Variable frequency drives

Variable Air Flow Fans. Variable air flow fans are needed ia the process iadustry for steam or vapor condensing or other temperature critical duties. These also produce significant power saviags. Variable air flow is accompHshed by (/) variable speed motors (most commonly variable frequency drives (VFDs) (2) variable pitch fan hubs (J) two-speed motors (4) selectively turning off fans ia multiple fan iastaHations or (5) variable exit louvers or dampers. Of these methods, VFDs and variable pitch fans are the most efficient. Variable louvers, which throttle the airflow, are the least efficient. The various means of controlling air flow are summarized ia Table 3. 

Rietz disintegrators are normally supplied in rotor diameters from 10 to 60 cm (4 to 24 in), with rotational speeds to produce hammer tip speeds in ranges of 300 to 6700 m/min (1000 to 22,000 ft/min) and power ranges from 0.4 to 1.50 kW (V2 to 200 hp). Higher speeds and higher power are available. AC variable-frequency drives can eliminate belts and provide easier variation of speed. Models are available... 

The merits of a variable-speed motor would appear to be obvious, as many compressors in the past have benefited from the variable speed available in a steam turbine. A compressor may be adjusted as required to meet the process needs. The advent of the variable-frequency drive returns some of the benefits to the process operator that were lost when the more favor able electric energy caused motors to replace steam turbines. 

Application, relative to size, is not too much of a problem as the variable frequency drive has been used to 40,000 hp. High horsepower drives have been in service in Europe for a significant period of time [4J. In the U.S., the applications have been somewhat more modest in size but, as the popularity grows, the size will doubtless follow. 

Variable-frequency drive technology is constantly improving in step with the advances in power electronic device technology and with the associated microprocessor controls. The following list of desirable fea tures is offered ... 

Electric Motor Variable Frequency Drive No. of Pulaas " Motor Una Fraquancy Steady... 

Molecular weight, effect on centrifugal sizing, 159 Mollier charts, 27 Monitoring system, 356 Motor, 146 enclosure, 260 equations, 267 insulation, 257 locked rotor torque, 270 selection, 270 service factor, 262 starting characteristics, 270 starting time, 273, 274 synchronous vs induction, 265 variable frequency drives, 27/, 280 voltage, 258 Motors... 

One industrial facility that has done exhaust system retrofit is the Eldec Corporation, an aerospace electronic manufacturer. With the help of the local utility, Eldec implemented a control project to reduce exhaust air by up to 30% for the first shift and 60% for the rest of the time and achieved great savings with one year simple payback. The project closed the exhaust inlets with dampers and controlled the exhaust fan speeds with variable frequency drives (VFD). The exhaust fans are now monitored and controlled by the building direct digital controls (DDC) system to ensure proper operation and save energy. 

During restart, the feed auger variable-frequency drive failed as a result of overloads, necessitating replacement of the drive. 

Variable Frequency Drives. An important energy by-product of solid-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 directly to deliver the right flow without the need for a control valve and its inherent pressure drop. Figure 11 shows that at rated load the variable speed drive uses only about 70% as much power as a standard throtde control valve system, and at half load, it uses only about 25% as much power. 

Variable frequency drives (VFDs) are sometime used to adjust the operation of typical (US) standard of 480VAC, 3PH, 60hz operation of the motor. The functionality of a VFD is to convert frequency measured in Hertz (Hz) to motor speed. One Hz equals 1 cycle per second. When voltage is being received (input to the VFD), it is in the sinusoidal waveform. The sine wave is converted to a digital square wave that now controls the revolutions per minute (RPM) of the motor. 

Figure 6.5 Variable frequency drive (VFD) control cabinet. Courtesy of Nalco-Crossbow Water LLC.

A variable frequency drive (VFD) adjusts the speed of a motor to alter the discharge pressure (see Chapter 6.2). This is useful when... 

Variable frequency drives generate harmonic other than the triplets, the highest of which are the 5th, 7th, 11th, and 13th harmonic currents, on the three-phase circuits that feed them. Drive transformers have been specifically designed for VFD loads. 

The use of variable-speed drives in pilot plant and manufacturing plant vessels is recommended for development and scale-up of crystallization processes. This capability provides the opportunity for critical experimentation at the pilot plant scale to determine the effect of impeller speed on PSD and other variables. On the manufacturing scale, the ability to change impeller speed is the most readily adjustable parameter for manipulation on scale-up. Modem variable-frequency drives provide an excellent means to vary speed over a wide range. The added cost of variable-speed capability is minimal compared to all other methods of changing mixing... 

The trend towards elimination of hydraulics in pharmaceutical process areas has turned maintenance over to the instrumentation and electrical specialists, as variable frequency drives become the standard. The Inverting Filter Centrifuge (Heinkel) eliminates all hydraulics from the centrifuge design to satisfy increasingly stringent cleanroom requirements by pharmaceutical companies. The risk of hydraulic oil in the process area has been a concern with respect to contamination with the product. 

A range of control options can be supplied, from simple variable frequency drives to fully integrated PLC recipe systems. 

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