Maintenance Technology May 2015 - (Page 39)

MOTOR MANAGEMENT The allowable starts per hour is the lesser of the value in Column A or Column B, divided by the load inertia (if known). The values in Column B are nearly identical to those for the same ratings in Table 12-7 of NEMA Stds. MG 1. In Table 12-7, the load inertia of a 100 hp, 4-pole motor is 441 lb/ft2, which is the same value given for a 100 hp, 4-pole motor in NEMA Stds. MG 10. This means for the allowable inertia, the safe starts would be one, the same as stated in NEMA Stds. MG 1, 12.54.1. If the inertia is known and is less than the value given in Table 12-7, additional starts might be allowable. If the motor is larger than 250 hp or if the load inertia is not known, the application should use the "2 cold/1 hot start" rule unless the manufacturer is consulted. Impacted by voltage available As Fig. 1 shows, the starting current is affected by the amount of voltage available during start-up. When the power supply is limited, it is often necessary to employ a starting method such as reduced voltage, wye-start/delta-run or soft starting to limit the starting current and avoid voltage sag to other loads on the supply. This reduces the starting current (and therefore the rate of heating), but extends acceleration time so the extra heating lasts longer. This results in the same amount of kW being injected into the motor. Another way to look at this is to consider the acceleration of the load as work accomplished. Whether the load is accelerated in 5 seconds or 30 seconds, the same amount of work has been accomplished, therefore heat accumulation is the same. Conclusion A wise motor engineer once noted that since every motor has a specific number of starts in its life, it would be better to spread them out than to use all of them the first year. To achieve the best and longest motor performance, it is important to recognize the stress imposed by starting and to limit the number of starts. Even the limits defined here represent an extreme in application. Unless that many starts are necessary, they should be avoided. Starting stress can also be mitigated through the use of alternative applications. For example, in the case of flow demand for a pump or fan, for instance, variable-speed control may be able to provide a constant, correct flow without starting and stopping to adjust the availability of the material. With repetitive operations like punch presses or load positioning, an eddy current or fluid clutch may help. The NEMA and IEC guidelines referenced here apply only to usual conditions. Unusual applications-such as elevators that may start 40 or 50 times per hour during peak operation-must be specifically addressed by the manufacturer's design team. MT Jim Bryan is a technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis, MO; 314-993-2220. EASA is an international trade association of more than 1900 ﬁrms in 62 countries that sell and service electrical, electronic and mechanical apparatus. For more information, visit easa.com. MAY 2015 LEADING EDGE POWER TRANSMISSION Companies continue to introduce innovative new products for the robust manufacturing sector. Timing Pulleys, Bushings and Belts AutomationDirect's SureMotion family of synchronous-drive components lets personnel change speed and torque while connecting mechanically rotating components. Items include SureMotion XL timing pulleys in aluminum or steel, with or without hubs and with smooth bores and setscrews. Aluminum and steel SureMotion L timing pulleys are available without hubs and with smooth bores and setscrews. SureMotion L steel pulleys that fit Taper-Lock or QD-style-drive steel bushings are also available, and fiberglass-reinforced Neoprene SureMotion timing belts are available in several pitches and widths. AutomationDirect Cumming, GA automationdirect.com Intelligent Chain Sprockets U.S. Tsubaki has introduced SMART TOOTH chain sprockets with the company's patented Wear Indicator Technology that lets users detect drive-system-component problems and schedule maintenance before failures occur. Special, strategically placed wear-indicator pins on one or more sprocket teeth make it easy to visually confirm that a sprocket is still within allowable wear tolerance or that it needs to be replaced. These patented pins come standard on three sprocket teeth, which ensures they can be seen, regardless of what part of the sprocket is covered. SMART TOOTH technology is available on engineering-class and roller-chain sprockets for a range of applications, including mining, cement, asphalt and oil & gas, among others. U.S. Tsubaki Power Transmission, LLC Wheeling, IL ustsubaki.com MAINTENANCETECHNOLOGY.COM | 39 http://www.automationdirect.com http://www.easa.com http://www.ustsubaki.com http://www.MAINTENANCETECHNOLOGY.COM

Maintenance Technology May 2015

Table of Contents for the Digital Edition of Maintenance Technology May 2015