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Introduction
Prior to the advent of adjustable speed electric drives, the operational speeds of a Western States centrifuge were accomplished through the use of 2-speed motors. These highly specialized motors utilized a dual set of internal windings such that the motor could operate at either a low or a high speed. Since the typical centrifuge cycle involved holding speed at some intermediate value on the low speed windings, the only means of accomplishing this task was to repeatedly open and close a set of electrical contacts which energized and de-energized the low speed windings.
In addition to operating the centrifuge in the forward direction, Western, States developed a system for reverse direction discharging the centrifuge basket. The primary purpose for reverse direction discharging was one of operator safety. This however induced additional complexities in controlling the 2-speed motor.
It was felt at the time of initial design that the 2-speed motor would not have the sufficient torque requirements to discharge a centrifuge basket at the low RPM needed for discharging. This situation necessitated the development of the Western States "Turn-Tork" centrifuge control. This device mounts on top of the 2-speed motor. When the centrifuge is mechanically braked from approximately 600 RPM to zero RPM, the main 2-speed motor is electrically disengaged. At zero RPM the "Turn-Tork" device is engaged to rotate the centrifuge basket in the reverse direction. This device utilizes a small electric motor and appropriate gear reduction to provide low basket speed with high shaft torque. Upon completion of the discharge portion of the centrifuge cycle, the "Turn-Tork" disengages and the 2-speed motor will reengage to power the centrifuge to loading speed.
Summary
The motor controller panel utilizes a control scheme and a series of SCR's and contactors to replace the original electrical contactors provided with the centrifuge. This new method of 2-speed motor centrifuge control provides the end user with several significant benefits in both operation as well as maintenance. During normal operation of the centrifuge, there is significant contactor wear during the energizing/de-energizing loading phase of the process cycle, resulting in machine down time and high maintenance costs associated with contactor replacement.
Another benefit is a significant cost savings realized by the elimination of the "Turn-Tork" unit on top of the 2-speed motor. The entire discharging portion of the centrifuge cycle is now performed by the 2-speed motor via the motor controller panel. There is an additional safety benefit by the elimination of the "Turn-Tork" unit. On older units where maintenance has been neglected, the clutch mechanism of the "Turn-Tork" could engage at high speed thereby causing catastrophic failure of the device.
This motor controller panel also has the benefit of more precise control on the centrifuge motor power demand on the end user's electrical transformer. Under the old system, engaging the 2-speed motor low or high speed windings amounted to "across-the- line" starting of the motor. This has the effect of huge current demand on the electrical transformer during motor acceleration. The new motor controller panel can be configured to ease these peak electrical demands on the transformer. The addition of a tachometer speed signal provides more precise control of the centrifuge cycle by controlling brakes, valve lifter, etc. based on actual RPM vice being timer controlled. This results in increased efficiency of operation and throughput.
Detailed Description
Western States drawing number 064-1311-00-4 details the physical layout of the panel along with the power and control wiring and the integration into the existing 2-speed motor system. The major components are the PLC, Power Cells, Logic Control Module, and mechanical contactors. Additional components include a tachometer speed sensing device, voltage suppression components, and current transformers for current sensing.
The PLC provides overall system control. The PLC program incorporates the tachometer speed signal and controls the Logic Control Module and various centrifuge control and process functions. The Power Cells are the heart of the system. The Power Cells control the voltage being supplied to the 2-speed motor during acceleration and deceleration providing a ramping action. There are 3 Power Cells, one for each phase. A Power Cell consists of 2 Silicon Controlled Rectifiers (SCR's), which are solid state switches able to control large amounts of current flow. The SCR's limit the amount of voltage or current being supplied to the 2-speed motor by turning on and off in rapid succession.
The Logic Control Module is the brain of the system. The Logic Control Module monitors the voltage and current levels being supplied to the 2-speed motor and directly controls the Power Modules. The Logic Control Module produces a voltage ramp-up during acceleration that provides a smooth start and eases transients on the incoming power system. To maintain a low speed for centrifuge loading or discharging, the Logic Control Module will turn the Power Cells on and off based on a tachometer speed signal, maintaining the centrifuge speed within a pre-selected speed band. Precise speed is maintained without repeated cycling of the low speed contactor. The new system will continue to use mechanical contactors to route the output voltage of the Power Cells to the low or high speed motor windings and to reverse the low speed motor windings for normal reverse plowing discharge operations. Optional forward plowing discharge operation is accomplished by a simple change to the PLC programming. It is important to note that the Logic Control Module ensures that the Power Cells are off during actual contactor cycling. This prevents the contactors from being opened or closed while energized and under load as in the old system.
The motor controller panel also includes voltage surge suppression in the form of a Metal Oxide Varistor (MOV). The MOV prevents damage to the Logic Control Module by clamping short duration, high voltage spikes.
Tachometer speed sensing is accomplished by a magnetic pickup mounted in the original "Turn-Tork" base assembly on top of the 2-speed motor. The magnetic pickup senses the speed and direction of a toothed gear mounted on the 2-speed motor shaft and sends a pulsed speed signal to a signal conditioning module, which in turn provides a 4-20ma signal corresponding to various motor speeds to the control system. The magnetic pickup also drives an RPM readout meter located near the existing pushbutton station.
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