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Safe starting time--what does it mean? With
trilingual summary
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.
From “Understanding
Cable Tray Usage," to
be published in Electrical Apparatus
March 2006
. Visit our online webstore to
order a
copy. © 2006
Barks Publications, Inc. All Rights Reserved.
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