Maintenance Technology May 2015 - (Page 35)

MOTOR MANAGEMENT THE MOST STRESSFUL TIME for an electric motor is during starting, when the shaft speed is zero and the motor current is at its maximum. This condition is termed starting or locked-rotor current. Figure 1 illustrates its effects, as well as the impact of the applied voltage on the current characteristics. As shown, the starting current is directly proportional to the voltage and inversely proportional to the speed. Although many motor-performance parameters are directly proportional to the current, the main concern here is the extra heat produced during starting. The power lost as heat (P), measured in kilowatts (kW), is proportional to the square of the current flow (I2) through a resistance (R): P = I2R Once a motor has successfully started and reached its load current level, its cooling circuit can dissipate the additional heat produced by the starting current. Restarting the motor before all extra heat has been dissipated, however, will add more heat (kW) to the heat already there. In that case, each subsequent start will add even more heat, raising the motor temperature until some component fails. Guidance on starts Depending on design, the thermal "weak link" for a squirrel-cage induction motor could be the winding, the rotor bars or the rotor shorting end rings. Thermal protection located in the windings might not be sufficient to prevent rotor bar or end ring damage. For this reason, both the National Electrical Manufacturer's Association (NEMA) and the International Electrotechnical Commission (IEC) limit the number of times a motor can be safely started in a given amount of time, as noted below. NEMA Std. MG 1-2011, 12.54.1: Normal Starting Conditions Design A and B squirrel-cage induction motors having horsepower ratings given in 10.32.4 and performance characteristics in accordance with Part 12 shall be capable of accelerating, without injurious heating load, a load inertia (Wk2) connected to the motor shaft that is equal to or less than the values listed in Table 12-7 under the following conditions: a. Applied voltage and frequency in accordance with [NEMA Standard] 12.44. b. During the accelerating period, the connected 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-load torque at rated speed. c. Two starts in succession (coasting to rest between starts) with the motor initially at the ambient temperature or one start with the motor initially at a temperature not exceeding its rated load operating temperature. MAY 2015 Fig. 1 Speed-current curve Squirrel-Cage Induction Motors Synchronous Speed, RPM RPM Hp 3600 1800 1200 900 720 600 514 53 82 118 Load Wk (Exclusive of motor Wk ) lb/ft 2 2 2 1 ... 5.8 15 31 1½ 1.8 8.6 23 45 77 120 174 2 2.4 11 30 60 102 158 228 3 3.5 17 44 87 149 231 335 6 6.7 27 71 142 242 375 544 7½ 8.3 39 104 208 355 551 799 10 11 61 137 273 467 723 1050 15 16 75 200 400 684 1060 1540 20 21 99 262 525 898 1390 2020 25 26 122 324 647 1110 1720 2490 30 30 144 384 769 1320 2040 2960 40 40 189 503 1010 1720 2680 3890 50 49 232 620 1240 2130 3300 4790 60 58 275 735 1470 2520 3920 5690 75 71 338 904 1810 3110 4830 7020 100 92 441 1180 2370 4070 6320 9190 125 113 542 1450 2920 5010 7790 11300 150 133 640 1720 3460 5940 9230 ... 200 172 831 2240 4510 7750 ... ... 250 210 1020 2740 5540 ... ... ... 300 246 1200 3240 ... ... ... ... 350 281 1370 3720 ... ... ... ... 400 315 1550 ... ... ... ... ... 450 349 1710 ... ... ... ... ... 500 381 1880 ... ... ... ... ... MAINTENANCETECHNOLOGY.COM | 35 http://www.MAINTENANCETECHNOLOGY.COM

Maintenance Technology May 2015

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