrical connection is made between the at least one power cell and the motor, and a second state wherein an electrical connection is broken between the at least one power cell and the motor. The second contactor is arranged such that, when the voltage is applied to the motor through the second contactor, the rotation of the motor is opposite the rotation of the motor when the voltage is applied to the motor through the first contactor.




The motor controller may further include a third contactor coupling the power cell to the motor. The third contactor is switchable between a first state wherein an electrical connection is made between the at least one power cell and the motor, and a second state wherein an electrical connection is broken between the at least one power cell and the motor. The motor comprises high speed windings and low speed windings, the first contactor is connected to the low speed windings and the second contactor is connected to the high speed windings.




Additionally, the motor controller may include a voltage suppression device arranged to prevent voltage spikes from reaching the first power cell. For example, a varistor may be used to absorb voltage spikes and transients. Likewise, a current sensing device such as a transformer may be connected to the logic control module to monitor current draw by the motor.




According to yet another embodiment of the present invention, a motor controller for controlling a three phase, 2-speed AC motor comprises three power cells, each of the three power cells connected to a different one phase of a three phase power supply. A first contactor is connected between each of the three power cells and first windings of the 2-speed AC motor, and is arranged to bias the 2-speed AC motor to operate in a first direction. The first contactor is switchable between a first state wherein an electrical connection is made between the at three power cells and the 2-speed AC motor, and a second state wherein an electrical connection is broken between the three power cells and the 2-speed AC motor. A second contactor is connected between each of the three power cells and the first windings of the 2-speed AC motor, in parallel with the first contactor. The second contactor is switchable between a first state wherein an electrical connection is made between the at three power cells and the 2-speed AC motor, and a second state wherein an electrical connection is broken between the three power cells and the 2-speed AC motor. The second contactor is arranged to bias the 2-speed AC motor to operate in a second direction. A third contactor is connected between each of the three power cells and second windings of the 2-speed AC motor. The third contactor is switchable between a first state wherein an electrical connection is made between the at three power cells and the 2-speed AC motor, and a second state wherein an electrical connection is broken between the three power cells and the 2-speed AC motor. The third contactor is arranged to bias the 2-speed AC motor to operate in the first direction on the high speed windings. A logic control module is connected to the three power cells and the first, second and third contactors, arranged to control the amount of power the three power cells supply to the motor. A speed determining device is coupled to the logic control module, the speed determining device arranged to provide data concerning the rotational speed of the 2-speed AC motor to the logic control module.











BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS




The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals, and in which:





FIG. 1

is a partially sectioned, perspective schematic view of portions of a cyclic centrifugal machine to schematically illustrate apparatus operable in accordance with the present invention;





FIG. 2

is a graph illustrating a theoretical plot of revolution speed of a centrifugal machine basket versus time;





FIG. 3

is a block diagram of the centrifugal machine motor control according to the present invention; and,





FIG. 4

is a schematic illustration of the centrifugal machine motor control of

FIG. 3

; and,





FIG. 5

is a graph of a typical plot of revolution speed of a centrifugal machine basket versus time.











DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS




In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, specific preferred embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention.





FIG. 1

schematically illustrates several features of a heavy cyclical centrifugal machine


100


. A loading gate assembly


102


cooperates with a loading controller


104


to allow a slurry to enter the centrifugal machine


100


. The loading gate assembly


102


and loading controller


104


receive signals generated by an ultrasonic probe


106


or other means for linearly measuring a charge wall as it builds up in the centrifugal machine


100


. A variety of valve constructions can be used in the present invention as the loading or infeed gate including, for example, knife valves, butterfly valves, and other appropriate valves as will be apparent to those skilled in the art. Further, while the centrifugal machine


100


is shown with an ultrasonic probe


106


, other sensors may be used, including capacitive sensors or mechanical (feeler)-style cake sensors (not shown).




The centrifugal machine


100


includes a perforated cylindrical basket


108


carried on a spindle


110


that is suspended from a gyratory head (not shown) and is rotated in a conventional manner by a 2-speed motor


111


. For example, the 2-speed motor


111


may be an inverter duty AC motor suitable for use with three phase power sources. The 2-speed motor includes two sets of windings, a first set of windings for low velocity, for example, up to 600 r.p.m., and a second set of windings for high velocity, for example, up to 1200 r.p.m. The spindle


110


and basket


108


are driven at high centrifuging speeds for processing a load of charge material in the basket


108


and at lower speeds during other operating phases of cyclic machine operation, including loading and discharging phases.




Charge material is delivered into the basket


108


, from a storage or supply tank


112


through operation of the loading gate assembly


102


. For example, the charge material may be massecuite for sugar manufacture and refining. The loading gate assembly


102


is mounted at the mouth of a spout


114


extending from the tank


112


. The charge material flowing from the loading gate assembly


102


passes into the basket


108


through a central opening


116


in a top


118


of the basket


108


reaching the basket


108


through a central opening


120


in a top


122


of a cylindrical curb structure including an outer wall


124


which surrounds the basket


108


.




The operation of the above described centrifugal machine


100


will now be described by reference to

FIGS. 1 and 2

.

FIG. 1

illustrates components of the centrifugal machine


100


while

FIG. 2

illustrates timing of events and rotation of the cylindrical basket


108


for one complete cycle. Referring to

FIG. 2

, the centrifugal machine


100


is accelerated through time A, to a predetermined rotational velocity B, and the centrifugal machine


100


is held at velocity B while the cylindrical basket


108


is loaded. Referring back to

FIG. 1

, the charge material is made up of both cake and filtrate components and is delivered into the cylindrical basket


108


while the cylindrical basket


108


is rotating. The velocity B is a predetermined speed suitable for forming a charge wall


126


. The charge wall


126


is formed in a charge space S along an inner sidewall


128


of the cylindrical basket


108


by centrifugal force. As centrifugal force drives the mother liquor through the deposited cake, filter media and inner sidewall


128


of the perforated cylindrical basket


108


, a cake of charge material builds up on the filter media wall. (The filter media is not shown). The rotational velocity B of the loading process may be 40% to 60% of full speed.




The controller


104


receives input signals from an encoder


136


and from probe control circuitry within a probe control circuit housing


138


(alternately, the probe control circuitry can be housed within the controller


104


) of the ultrasonic probe


106


and also from operator settable controls


140


,


142


associated with the controller


104


. An operator of the centrifugal machine


100


can set an appropriate final thickness for the charge wall


126


to be loaded into the machine


100


by the settable control


140


.




In response to signals from the probe control circuitry within the housing


138


and the gate member position signal, the loading controller


104


controls the movable gate member


130


. The loading controller


104


may be embodied in a programmable logic control module (PLC) or in one of a large variety of commercially available microprocessors. Referring to

FIG. 2

, the loading process continues for a duration designated by reference to time period C.




Due to varying crystal sizes and different solid/liquid ratios from one batch of massecuite to the next, purge rates vary. Therefore, the amount of solids and the thickness of the charge wall or cake at process revolution speed will vary also. Because a portion of the cake is dissolved by the wash, the amount of wash time is set at an optimum level to perform the purge. Excessive wash time merely wastes product however. Accordingly, the controller


104


thus automatically adjusts for different amounts the cake settles during centrifugal machine


100


processing.




After the charge wall reaches a desired thickness, the centrifugal machine


100


is further accelerated to velocity D, over time period E. Velocity D may be full speed for the centrifugal machine


100


for example. At full speed, or velocity D, the cake is washed, and dried over time period F. It should be appreciated that the wash cycle may actually start prior to completing the acceleration of the centrifugal basket


108


to full speed, or velocity D. The retained solids are accelerated to spin drying speed (corresponding to duration F). After spin drying, the centrifugal machine


100


decelerates to discharge speed and the discharger removes the material from the centrifugal basket


108


. Alternatively, the material may removed by lifting the top of the centrifugal machine


100


an removing the product in a filter bag (Not shown). Referring to

FIG. 2

, the centrifugal machine


100


is decelerated during time period G to velocity H where the charge material is removed from the centrifugal machine


100


during time period


1


. It should be observed that the cycle times may vary from a few minutes up to one half of an hour or more. Further, the time periods required for the phases of loading, drying and discharging may vary. As such, the graph in

FIG. 2

is not necessarily drawn to scale in terms of either relative rotational velocity, or in terms of relative time periods between respective phases.




Referring back to

FIG. 1

, the loading controller


104


may be a computer, including a general purpose computer, or a specialized computer-type of processing unit. For example, a central processing unit (CPU), in conjunction with, or in lieu of a programmable logic control (PLC) may be used. Further, motor controller


144


communicates with the loading controller


104


and the 2-speed motor


111


to provide an intelligent system to control the operation of the 2-speed motor


111


. By intelligent system, it is meant that the motor controller


144


may be implemented by neural networks, logic, fuzzy logic, expert systems, statistical analysis, signal processing, pattern recognition, categorical analysis, any combination thereof, or any combination of known processing techniques.




As shown in

FIG. 3

, the motor controller


144


is comprised of a logic control module


146


, power cells


148


and contactors


150


. The logic control module


146


receives information from input/output (I/O) devices, and relies on internal processing to control the power cells


148


and the contactors


150


. The main power


152


passes through the power cells


148


, to the contactors


150


, and on to the 2-speed motor


111


. The power cells


148


condition the main power


152


as more fully explained herein, so that the 2-speed motor


111


can be efficiently controlled. Further, the contactors


150


act as switches to determine which of the windings the are energized by the power cells


148


.




As shown in

FIG. 4

, a motor controller


144


is schematically illustrated. The logic control module


146


monitors the voltage and current levels being supplied to the 2-speed motor


111


and directly controls the power cells


148


. Further, the logic control module


146


communicates with other controllers, such as the loading controller


104


, and further obtains information from I/O devices such as speed sensing device


160


as more fully explained herein. The logic control module


146


cooperates with the power cells


148


to produce a voltage ramp-up during acceleration that provides a smooth start and eases transients on the incoming power from the main power system


152


. It should be appreciated that, while illustrated in

FIG. 4

as a dedicated integrated circuit chip, the logic control module


146


may be implemented as a circuit of discrete components, a general purpose computer, or a specialized computer-type of processing unit.




The power cells


148


control the voltage being supplied to the 2-speed motor


111


during acceleration and deceleration operations, thus providing a ramping action. There are three power cells (PC


1


, PC


2


, and PC


3


) as illustrated in

FIG. 4

, one for each phase of the AC main power supply


152


. Each power cell PC


1


, PC


2


, and PC


3


consists of two silicon controlled rectifiers (SCR's). The SCR's (not shown) are solid state switches able to control large amounts of current flow and function to limit the amount of voltage or current being supplied to the 2-speed motor


111


by turning on and off in rapid succession. Six SCR devices connect in three sets of inverse parallel configuration to provide full wave voltage and current control for the 2-speed motor


111


. While illustrated in

FIG. 4

with three power cells


148


, it is to be understood that any number of power cells may be implemented. Additionally, devices and structures other than the use of SCR's may be realized. Further, while not shown in

FIG. 4

, it is to be understood that additional components such as heat sinks, cooling fans and the like may be required.




The contactors


150


route the output voltage of the power cells


148


to the low or high speed motor windings of the 2-speed motor


111


, and further serve to reverse the direction of the 2-speed motor


111


for discharge operations. The logic control module


146


ensures that the power cells


148


are off during actual contactor cycling. This prevents the contactors


150


from being opened or closed while energized and under load. As illustrated in

FIG. 4

, the contactors


150


include a forward contactor


150


-FOR, a reverse contactor


150


-REV, a first high speed winding contactor


150


-H


1


, and a second high speed winding contactor


150


-H


2


.




During initial acceleration of the centrifugal machine


100


, and during basket loading operations, the logic control module


146


turns on the forward contactor


150


-FOR. The logic control module


146


turns off the reverse contactor


150


-REV, as well as the high speed contactors


150


-H


1


and


150


-H


2


. As such, the forward contactor


150


-FOR couples the output of the power cells


148


to the low speed windings


111


-LOW of the 2-speed motor


111


. As illustrated in

FIG. 4

, when the 2-speed motor


111


is operating in the forward direction, the output of PC


1


is coupled to the low speed windings


111


-LOW of the 2-speed motor


111


along connection


154


. The output of PC


2


is coupled to the speed windings


111


-LOW of the 2-speed motor


111


along connection


156


, and the output of PC


3


is coupled to the low speed windings


111


-LOW of the 2-speed motor


111


along connection


158


. When the centrifugal machine


100


ramps up to full speed for the drying phase of the cycle, the logic control module


146


turns off the forward contactor


150


-FOR, and turns on the high speed forward contactors


150


-H


1


and


150


-H


2


. The low speed reverse contactor


150


-REV remains off during this phase of the cycle. The high speed forward contactor


150


-H


1


couples the output of the power cells


148


to the high speed windings


111


-H


1


of the 2-speed motor


111


. Both the low speed forward contactor


150


-FOR, and the low speed reverse contactor


150


-REV are off, creating an open circuit between the power cells


148


and the low speed windings


111


-LOW of the 2-speed motor


111


. The high speed contactor


150


-H


2


is turned on to tie together the low speed windings


111


-LOW of the 2-speed motor


111


. After the dry phase of the cycle, the centrifugal machine


100


is operated in a low speed, reverse direction phase of the cycle while the cake is discharged from the basket of the centrifugal machine


100


. During this operation, the high speed contactors


150


-H


1


and


150


-H


2


are turned off, the low speed forward contactor


150


-FOR remains off, and the low speed reverse contactor


150


-REV is turned on. This couples the power cells


148


to the low speed windings


111


-LOW of the 2-speed motor, and further biases the power supplied to the low speed windings


111


-LOW to operate the 2-speed motor in the reverse direction. As illustrated in

FIG. 4

, when the logic control module


146


operates the 2-speed motor


111


in the reverse direction, the output of PC


1


is coupled to the low speed windings


111


-LOW along connection


156


, and the output of PC


2


is coupled to the low speed windings


111


-LOW


111


along connection


154


, while the output of PC


3


continues to couple to the low speed windings


111


-LOW along connection


158


. While the operating direction of the 2-speed motor


111


as illustrated in

FIG. 4

, can be reversed by swapping the connections


154


and


156


on the low speed windings


111


-LOW, it will be appreciated that other, or additional modifications may be required depending upon the motor actually used.




The contactors


150


may be electrical or mechanical contactors. Electrically held contactors require a continuous application of voltage to the holding coil (not shown) that maintains contact closure. These units are frequently used in applications where a high number of operations may be run, the contacts will open whenever the coil voltage is released. Electrical contactors are known to be used in centrifugal machines


100


to vary the power delivered to a motor. The contactor is switched on and off in rapid succession to vary the power delivered to the motor. The repeated switching wears out the solenoid, causing maintenance and frequent repairs.




Mechanical contactors use a momentary application of voltage to close or open main contacts. Since the contacts are held closed mechanically, the AC hum associated with holding coils is eliminated. Because the motor controller


144


relies on the power cells


148


to adjust the power delivered to the 2-speed motor


111


, and not the contactors


150


of the present invention, the contactors


150


are not switched on and off in rapid succession, and as such, the contactors


150


may be mechanical or electrical.




To ensure that the centrifugal basket


108


(not shown in

FIG. 4

) rotates at the programmed rotational speed, the motor controller


144


further incorporates a speed sensing device. For example, a tachometer speed sensing device may be used. The tachometer includes a magnetic pickup (not shown) mounted to the 2-speed motor


111


. The magnetic pickup senses the speed and direction of a rotating portion of the motor, such as a toothed gear (not shown) mounted on the motor shaft, and sends a speed signal to the tachometer control unit


160


, which in turn provides various speed inputs to the logic control module


146


. While the tachometer circuit is described as using a toothed gear, it should be appreciated that other suitable devices may be used. Split and solid gears as well as tachometer tape may suitably be used to determine rotational velocity. An example of a suitable tachometer is the Tach Pak 3—digital process tachometer provided by Airpax Instruments of Cheshire Connecticut.




The motor controller


144


further includes voltage surge suppression


162


. For example, the voltage surge suppression may be implemented as a Metal Oxide Varistor. The voltage surge suppressor filters the voltage from the main power supply


152


that might otherwise damage to the motor controller


144


by clamping short duration, high voltage spikes.




Current monitoring devices are also utilized in the motor controller


144


to provide information to the logic control module


146


. For example, the current monitoring devices may be implemented as current transformers


164


,


166


. The current transformers


164


,


166


provide signals indicative of the current in the motor windings


111


-LOW and


111


-H


1


for input to the logic control module


146


.




The operation of the above-described motor controller


144


will now be described by reference to

FIGS. 4 and 5

.

FIG. 4

schematically illustrates components of the motor controller


144


, while

FIG. 5

illustrates timing of events and rotation of the cylindrical basket


108


for one complete cycle. Initially, the logic control module


146


sends a signal to the power cells


148


to produce voltage ramp-up to accelerate the 2-speed motor


111


, such that the acceleration provides a smooth start and eases transients on the incoming power system. The logic control module


146


monitors the speed of rotation of the 2-speed motor


111


until a predetermined speed is reached. Referring to

FIG. 5

, acceleration occurs over a period T


1


to a velocity of V


1


. For example, initially, over the course of about 5 seconds, the rotational velocity is increased from zero rpm to a relatively low loading speed of between about 200 rpm and about 300 rpm. Referring back to

FIG. 4

, if the 2-speed motor


111


is not adequately protected, the sudden change in rotation torque and speed that occurs on starting and stopping will jolt the equipment linked to it. Over the long-term this leads to increased mechanical wear. The logic control module


146


controls the voltage supplied to the 2-speed motor


111


during starting and stopping to ensure smooth acceleration and deceleration. The gradual supply of current to the 2-speed motor


111


also eliminates unwanted tripping, erratic current supply and motor overheating.




The loading controller


104


sends an input to the motor controller while the centrifugal machine


100


(not shown in

FIGS. 4 and 5

) is loaded. Referring to

FIG. 5

, loading occurs during time period T


2


. For example, once a loading speed of 250 r.p.m. to 300 r.p.m. is reached, a charge of material to be processed is loaded over the course of about 10 seconds. In operation, during the loading operation in time period T


2


, the velocity V


1


may be maintained or alternatively, the actual velocity may vary. For example, the 2-speed motor


111


may be allowed to coast, or alternatively, the 2-speed motor


111


may be maintained within a predetermined speed band V


1


-V


2


. Referring to

FIG. 4

, to maintain a low speed for loading, the logic control module


146


turns the power cells


148


on and off based on maintaining the centrifugal machine


100


speed within a pre-selected velocity, or alternatively, within a predetermined speed band (V


1


-V


2


as illustrated in FIG.


5


). Precise speed is maintained without repeated cycling of the contactors


150


because the power cells


150


provide the power conditioning. The logic control module


146


monitors the voltage and current levels being supplied to the two-speed motor


111


through communication with the power cells


148


, and the current monitoring devices


164


,


166


, and further obtains information from the speed sensing device


160


to determine suitable power to be supplied to the 2-speed motor


111


via the power cells


148


.




Referring to

FIG. 5

, upon completion of the loading phase, the rotational velocity is increased for a drying phase of cyclical operation. The velocity increases over time period T


3


to velocity V


3


, and over time period T


4


to velocity V


4


. Following loading, the centrifugal machine


100


motor is accelerated over the course of about 70 seconds, to a relatively high rotational speed of about 1200 rpm. Referring to

FIG. 4

, as the velocity increases, eventually, the maximum rated rotational velocity of the low speed windings


111


-LOW will be reached (illustrated in

FIG. 5

as velocity V


3


). At that point, the logic control module


146


turns off the power cells


148


, switches off the forward contactor


150


-FOR, and turns on the high speed contactors


150


-H


1


and


150


-H


2


. The power cells


148


are turned off to avoid switching the contactors


150


while energized. The power cells are turned back on to continue accelerating the 2-speed motor


111


with the high speed windings


111


-H


1


engaged. The logic control module


146


may control the 2-speed motor


111


when the 2-speed motor


111


is not operating at full speed. When operating at full speed, such as during the drying phase of a cycle, the 2-speed motor


111


is supplied directly from the main power supply


152


. Alternatively, the logic control module


146


may keep control of the 2-speed motor


111


at all times.




Referring to

FIG. 5

, after completion of the drying phase of the cycle, the velocity is decelerated over time periods T


5


and T


6


, until the rotational velocity is reversed. Operation is maintained at velocity V


5


during the discharge phase. V


5


is illustrated below the zero line to indicate that the rotational velocity is in the opposite direction as that used in the loading and drying phases. For example, following the drying phase, where the rotational velocity is around 1200 r.p.m., the rotational speed is decelerated and reversed over the course of about 20 seconds. The reverse drive of the motor is executed at a relatively low velocity, such as 50 r.p.m. It is contemplated by the present invention that the 2-speed motor


111


need not be reversed if an appropriate mechanical modification is made to the centrifugal basket


108


(not shown in

FIG. 4

) to allow for charge unloading in the forward direction. Following charge removal, the 2-speed motor


111


is accelerated to the loading speed and the process is repeated.




Referring to

FIG. 4

, the motor controller


144


has the benefit of more precise control on the 2-speed motor


111


, and thus the power demand on the user's electrical transformer. In prior centrifugal machines, engaging the low speed or high speed windings of the motor amounted to “across-the-line” starting of the motor. This has the effect of large current demand on the electrical transformer of the main power supply during motor acceleration. The motor controller


144


eases these peak electrical demands on the transformer by providing ramping action through the control of the power cells


148


.




Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. For example, the present invention is not limited to the specific rpm and timing ranges noted herein and it is contemplated that a variety of suitable rpm and timing values may be effective in the motor control scheme of the present invention.



Claims
  • 1. A centrifuge comprising:a basket arranged to receive materials; a motor interconnected to said basket; and, a motor controller coupled to said motor, said motor controller comprising: at least one power cell coupled to a main power supply and arranged to control a voltage applied to said motor, wherein said voltage adjusts the rotational speed of said motor, a first contactor coupling said at least one power cell to said motor, said first contactor switchable between a first state wherein an electrical connection is made between said at least one power cell and said motor, and a second state wherein an electrical connection is broken between said at least one power cell and said motor, and, a logic control module coupled to both said at least one power cell and said first contactor, said logic control module arranged to control said at least one power cell and said first contactor to selectively apply power to said motor during at least acceleration operations of said motor.
  • 2. A centrifuge according to claim 1, wherein said voltage is a three phase voltage, and said motor controller further comprises three power cells, one power cell arranged to control an associated one phase of said three phase voltage.
  • 3. A centrifuge according to claim 1, wherein said power cells comprise silicone controlled rectifiers.
  • 4. A centrifuge according to claim 1, wherein said motor controller communicates with said at least one power cell to produce a voltage ramp-up to accelerate said basket.
  • 5. A centrifuge according to claim 1, wherein said motor controller adjusts the speed of rotation of said basket by selectively turning on and off said at least one power cell.
  • 6. A centrifuge according to claim 1, further comprising a speed determining device coupled to said logic control module.
  • 7. A centrifuge according to claim 6, wherein said speed determining device comprises a tachometer.
  • 8. A centrifuge according to claim 7, wherein said tachometer further comprises a magnetic pickup positioned to sense the speed and direction of a toothed gear mounted on a shaft of said motor.
  • 9. A centrifuge according to claim 8, wherein said magnetic pickup is mounted on said motor.
  • 10. A centrifuge according to claim 7, wherein said tachometer sends speed control data to a tachometer control unit, said tachometer control unit sending information to said logic control module.
  • 11. A centrifuge according to claim 10, wherein said speed control data is transmitted as a pulsed speed signal.
  • 12. A centrifuge according to claim 1, wherein said motor controller controls the speed of rotation of said basket by selectively turning on and off said at least one power cell.
  • 13. A centrifuge according to claim 1, wherein said first contactor comprises a mechanical contactor.
  • 14. A centrifuge according to claim 1, wherein said motor controller further comprises a second contactor coupling said at least one power cell to said motor, said second contactor switchable between a first state wherein an electrical connection is made between said at least one power cell and said motor, and a second state wherein an electrical connection is broken between said at least one power cell and said motor, said second contactor arranged such that, when said voltage is applied to said motor through said second contactor, the rotation of said motor is opposite the rotation of said motor when said voltage is applied to said motor through said first contactor.
  • 15. A centrifuge according to claim 14, wherein said logic control module turns off said voltage at said at least one power cell prior to switching either said first or second contactors.
  • 16. A centrifuge according to claim 1, further comprising a second contactor coupling said at least one power cell to said motor, said second contactor switchable between a first state wherein an electrical connection is made between said at least one power cell and said motor, and a second state wherein an electrical connection is broken between said at least one power cell and said motor wherein said motor comprises high speed windings and low speed windings, said first contactor connected to said low speed windings and said second contactor connected to said high speed windings.
  • 17. A centrifuge according to claim 1, further comprising voltage suppression device.
  • 18. A centrifuge according to claim 17, wherein said voltage suppression device comprises at least one varistor.
  • 19. A centrifuge according to claim 17, wherein said voltage suppression device is arranged between said main power supply and said at least one power cells.
  • 20. A centrifuge according to claim 1, wherein said motor controller integrates with a programmable logic control module programmed to control the cycling of said centrifuge.
  • 21. A motor controller for a centrifuge machine comprising:a first power cell having an input coupled to a main power supply and an output, wherein said first power cell is switchable between an on state where power is supplied to said output, and an off state where no power is supplied to said output; a first contactor connected between the output of said first power cell and first windings of a motor, said first contactor switchable between a first state, wherein an electrical connection is made between said first power cell and said motor, and a second state wherein an electrical connection is broken between said first power cell and said motor; and, a logic control module coupled to both said first power cell and said first contactor, said logic control module arranged to interface with the controls of said centrifuge machine, said first contactor and said first power cell to selectively apply power to said motor at least during acceleration operations of said motor.
  • 22. A motor controller for a centrifuge machine according to claim 21, further comprising second and third power cells, wherein said power supply comprises a three phase power supply and each of said first, second and third power cells couple a respective phase of said three phase power supply to said first contactor.
  • 23. A motor controller for a centrifuge machine according to claim 21, wherein said first power cell comprises two silicone controlled rectifiers.
  • 24. A motor controller for a centrifuge machine according to claim 23, wherein said logic control module controls the amount of power said first power cell supplies to said motor by varying the rate at which said logic control module switches said first power cell between said on and off states.
  • 25. A motor controller for a centrifuge machine according to claim 21, further comprising:a second contactor connected between said first power cell and second windings of said motor, said second contactor switchable between a first state, wherein an electrical connection is made between said first power cell and said motor, and a second state wherein an electrical connection is broken between said first power cell and said motor, said second contactor further coupled to said logic control module, wherein said logic control module is further arranged to control said first and second contactors to selectively supply power to a select one of said first and second windings of said motor.
  • 26. A motor controller for a centrifuge machine according to claim 25, wherein said first windings are low speed windings, said second windings are high speed windings.
  • 27. A motor controller for a centrifuge machine according to claim 25, wherein said logic control module switches said first power cell to said off state prior to switching either said first or second contactors.
  • 28. A motor controller for a centrifuge machine according to claim 21, further comprising:a second contactor connected between said first power cell and said first windings of said motor, said second contactor switchable between a first state, wherein an electrical connection is made between said first power cell and said motor, and a second state wherein an electrical connection is broken between said first power cell and said motor, said second contactor further coupled to said logic control module, wherein said second contactor is arranged to supply power to said motor such that said motor rotates in a direction opposite of the direction said motor rotates when powered through said first contactor.
  • 29. A motor controller for a centrifuge machine according to claim 21, wherein said logic control module supplies power through said first power cell to said motor during acceleration operations of said motor, and said main power supply does not supply power to said motor through said first power cell during full speed operations of said motor.
  • 30. A motor controller for a centrifuge machine according to claim 21, wherein said logic control module supplies power to said motor while said motor is rotating at a speed less than the maximum rated speed of said motor.
  • 31. A motor controller for a centrifuge machine according to claim 21, further comprising a speed determinative device connected to a first input of said logic control module.
  • 32. A motor controller for a centrifuge machine according to claim 31, wherein said speed determinative device comprises a tachometer.
  • 33. A motor controller for a centrifuge machine according to claim 31, wherein said logic control module is programmable to maintain the rotation speed of said motor to within a speed band range during at least a portion of a load cycle.
  • 34. A motor controller for a centrifuge machine according to claim 21, wherein said first contactor comprises a mechanical contactor.
  • 35. A motor controller for a centrifuge machine according to claim 21, further comprising a voltage suppression device arranged to prevent voltage spikes from reaching said first power cell.
  • 36. A motor controller for a centrifuge machine according to claim 35, wherein said voltage suppression device comprises a varistor.
  • 37. A motor controller for a centrifuge machine according to claim 21, further comprising a current sensing device connected to said logic control module.
  • 38. A motor controller for controlling a three phase, 2-speed AC motor, comprising:three power cells, each of said three power cells connected to a different one phase of a three phase power supply; a first contactor connected between each of said three power cells and first windings of said 2-speed AC motor, said first contactor switchable between a first state wherein an electrical connection is made between said at three power cells and said 2-speed AC motor, and a second state wherein an electrical connection is broken between said three power cells and said 2-speed AC motor, said first contactor arranged to bias said 2-speed AC motor to operate in a first direction; a second contactor connected between each of said three power cells and said first windings of said 2-speed AC motor, said second contactor switchable between a first state wherein an electrical connection is made between said at three power cells and said 2-speed AC motor, and a second state wherein an electrical connection is broken between said three power cells and said 2-speed AC motor, said second contactor arranged to bias said 2-speed AC motor to operate in a second direction; a third contactor connected between each of said three power cells and second windings of said 2-speed AC motor, said third contactor switchable between a first state wherein an electrical connection is made between said at three power cells and said 2-speed AC motor, and a second state wherein an electrical connection is broken between said three power cells and said 2-speed AC motor, said third contactor arranged to bias said 2-speed AC motor to operate in said first direction; a logic control module connected to said three power cells and said first, second and third contactors, said logic control module arranged to control said three power cells to control the amount of power said three power cells supply to said motor, said logic control module further arranged to control said contactors to selectively control the speed and direction of said motor; and, a speed determining device coupled to said logic control module, said speed determining device arranged to provide data concerning the rotational speed of said 2-speed AC motor to said logic control module.
  • 39. A motor controller for controlling a three phase, 2-speed AC motor according to claim 38, further comprising a fourth contactor in series between said first and second contactors, and said motor, said fourth contactor coupled to said logic controller and arranged to tie said first windings of said motor together when said third contactor is in a first state and power is supplied to said second windings through said third contactor.
CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/197,240 filed Apr. 14, 2000 that is incorporated herein by reference.

US Referenced Citations (25)
Number Name Date Kind
2427101 Kochli Sep 1947 A
2551838 Huser et al. May 1951 A
2752044 Olcott Jun 1956 A
3970245 Aeschlimann Jul 1976 A
4132349 Khoja et al. Jan 1979 A
4224558 Hays Sep 1980 A
4450391 Hara May 1984 A
4464161 Uchida et al. Aug 1984 A
4482853 Bhavsar Nov 1984 A
4700117 Giebeler et al. Oct 1987 A
4752283 Copeland et al. Jun 1988 A
4903191 Fries Feb 1990 A
4994725 Gschlossl Feb 1991 A
5203762 Cooperstein Apr 1993 A
5254241 Bange et al. Oct 1993 A
5485066 Zeigler Jan 1996 A
5608301 Inaniwa et al. Mar 1997 A
5726550 Inaniwa et al. Mar 1998 A
5731681 Inaniwa et al. Mar 1998 A
5857955 Phillips et al. Jan 1999 A
5897786 Henkel et al. Apr 1999 A
5905348 Nolan May 1999 A
5919123 Phillips Jul 1999 A
6063292 Leung May 2000 A
6213929 May Apr 2001 B1
Foreign Referenced Citations (5)
Number Date Country
SU 921008 Apr 1982 DE
2133233 Jul 1984 DE
1055971 Jan 1967 GB
2133233 Jul 1984 GB
WO 8700770 Feb 1987 WO
Non-Patent Literature Citations (1)
Entry
Baldor Motors and Drives; Multipurpose Soft Starter Sizes 8 to 840 AMP; Instillation & Operating Manual; 9/99.
Provisional Applications (1)
Number Date Country
60/197240 Apr 2000 US