Each time an a-c motor starts from rest, then accelerates some driven machine to full speed, the motor is stressed three ways. First, the sudden inrush of starting current (typically 6 to 8 times the nameplate amperes) causes high electromagnetic forces tending to tear the winding apart. Second, that current also causes severe thermal expansion in the winding. Third, similar short-time overheating and mechanical stress occur in the rotor squirrel cage. The larger the motor, the more severe the stresses.
That will eventually lead to fatigue failures. This is a “wearout” mechanism. When we ask how many starts a motor can handle before breaking down, we’re asking the same sort of question as “How many times can Nolan Ryan throw that fastball?” (We got the answer near the close of the 1993 baseball season — but until then no one could have predicted exactly when the end would come.)
NEMA offers general answers about motor starting frequency — for standard motors only. First, NEMA recognizes that starting stress depends upon the nature of the driven machine — the load inertia, and the load torque during acceleration. For a 3-phase motor 1 through 250 hp, 1200 through 3600 rpm, NEMA Standard MG10 stipulates the maximum number of starts per hour, and the required cool-down time between successive starts, for loads possessing specific inertia and torque values. A 10 hp motor, for example, may be able to start 6 times per hour; a 200 hp motor, only twice per hour. In all cases, the presumption is that 24 such hours occur each and every day.
For larger machines, NEMA MG1 gives only the number of starts in succession —usually 2 with the motor initially cold, or one restart if the motor has been stopped while hot. How many starts per hour, or per day, isn’t stated. But, 4 to 6 per 24-hour day is a common limit.
Exceeding such numbers will reduce motor life on the average, just as overwork may reduce human life — even though a specific individual may survive, most individuals won’t; the same for motors.
If a single start is not severe, a large motor may be able to sustain 50,000 starts over its lifetime; only 35,000 for a difficult start. Obviously, starting 10 or 20 times daily can use up that fatigue life long before the motor would otherwise be at risk. Starting any motor more than a few times daily, over a long period, should always be checked with the manufacturer.
“Reduced-voltage” starting methods, such as the solid-state “soft starter,” do not ease the starting burden on the motor. By lowering motor torque output during acceleration, they may actually make the start much more severe. Allowing the motor to safely start more often is not among the several valid reasons for using such starting methods.
Richard L. Nailen, P. E