6.7
Adjustable-speed drives can be used in every environment where variable speed and torque are needed. ASDs are currently used in elevators, water and wastewater pumps, boiler fans, HVAC systems, wind turbines where speed of wind is not constant, hydroelectric plants where speed of water is not constant, and the automobile industry. Some of these applications will be discussed in detail below.
Over the last few years there has been an increase in demand for electrical energy that has been made by wind energy,
an alternative energy source. Wind speed is variable and applies variable speeds to the generator at the turbine, which creates different frequency levels. To increase productivity and get 60-Hz frequency at the output, adjustable-speed drives can be used in wind turbines. In advanced systems, wind is converted to electrical energy via an AC generator with variable frequency and voltage levels. By using a converter, this AC is converted to DC; then, using a DC link line, this DC is transmitted to a DC/AC inverter. The AC utility line has fixed voltage and frequency with better power quality. Illinois, Texas, and Arizona in the United States as well as Denmark and others use wind energy. Other countries are sure to follow.
As with wind energy, water flow also has variable speed.
This again creates variable voltage, frequency, and a poor power factor. By using ASDs, these problems can be fixed and the AC utility line can have fixed values. In such advanced systems, water flow is converted into electrical energy via an AC generator with variable frequency and voltage levels. By using a converter, this AC is converted to DC; then, using a DC link line, this DC is transmitted to an inverter connected to the AC utility line, which has fixed voltage and frequency with better power quality. This increases quality and efficiency. Hydroelectric power plants are one of the oldest and biggest electrical energy creation systems.
Centrifugal pump and fan applications are among of the most common areas for ASDs. Gas or liquid flow can be regulated using ASDs. Adjustable-speed drives can reduce maintenance costs and increase the efficiency of pumps to almost the best efficiency point.
In Table 6.6, the relationships among the speed of a pump, liquid flow, and horsepower needs are shown. As seen in the table, increments in the flow and speed will greatly decrease power requirements. If the speed of the pump decreases by 50%, the power requirements will go down to 13%. This shows that ASDs can be very efficient and save money.
In conclusion,
ASDs and their applications have been rapidly growing over the last few years. There are many applications, like fans, compressors, pumps, automobile industry applications, and many other motor areas, where variable-speed drives are desirable and can be readily found. There are some downsides to ASDs, such as harmonics, voltage notching, and complexity of design and cost, but these can be fixed using harmonic filters, voltage regulators, and switching control mechanisms. Use of ASDs will increase the efficiency of machinery, lower maintenance costs, and reduce machine size. Faster-switching semiconductor devices and a decrease in semiconductor prices make this field very popular for engineers and the industry. Available ASDs can be purchased as AC or DC drives. AC drives can adjust the frequency and voltage to change the speed of the electric machine. Creating a voltage and frequency ratio constant will create a constant torque. These drives are newer and simpler compared to the DC drives, but they are not as efficient. AC machines need less maintenance than DC machines and motor speed can reach almost four times more than DC machines. Another advantage of using vector-controlled AC machines is that fast-changing load applications can be used. DC
TABLE 6.6 Flow-Speed-Power Relationships of Typical Pumps
| Flow, % | Speed, % | Horsepower, % |
| 30 | 30 | 3 |
| 50 | 50 | 13 |
| 70 | 70 | ‘■V: |
| 80 | SO | 51 |
| 90 | 90 | 73 |
| 100 | 100 | 100 |
drives need converters to convert AC utility voltage into DC voltage, and by changing this the voltage speed of the machine can be altered. DC machines require better maintenance and therefore its environment needs to be clean and dry. Adjustable-speed drives are one of the newest and fastest growing technologies in electric machines, and their future will certainly be dynamic.
