Maintenance Technology August 2014 - (Page 25)

BEST-PRACTICE BASICS on the stator windings or externally on the frame and the fan cover (if applicable) may inhibit airflow. Damaged or missing fans also significantly reduce the flow of cooling air. The solutions for these types of problems include keeping the motor clean and repairing or replacing damaged or missing fans. To ensure that heated air is not recirculated to the motor's air inlet, it may be necessary to supply a sufficient volume of filtered air from an external source. If the motor is an open enclosure in a dirty environment, consider replacing it with a totally enclosed fan cooled (TEFC) model. It's much easier and faster to remove dirt from the exterior of a TEFC motor than from the inside of an open-enclosure motor. Winding failures can also result from transient voltages. These voltage "spikes" can reach levels many times higher than line voltage within microseconds. Transient voltages may occur as a single event (e.g., from lightning strikes, rapid switching of the motor or utility bus transfers) or continuously (e.g., highfrequency transients from the "chopped" waveform output that VFDs use to simulate variable-voltage and variable-frequency AC supplies). While the effect of single-event transients is often dramatic, the partial discharge (corona) from continuous VFD transients can literally eat away the insulation of a stator winding. Although "prevention" would be the ideal solution for singleevent transients, the practical remedy is to install transientvoltage protection in the motor terminal box. Similarly, the only true solution for repetitive transients from VFDs would be Fig. 2. Symmetrical overheating of the entire winding caused by overcurrent a VFD output without transient voltages. Until that becomes available, common preventive measures include installing filters or line reactors and inverter-duty (VFD-rated) motor windings. It is also important to use inverter-duty supply cables and to keep the cable length within the motor manufacturer's guidelines. Capturing the reward Since most motor failures can be traced to damaged bearings or stator windings, it makes sense to leverage techniques and solutions to protect these components. Your reward: longer, more reliable motor life and increased productivity. MT Thomas H. Bishop, P.E., is a Senior Technical Support Specialist at the Electrical Apparatus Service Association (EASA), in St. Louis, MO, 314-993-2220. EASA (easa.com) is an international trade association of more than 1900 firms in 62 countries that sell and service electrical, electronic and mechanical apparatus. REVOLUTIONARY VIBRATION AND BEARING ANALYSIS Leonova Diamond is the latest proof of our commitment to developing ﬁrst class condition monitoring products for more proﬁtable maintenance. Use SPM HD for accurate rolling element bearing analysis. Reduce data collection time with tri-axial vibration measurements. Add balancing, laser alignment, orbit analysis and much more, all in a rugged and lightweight instrument. Ex version available. Experienced Manufacturers Reps Wanted, Please call 1-800-505-5636 Tel. 1-800-505-5636 leonovabyspm.com spminstrument.com AUGUST 2014 MAINTENANCETECHNOLOGY.COM | 25 http://www.easa.com http://www.spminstrument.us/ http://www.leonovabyspm.com http://www.spminstrument.com http://www.MAINTENANCETECHNOLOGY.COM

Table of Contents for the Digital Edition of Maintenance Technology August 2014

Maintenance Technology August 2014