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:
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. 
    
    
     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&#39;s). The SCR&#39;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&#39;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&#39;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.