An ASD is an electronic device connected between the source of power and the motor (Figure 1) for both AC and DC motors. In the case of the DC ASD, the ASD converts AC power to DC and then varies the voltage and current supplied to the motor to vary the motor speed. In the case of the AC ASD, the ASD takes AC power, rectifies it to DC and then switches the voltage and frequency of its output to vary the speed of the AC motor. AC ASDs are replacing more and more DC drives in industry; therefore this article focuses on AC drives.

Figure 1: ASD-Motor Circuit

Uses and benefits of ASDs

Although ASDs are used to change motor speed, they also may be used for other purposes, such as

• to accelerate or decelerate machines in a smooth and predictable fashion,.
• to provide motor direction reversal on a regular basis to satisfy the needs of a certain processes.
• to regulate conveyor speeds to comply with varying load requirements.

However, the most common use of ASDs is to vary the speed of a fan or pump to meet varying fluid flow requirements. When operated in this mode, ASDs can save considerable amounts of electrical energy compared with running the fan or pump at full motor speed. This savings is because centrifugal pumps and fans accept load in a cubic relationship relative to the change in speed.

For example, consider a fan and motor running at full motor speed. Assume that a 100 Hp motor runs the fan and that the motor is fully loaded. Sometimes only half the airflow is needed. This can be accomplished by at least three methods:

• by putting a damper on the inlet side of the fan,
• putting a damper on the outlet side of the fan, or
• reducing the speed of the fan to ½ of the full load speed.

Adding an ASD is the most effective method of adjusting fan speed. When the fan is reduced to ½ of full load and the power the motor is consuming is measured, the result shows that the power is not 50 Hp, or ½ of 100 Hp but is actually 12.5 Hp (? of the 100 Hp, or ½ x ½ x ½). Compare this to the energy consumed by a dampered system, and there are dramatic reductions.

This dramatic reduction in power illustrates how ASDs can be used to reduce energy consumption in many process situations. When full load or flow is only needed for short periods of time, the most savings are possible. It is important to remember that these rules do not apply to all types of pumps and fans. For example, they do apply to centrifugal devices, but do not apply to positive displacement pumps.

ASDs can have other benefits. The ASD can serve as a motor starter in most instances and thus replace that device. A particular benefit is that if the load allows, the ASD can "soft start" the motor, dramatically reducing the inrush of electriccurrent to the motor during starting.

Potential disadvantages

An ASD typically costs several times the cost of the motor it is controlling. In addition, ASDs are available in different levels of sophistication, such as those that can very precisely control motor speed and can sense the actual position of the motor/load shaft, if necessary, to control the process. Of course more sophisticated ASDs do cost more.

There are other considerations that must be addressed when installing ASDs. The ASD installed between the power supply and the motor means another piece of equipment has been inserted into the system. Installing an ASD will naturally decrease system reliability, as now there are two pieces of equipment that can fail rather than just one. Also the ASD is an electronic device and is more susceptible to negative environmental conditions, such as dust and temperature changes, than are electromechanical devices, such as motors.

Furthermore, ASDs can cause and/or be affected by power quality problems, and in some rare cases cause motor damage. As more drives are utilized, the possibility of harmonic problems increase. Because of converting AC power to DC, the ASD can inject harmonics back onto the AC line. The ASD is a high-speed switching device. The wave-form it produces and supplies to the motor is very different from the sine-wave supplied by the utility. If the drive/motor system is not properly designed, this wave-form can cause premature motor failure. Such problems are usually small and infrequent; however, it is important to consider the potential for such problems.

Conclusion

In conclusion, the ASD is an important technology that has increased our ability to operate motors/processes more flexibly as well as offering considerable energy savings if properly applied. In spite of the potential problems associated with the installation of ASDs, it is worth considering. Involve your Progress Energy representative in choosing ASDs for your operation.