Constant Torque Characteristics

Most extruders have a so-called constant torque characteristic. This means that the maximum torque obtainable from the drive, for all practical purposes, remains constant over the range of screw speed. The torque-speed characteristic can be used to determine the power-speed characteristic by using the well-known relationship between torque and power  

if torque is constant with speed, then it follows directly from Eq. 3.1 that the power is directly proportional to speed see Fig. 3.8. 

This means that the maximum power of the drive can only be utilized if the motor is running at full speed. Whenever the extruder output is power-limited, it is good practice to make sure the motor is running at full speed. If it is not, then a simple gear change can often alleviate the problem. 

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Beyond /V. the h.p can be kept constant by keeping the voltage fixed and raising/(h.p. T.N). The speed-torque characteristic is similar to a d.c. machine as shown in Figure 6.51... 

The synchronous motor is a constant-speed machine. Unlike the induction motor which inherendy has slip from losses, the synchronous motor uses an excitation system to continually keep the rotor in synchronous speed with current flowing through the stator. Within its designed torque characteristics, it will operate at synchronous speed regardless of load variations. 

The operating speed characteristics of the process equipment dictate the type of motor and control to be applied. Most drives operate at a speed lower than that of the motor, thus requiring some form of speed reduction. The gearing may be via direct-cpnnected coupled motor or a speed reducer may be used. Variable or adjustable speed performance must be definitely established as to speed range, degree of speed adjustment, and load requirements at all speeds. Constant-torque or constant-horsepower drives both require variable-speed or multispeed motors with suitable control equipment. 

The ratio of voltage to frequency must be held constant to maintain a constant torque capability as the motor speed is varied. Almost any speed/torque characteristic can be obtained by varying the voltage to frequency ratio. Because of limitations of the SCR cells, the maximum rating of adjustable frequency drives is presently around 300 hp. As better SCR cells are developed, this maximum rating is likely to increase. 

The Cyclo gearbox torque characteristics are different from those of the epicyclic. Its torque capability varies with differential speed while the maximum torque transmission of the epicyclic remains essentially constant for all differentials used. According to the manufacturer s performance data, the torque capacity of the Cyclo gearbox approximately halves when differential is increased from minimum to maximum. 

Such a control is good for machines that are required to operate at low speeds with a high accuracy. Now the phasor /, in terms of /, , is varied according to the speed required. Figure 6.2 now changes to Figure 6.8, which is a marked improvement on the earlier characteristics. The torque variation with speed is now almost constant, except at very low speeds. The reason for poor torque at low speeds is the method of speed variation which is. still based on Vlf. Now a motor s mathematical model is used... 

The phasor /, and /, are separated and then controlled separately as discussed later. For more precise speed control a pulse encoder feedback device can also be employed. The characteristics now improve to Figure 6.10. The torque can now be maintained constant at any speed, even at zero speed. 

Figure 16-16 shows the performance characteristic of a split-shaft turbine where the only power output limitation is the maximum allowable temperature at the inlet of the turbine section. In actual practice a torque limit, increased exhaust temperature, loss of turbine efficiency, aud/or a lubrication problem on the driven equipment usually preclude operating at very low power turbine speeds. The useful characteristic of the split-shaft engine is its ability to supply a more or less constant horsepower output over a wide range of power turbine speeds. The air compressor essentially sets a power level and the output shaft attains a speed to pnivide the required torque balance. Compressors, pumps, and various mechanical tinvc systems make very good applications for split-shaft designs. 

A turbine device has the unique characteristic that it will allow circulation independent of what torque or horsepower the motor is producing. In the example where the turbine motor has a 10-lb/gal mud circulating at 400 gal/ min, the pressure drop through the motor is about 1,324 psi. This pressure drop is approximately constant through the entire speed range of the motor. 

Synchronous motors are made in speeds from 1800 (two-pole) to 150 rpm (48-pole). They operate at constant speed without slip, an important characteristic in some applications. Their efficiencies are 1-2.5% higher than that of induction motors, the higher value at the lower speeds. They are the obvious choice to drive large low speed reciprocating compressors requiring speeds below 600 rpm. They are not suitable when severe fluctuations in torque are encountered. Direct current excitation must be provided, and the costs of control equipment are higher than for the induction types. Consequently, synchronous motors are not used under 50 HP or so. 

Control systems are used to monitor and regulate temperature, pressure, flow rates, stirring speeds, and compositional characteristics in a pressure reactor. Commercially available process controllers can sense and display temperatures and regulate heaters for temperature control. Controllers may also be specified to sense pressure and operate control valves for pressure control, adding reagents or relieving contents. Motor controllers are available for both AC and DC motors. They can vary from those with simple manual adjustment of speed settings to closed loop controllers that will maintain constant revolutions per minute, torque, or power to the motor. 

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