# Maintenance Technology May 2015 - (Page 38)

Keep workers safe & compliant with avo's electrical safety for industrial facilities course. MOTOR MANAGEMENT above 200 hp/150 kW) to nine starts per day or fewer in certain circumstances. Although manufacturers may relax these limits during commissioning to allow for alignment or balancing procedures, they should be consulted beforehand to verify that it is safe. Note that these limits are based on the load inertias (Wk2) in NEMA Table 12-7 (or Table 20-1 for large motors) and IEC Table 3, all of which were calculated from the motor horsepower or kW rating and speed. Load inertias are important because they determine how long it will take to accelerate the load to full speed. The higher the inertia, the longer the acceleration time, and therefore the longer the motor will draw the increased current necessary for acceleration. This increased time at elevated current results in more heating of the motor. NEMA Std. MG 10-2001, Table 7, also lists allowable starts for motors through 250 hp based on size and speed. At first glance, these limits seem to conflict with those in NEMA Stds. MG 1, but closer examination shows they are in harmony. The limits in this table are based on the following conditions: ■ Applied voltage and frequency must be within the limits set in NEMA Stds. MG 1-2011, 12.44, which is a combined value of ±10% of rated voltage and frequency. For instance, an 8% voltage variation and 2% frequency variation would be a combined 10% variation. ■ During acceleration, the load torque is equal to or less than a torque that varies as the square of the speed and is equal to 100% of rated torque at rated speed. ■ External load inertia is equal to or less than the values listed in NEMA Stds. MG 1-2011, Table 12-7. Allowable Starts and Starting Intervals NEMA Design A & B Motors Hp 2 Pole 4 Pole 6 Pole B C A B C A B C 1 15 1.2 75 30 5.8 38 34 15 33 1.5 12.9 1.8 76 25.7 8.6 38 29.1 23 34 2 11.5 2.4 77 23 11 39 26.1 30 35 3 9.9 3.5 80 19.8 17 40 22.4 44 36 5 8.1 5.7 83 16.3 27 42 18.4 71 37 7.5 7.0 8.3 88 13.9 39 44 15.8 104 39 10 6.2 11 92 12.5 51 46 14.2 137 41 15 5.4 16 100 10.7 75 50 12.1 200 44 20 4.8 21 110 9.6 99 55 10.9 262 48 25 4.4 26 115 8.8 122 58 10.0 324 51 30 4.1 31 120 8.2 144 60 9.3 384 53 40 3.7 40 130 7.4 189 65 8.4 503 57 50 3.4 49 145 6.8 232 72 7.7 620 64 60 Workers will learn safe work practices for maintenance on facility electrical systems, metal clad switchgear, substation equipment, motor control centers, and much more. A 3.2 58 170 6.3 275 85 7.2 735 75 75 5.8 338 90 6.6 904 79 220 5.2 441 110 5.9 1181 97 2.4 113 275 4.8 542 140 5.4 1452 120 2.2 133 320 4.5 640 160 5.1 1719 140 200 MAINTENANCE TECHNOLOGY 180 92 150 38 | 71 2.6 125 www.avotraining.com/mt Let us bring the training to you. Call 877-594-3156 for an On-site Quote. 2.9 100 2.0 172 600 4.0 831 300 4.5 2238 265 250 1.8 210 1000 3.7 1017 500 4.2 2744 440 Where: A=Maximum number of starts per hour. B=Maximum product of starts per hour times load Wk2 C=Minimum rest or off time in seconds between starts. MAY 2015 http://www.avotraining.com/mt http://www.avotraining.com/mt

# Maintenance Technology May 2015

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