Electrical Apparatus upcoming issue

Electrical Apparatus upcoming issue

This is a summary of the Electrical Apparatus December 2015 featured technical article, by Richard L. Nailen, P.E.    Receive this article with your subscription to Electrical ApparatusThe choice between replacing or repairing (or redesigning) an existing a-c motor is based on economics. Efficiency is now an important concern. It requires knowledge of what the drive load demands and how the existing motor is (or has been) performing. For the service center involved in this choice, here is some “do” and “don’t” advice: When making any measurements on an existing polyphase motor circuit, be sure to check all three phases. Voltage unbalance exceeding 1% affects any efficiency determination. Test instruments must have accuracy suited to the task and be within proper calibration. Take readings often enough to be sure of the motor load, which may be far below the motor rating and may vary widely from hour to hour or from day to day. The amount of “off” time is important; the efficiency of an idle machine is irrelevant. Know whether or not power factor will make any difference in utility charges. Take account of the effect on energy usage of the higher full-load speed of a more efficient motor driving a centrifugal load. In all calculations, allow for the effect of motor terminal voltage either above or below the motor nameplate value. On the negative side: Don’t assume that efficiency variation with load will be the same for all motors, or that performance will be within a certain range for any motor built during a particular era. Don’t assume that “soft starting” will either make starting easier on the motor or influence the utility bill. Don’t assume that efficiency will be lower at loads below the nameplate rating. For most standard motors, efficiency peaks at about 75% load, not at 100 percent. When a winding redesign appears to offer an efficiency increase, don’t rely on a calculation method that ignores the rotor. Finally, remember that not every motor of a standard design will exhibit exactly the nameplate efficiency. That number is an average, not necessarily met by all units. Instead, use the associated minimum efficiency value when evaluating operating cost. To order a back issue with the full article, “Do’s and don’t’s for evaluating motor operating cost” call 312-321-9440 or visit our online webstore.

Share Button