Fixed-speed drives

Two of the machines will operate full-time the third will serve as a standby. For such a situation, the suction header would be sized for the suction capacity of two machines. In practice, it may be better to run all three machines at reduced speed. Then when one comes down for some reason, the other two can speed up to carry the full load. Of course, this type of operation cannot be accomplished with a fixed speed drive. For the situation described, the design load is still the full capacity of two machines or four cylinders. 

Speed The speed of the impeller can affect the particle size of the product. Conical screening mills available have variable- or fixed-speed drives, however, the number of revolutions per minute vary depending on the size of the impeller. It is suggested that one keep the tip speed of the impeller the same on scale-up to achieve the same particle size distribution. 

Speed Devices. Many displacement pumps are connected by variable speed drives. When these pumps are used as a time device on a homogenizer, the setting is fixed, ie, the maximum speed is limited in order to meet the requirements of pasteurization. 

Most induction ac motors are fixed-speed. However, a large number of motor applications would benefit if the motor speed could be adjusted to match process requirements. Motor speed controls are the devices which, when properly applied, can tap most of the potential energy savings in motor systems. Motor speed controls are particularly attractive in applications where there is variable fluid flow. In many centrifugal pump, fan, and compressor applications mechanical power grows roughly with the cube of the fluid flow. To move 80 percent of the nominal flow only half of the power is required. Centrifugal loads are therefore excellent candidates for motor speed control. Other loads that may benefit from the use of motor speed controls include conveyers, traction drives, winders, machine tools and robotics. 

Fixed two-speed drive (usually for day and night operation). 

Figure 12-5F. Lubricated and nonlubricated balanced opposed process reciprocating compressors, designed to API 618 code. Fixed- and variable-speed drives using gas or diesel engines, steam or gas turbines, or electric motor. Note power drive to connect to right side of cross-head box in center. (Used by permission Bui. PROM 635/115/95-11. Nuovo Pignone S. P. A., Florence, Italy New York Los Angeles and Houston, Texas. All rights reserved.)...

A steam turbine is a machine with an ancient genealogy. It is a direct descendant of the overshot water wheel, used to kick off the industrial revolution in England, and the windmill still used in Portugal. Turbines are widely used in process plants to drive everything from 2-hp pumps to 20,000-hp centrifugal compressors. They are versatile machines, in that they are intrinsically variable-speed devices. Electric motors are intrinsically fixed-speed machines. It is true that there are a variety of ways to convert AC (alternating-current) motors to variable speed, but they are all expensive and complex. 

I hope you all understand the mistake Jane made. She trusted John. But if she had it all to do over again, what could Jane have done differently She has to drive the compressor by a fixed-speed motor. The gas molecular weight is going to be unpredictable. The pressure in the wet-gas drum has to be kept constant. 

In a third mode, the carriers are attached to the conveyor drive chain at fixed intervals and are conveyed through the labyrinth and between source plaques at a fixed speed. This mode is the most time-con-... 

Feed tank and metering pump the flow rate through such a pump can be controlled by a stroke adjusting mechanism or a variable speed drive acting on the stoke frequency. Control can be achieved by a fixed adjustment or through a flow meter. 

There are extruders that are utilized for just one process or a specific task. In this case, it suffices to define exactly one speed for their drive. A fixed-speed motor is used and operated directly from the mains. In such cases, a manual switching stage is provided on the gear box so that it can run at a second constant speed. The screw speed typically ranges between 200 and 300 rpm. 

It is possible to equip large extruders with fully variable drives. In this case, speed variability is often not the main intention, but rather the fact that the existing supply network does not permit a fixed-speed motor to be started, because other users on the same network would be starved by the drop in voltage. In such cases, a frequency converter offers the advantage that much less current is required for start-up. Starting currents can of course also be reduced by a starting transformer or a softstarter. 

These machines are widely used in fertilizer granulation and iron ore balling and consist of an inclined rotary cylinder powered by a fixed- or variable-speed drive. Agglomeration occurs when the feed contains an optimal amount of liquid phase and is subjected to the rolling, tumbling action of the... 

Speed/drives Variable or fixed speed Variable speed capability is essential. Consider over design to meet unpredicted performance... 

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