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Safe starting time--what does it mean?

Some specifiers and users of a-c machines assume that a "safe starting time" exists for any motor. Anything causing starting to take longer must be unsafe. That time limit is sometimes believed to equal the safe locked-rotor time (which does indeed have a specific value for any motor).

Those are misconceptions. Bringing any load up to speed develops heat within stator and rotor windings. Part of it results from acceleration of the rotating system inertia from standstill to rated rpm. Also, the motor's developed torque must overcome that demanded by the load. That develops additional heat.

The inertia-based heating is not a linear function of speed. Rather, heating at low rpm far exceeds that at higher speed. Since the effect of load torque is to multiply the inertia-based heating, it too will have greater influence at low rpm. Although also dependent upon net accelerating torque (motor minus load), acceleration time is a linear function-- that is, the same time applies for the same net torque regardless of rpm.

For one driven machine, that net value may be low at low speed, causing severe motor heating and long acceleration time; for a different load, the net torque may be low at a much higher rpm, producing much less heat but an equally long acceleration time. Hence, starting time alone cannot indicate a motor's thermal condition. Another misconception is that overcurrent protection is only possible when acceleration time is less than the safe locked-rotor time.

Reduced-voltage starting (though it increases motor heating by reducing motor torque) is sometimes thought to benefit a motor by spreading heat production over a longer period so that it can dissipate, thus lowering temperature rise. However, motor thermal time constants are so long that this seldom works.

Although motor users can calculate acceleration time from motor and load torque curves and the inertia, they cannot determine whether a start is "safe" or not. Only the motor designer has the information needed to determine short-time motor temperatures. Of course, if a motor's starting time begins significantly increasing (at the same voltage), something has changed. Investigation is appropriate. But the condition is not necessarily unsafe for the motor. So whereas safe locked-rotor time is independent of load, does not involve torque, and will have a specific value for any given machine, that's not true for starting time.

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