When it comes to motion control applications, don't forget to consider the benefits of ac drives. Some of these include variable speed, process efficiency improvement, acceleration and deceleration control, and demand reduction. The most obvious value that ac drives have to offer is their ability to vary speed, and it's important to understand the physics of what this means. A drive and motor constitute a constant torque system, which means that torque is available from very low speeds up to the motor's rated nameplate speed. This varies sharply from a constant hp system, where lower speeds mean higher torque capacities. Simply put, you can't expect an ac drive to be an electronic gearbox.
Variable speed can offer the ability to compensate for a number of factors in the real world, including differences in operator skill, materials, and even the process itself. This can result in tremendous process efficiency improvements. Variable speed can also be extended into process control fairly easily. With the help of a transducer monitoring a critical parameter such as temperature or speed, it's possible to regulate the process. This can improve the quality of the machine's output by eliminating the need for manual changes throughout the day. Keep in mind that the drive is a process regulator rather than a speed controller.
Acceleration control is another critical factor in many applications. All ac drives control the rate of increase in the output Hz, which is often specified in “seconds” alone for simplicity. Hertz can be related to motor speed with typical 1,800 rpm motors running at 30 rpm per Hz, 3,600 rpm motors at 60 rpm per Hz, and 1,200 rpm motors at 20 rpm per Hz. The “seconds” value is actually the time it takes the ac drive to move the output frequency from minimum to maximum limits. It is therefore indirectly looking at the true acceleration that would be specified in “Hz/sec.” Many applications can profit from acceleration control, with the principle benefits being waste control and reduction of mechanical wear and tear. Ac drives should be the first choice when true controlled acceleration is required, as opposed to what is termed “soft starting.”
Deceleration control is often a safety issue. If the load has any appreciable inertia, stopping the motor quickly can be a challenge, but one a drive can handle. Features like dynamic braking and dc injection braking can make this kind of process manageable. Frequently, if the drive doesn't have dynamic braking on board as standard, it can be easily added.
Finally, the benefit of demand reduction is achieved as a result of the acceleration control feature described above. When an ac motor is started directly online, current consumption and instant torque are both quite high. Many times the power company will monitor and bill users based on short-term peak demand, in addition to monitoring consumption over time. From the power company's perspective, if users need short-term power bursts to start motors, they should pay for them. The value provided by ac drives is a substantial reduction of demand charges through controlled acceleration.
Information provided courtesy of Tim Park of Vacon Inc. For more information, visit www.vacon.com.