Speed control constant torque

The inverter drive system that uses a current-controlled rectifier and parallel-capacitor commutation operates to both improve reliability and reduce cost. Such systems are built commercially for the ranges from 20 to 500 hp for the typical 20 1 constant-torque speed range. 

Note The speed of the motor ean be varied by vai-ying the frequeney alone but this docs not provide satisfactory performance, A variation in frequeney causes an inverse variation in the flux, for the same system voltage. The strength of magnetic field, p, develops, the torque and moves the rotor, but at lower speeds. / would be reduced, which would raise 0 , and lead the magnetic circuit to saturation. For higher s )ccds, / would be r.nised, but that would reduee which would adversely diminish the torque. Hence frequency variation alone is not recommended practice for speed control. The recommended practiee is to keep V/fas constant, to maintain the motor s vital operatin.c parameters, i.e. its torque and 0 ,. within acceptable limits. 

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. 

Moderate to accurate, depending upon the accuracy of controls. Stepless up to 20% of V, at constant h.p. and up to 33% of N, at constant torque is possible. Pumps, ID fans etc., that call for speed variation during a process need may not necessarily be too accurate. Or variation in flow of fluid, gas or temperature etc. not calling for very accurate controls, that such drives find their extensive use. It may be made more accurate, but at higher cost of controls... 

PrImary-Voltage-Control-AC Motor Driver. Induction motor torque at any slip s is proportional to primary V. Rotor-power dissipation is equal to s times the air-gap power. These two relationships define the boundary of operation of an induction motor with primary voltage control of speed. As the speed is reduced (s increased) at constant torque, the air-gap power remains fixed, but the power divides between rotor circuit dissipation and mechanical shaft power. 

Fluid motors may be either fixed or variable displacement. Fixed-displacement motors provide constant torque and variable speed. Controlling the amount of input flow varies the speed. Variable-displacement motors are constructed so that the working relationship of the internal parts can be varied to change displacement. The majority of the motors used in fluid power systems are the fixed-displacement type. 

Rotor speed control is an integral part of the process of rate of temperature rise. The modem mixer is generally supplied with variable speed, constant torque electric motors driving via gear reducers or by a hydraulic drive system which has a motor shaft mounted tachometer to control rotor speed. Systems that can monitor, adjust and record the mixer rotor speed provide improvement in resultant compound uniformity. Changes in rotor speed within a cycle can be programmed. 

Direct current motors are used for continuous operation at constant load when fine speed adjustment and high starting torque are needed. A wide range of speed control is possible. They have some process applications with centrifugal and plunger pumps, conveyors, hoists, etc. 

In rotational instruments, one member (e.g., the cup in a concentric cylinder viscometer) rotates while the other (e.g., the bob) remains stationary. The sample is held, and sheared, in the gap between the two. In a controlled shear rate measurement, the rotational speed is constant, and the torque on one member caused by the viscous resistance to flow exerted by the sample is measured. In a controlled stress measurement, a constant torque is applied to one member and its speed of rotation measured. Controlled stress instruments are particularly useful for measuring yield stress, the minimum stress causing flow of a plastic material. 

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. 

It may be seen that if this method is used to control the speed of a standard, almost constant-speed type of induction motor, then the actual range of speed control obtained will, in fact, be small before stalling occurs. The situation could be improved by using a motor with a high rotor resistance as shown in Figure 14.2. The rotor resistance at full-load has been increased by a factor of 10 in order to demonstrate the effect on the torque-speed characteristic. 

The controller is of the variable voltage, variable frequency type, thereby providing a complete speed range for the motor at constant torque. The system inherently provides soft start which is necessary to alleviate high torsional stresses within the motor-pump uniL that may otherwise damage the shaft and couplings. 

On delivery the speed control system adjusted the rotational speed of the drum at rope layer changes to keep the cage speed constant throughout the full speed zone. Motor torque... 

To facilitate model verification, experiments have been performed under similar conditions. The milling equipment (Thomas Wiley Mill, Thomas Scientific, Swedesboro, NJ) used in this study is a variable speed, digitally controlled, direct drive mill that provides continuous variation of cutting speeds from 600 to lldOipm with constant torque maintained throughout the speed range. Parametric studies were conducted with lactose granules to study the effect of impeller wall tolerance and feed rate on particle size reduction. 

Extruder drive motors must turn the screw, minimize the variation in screw speed, permit variable speed control (typically 50 to 150 r/min), and maintain constant torque. In selecting drive motors, the three major factors are (1) base speed variation, (2) the presence or absence of brushes, and (3) cost. The speed variation of a drive motor is based on the maximum speed available for the motor. Since this variation does not change when the speed is reduced, screw speed. 

The three basic types of drives are alternating current (ac), direct current (dc), and hydraulic. While a number of drives have been used in extruders, the most common are dc silicon control rectified (SCR) and ac adjustable frequency drives. A dc SCR drive is a sohd-state dc rectifier connected to a dc motor. The base speed is about 1 percent, but reduces to 0.1 percent when a tachometer is added to the drive. These drives are very reliable, can handle high starting torques, can maintain a constant torque through a speed range of 20 1, and are relatively easy to maintain (that is, replace brushes). However, since the drives have brushes, they are limited to noncorrosive polymers. 

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