a	Electronic Drives Technical Tips - Hot Spares and Load Testing
a	by Eddie Mayfield, President of Electronic Maintenance Associates
a	It's happened to everyone... you have a repaired (or even new) drive on the shelf, a problem develops with the one in service, you pull out the new/repaired one, and "uh oh..." it doesn't work. One suggestion we often make regarding critical applications is keep your spare drive powered up!
Many maintenance related products are available to you in our Products Showcase For other articles about motor drives  visit our Reference Library. top Need more training? We've got information about Motor Drive training and trade shows available to you if you visit our Events Calendar   top Browse the . . . Maintenance Resources Bookstore   where you'll find books on failure analysis.	While it wouldn't be very energy conscious to keep a drive fully loaded offline, it is possible to construct a fairly efficient load test to intermittently pull cuffent from spare units. Here are a couple of ways to do this.      Line regenerative AC motor... Most of you are aware that if an AC induction motor is pushed above its synchronous speed by an external force, the motor will become a generator. This sometimes happens inadvertently when attempting to decelerate a motor faster than the load inertia will permit. The resulting regeneration often causes an overvoltage trip within the AC drive powering the motor.      This property of a standard motor however, can be used to load a spare drive. It just requires two motors, coupled together. One motor is connected via a contactor to the power line, and the other to your spare drive. You start the drive, and spin up the motor to 60 hz, carrying the mechanically coupled motor with it. After it stabilizes, you simply close the line contactor, and then attempt to push the motor above 60 hz with the drive. The line-connected motor will begin to act as a generator, with the power line as a load. The primary caution: be certain that you have verified rotation of the line-start motor. You'll stand a chance of shearing the line shaft if the motor attempts to reverse when you close the line contactor.      DC motor load... A DC motor is also a generator. Again, you mechanically couple two motors together. This can be two DC motors, if you're testing DC drives, or an AC and a DC if you're testing AC drives.      You can create a load by varying the field strength of the coupled DC motor, while the armature is connected across a resistor bank. With little or no field excitation, there will be minimal loading, but as you increase the field strength, the motor will begin to generate. The advantage of this over the other system is that it is a little easier to control the loading, and you don't have to achieve any particular speed. The disadvantage is that you require a heat dissipating resistor bank, and an external, adjustable field supply of some sort.
top	This article is contributed by Eddie Mayfield, President of Electronic Maintenance Associates, and is reproduced for the Third Quarter, 1999 issue of Drive Lines, their newsletter dealing with electronic drives and controls. For more information on their services please visit their website at www.emainc.net. If you need a load stand built, please contact Dean Williams of EMA at dwilliams@emainc.net.

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