Variable speed drives

Pig. 6. Lurgi pressure gasifier A = variable speed drive B = coal distributor C = rotating grate. HP = high pressure. 

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. 

If the allowance for control can be reduced, it should be. One option is the use of variable-speed drives. This eliminates the control valve and its pressure drop and piping. Its best appHcation is where a large share of the head is required for friction and where process demands cause the required flow to vary. 

Other energy considerations for cooling towers include the use of two-speed or variable-speed drives on cooling-tower fans, and proper cooling-water chemistry to prevent fouling in users (see Water, industrial water treatment). Air coolers can be a cost-effective alternative to cooling towers at 50—90°C, just below the level where heat recovery is economical. 

A variable-speed drive is usually used on the feed and cross-belt drives to exercise control in separator operation, although the speed is not usually changed once the optimum operating condition is estabUshed. Feed rates and the selection of the number of magnetic poles are usually deterrnined by preliminary laboratory tests. The mineral types involved in the feed largely determine the number of poles selected. High intensity cross-belt separators are frequendy used in combination with induced-roU or electrostatic separators. 

V ri ble Frecjuency Drives. An important energy by-product of soHd-state electronics is the relatively low cost variable speed drive. These electronic devices adjust the frequency of current to control motor speed such that a pump can be controlled direcdy to deUver the right flow without the need for a control valve and its inherent pressure drop. Eigure 11 shows that at rated load the variable speed drive uses only about 70% as much power as a standard throttle control valve system, and at half load, it uses only about 25% as much power. 

Eig. 11. Power saving for variable speed drives. Power input for variable speed adjusts with flow to naturally match the frictional losses. FIC = flow... 

In addition to energy conservation, the variable speed drives offer better control because of a faster response, ie, reduced dead band. They are also sometimes chosen for safety reasons because of elimination of the control station and accompanying valving. The capital saved by use of a smaller motor and elimination of the control valve partially compensates for the cost of the drive. 

Most drum fUters operate at a rotation speed in the range of 0.1 to 10 r/min. Variable-speed drives are usuahy provided to ahow adjustment for changing cake-formation and drainage rates. 

Variable-speed drives on all shafts Screw, 64.4 Orbit, 2.2... 

Direct-Current Motor Control Control for dc motors runs the gamut from simple manual line starters to elaborate regulating systems. Only the starting problems are considered here since variable-speed drives and regulating systems are discussed elsewhere. 

Although motors and controllers can be bought separately, the trend is to purchase a system from a given manufacturer. There are six types oi variable-speed drives, as own in F ig. 29-73. Dynamic response indicates the ability of the drive to respond to a change in command it is measured in radians/second the higher the number, the faster the drive response. 

Moreover, high-temperature variation may result in large variations in the resistance of the grid and may vary the performance of a variable-speed drive if care is not exercised in selecting a proper alloy of the resistance. An alloy such as aluminium-chrome steel should be preferred when it is required to perform a speed control or a speed varitition. as discussed above. 

The development of this hybrid combination in a bipolar transistor has greatly enhanced the application of power transistors in the field of power conversion and variable-speed drives. It possesses the qualities of both the power bipolar transistor (BJT) and the power MOSFET. Like a power MOSFET, it is a voltagc-eontrollcd switching device... 

Variable-speed drives are essential for many industrial applications requiring variable operating parameters during the course of operation. Such variations can be in the flow of fluid and pressure of air or gas etc. The con-... 

Nath, R., Saving energy with variable speed drives, Siemens Circuit. XXII, Oct. (1987). 

Temperature control of the process fluid may be accomplished easily through the use of shutters, variable pitch fan blades, variable speed drives, or, in multiple fan installations, by shutting off fans as required. 

Another version of the dewatering screw that has been used successfully in a number of installations (Figure 40) consists of an inclined screw with a box-like back end and an internal overflow weir. The drive is usually a variable-speed motor reducer. Rotary air lock fitted with a variable-speed drive is typically used as shown in Figure 37 in connection with the plain receiving tank and an inclined screw conveyor. Materials of construction can be chrome-plated cast iron or nickel-hard casing. The rotary valve is made of hard faced tips or stainless steel replaceable blades. 

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