AC drives have been around for many years in one form or another. Dating back into the 50’s and 60’s they we operated by vacuum tubes. Of course today we have IGBT’s (Insulated Gate Bi-Polar Transistors) that allow the usage of 460 Volt and higher voltages to be applied to motors. You may not be able to imagine the size of the older drives and how they occupied large amounts of space for even a small amount of Horsepower. Take for example an 100 HP drive would have required a room 12 or 14 foot long whereas today the same 100 HP would only require 3 to 4 foot of wall space, and they are getting smaller everyday. Another area of advancement has been in the performance area. The older drives were limited as to the load they could start. The stating torque was limited to on 100% of full load torque or less. Today drives can do 200% starting torque or more. This has benefited in the use of AC Drives on heavy equipment and difficult hard starting applications.
AC Drives have opened up many applications that use to be driven by DC Drives or large Mechanical Drives. Now the applications are limited only to a few very large systems that still require, mainly from a dollar standpoint, the use of the other types of systems. In the HVAC, Water, and Waste Water industries the use of AC Drives has not only brought about more efficient process control, but when used with centrifugal pumps and fans there is significant electrical energy savings achieved in operational costs. The drives actually pay for themselves in a short period of time through electrical energy savings. Still the main thing that the AC Drive has done is to optimize the control of a process. By optimizing a process to the right speed for the process, or being able to change the speed for various processes used by the same piece of equipment, the AC Drive saves in productivity and ease of operation which adds dollars to the bottom line of a business. Some examples of these would be in Mixing, Conveying, Cold Forging, and Presses.
Over the years we have seen AC Drives also become less expensive. This has lead to even more applications now using AC Drives. In some cases they have taken the place of Motor Starters. Because of the unique way an AC Drive starts an AC Motor by varying the voltage and frequency, the drive allows for a controlled acceleration of a load or soft starting. Even if the application does not need variable speed but would benefit from a controlled start the AC Drive has advantages over the basic Motor Starter. Even though the across the line Motor Starter will be less expensive, an AC Drive may save the user by providing less mechanical stress on the driven equipment. There are also Solid State Starters that will give a Soft Start, but they may have a problem starting some loads due to the nature of how they operate. I’ve often used an illustration to show the difference in across the line starting versus using an AC Drive. Have you ever had to push a car? You don’t back up 30 feet and run toward the car as fast as you can, because you will end up with something broken and in a Hospital and the car will not have moved an inch. The correct way is to lean against the car and gradually apply pressure until the car begins to move and then you can gradually pick up speed until you get the car off of the road. This is how an AC Drive starts a load, by applying voltage and frequency together in the right proportion that the load begins to accelerate. So, for a few dollars more you eliminate shock loads on coupling, gears, and other components in a drive train.
AC Drives may have many benefits for the user, but they still need to be applied properly depending on the application. If you think you have an application that an AC Drive would benefit your process then you should talk with an AC Drive Applications Engineer (I just happen to know one, ME). Don’t listen to a salesman say that this drive or that drive will work if they don’t know the application. And don’t be fooled that all AC Drives are all alike, they aren’t. One common mistake is sizing a drive by Horsepower only. There are some motors that have a given Horsepower, but their full load amps and starting amps are way above the amperage capacity of a drive of the same HP rating. Most distributors have inside sales people who only know part numbers and very little else. Know who you are talking with and discover if they are just giving you what you want, or are they offering you a solution for your application and a drive that will meet your needs, and one that will give you years of excellent service. My grandfather use to tell me that, “you get what you pay for”, and this is very true when it comes to AC Drives. Price alone should never be the deciding factor when purchasing an AC Drive.
AC Drives do wonderful things, but there are some problems that you should be aware of when you are making your selection. The installation and location of a drive are very important. There are some many potential problems that I don’t have time to cover in this format, but you should again find an expert and discuss the down side of AC Drives and some of the pitfalls that you might encounter when installing and operating a drive.
I’ve got 30 plus years in this business and if you need good application help please give me a call and ask for Syd, not many guys left that go by that name, or with that amount of experience.
www.sordselectric.com
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Acceleration sensitive—measures total
