4–34 369 MOTOR MANAGEMENT RELAY – QUICKSTART GUIDE
S3 OVERLOAD PROTECTION CHAPTER 4: KEYPAD SETPOINTS
4.4 S3 Overload Protection
4.4.1 Description
Heat is one of the principle enemies of motor life. When a motor is specified, the purchaser
communicates to the manufacturer what the loading conditions, duty cycle, environment
and pertinent information about the driven load such as starting torque. The manufacturer
then provides a stock motor or builds a motor that should have a reasonable life under
those conditions. The purchaser should request all safe stall, acceleration and running
thermal limits for all motors they receive in order to effectively program the 369.
Motor thermal limits are dictated by the design of the stator and the rotor. Motors have
three modes of operation: locked rotor or stall (rotor is not turning), acceleration (rotor is
coming up to speed), and running (rotor turns at near synchronous speed). Heating occurs
in the motor during each of these conditions in very distinct ways. Typically, during motor
starting, locked rotor, and acceleration conditions, the motor is rotor limited. That is, the
rotor approaches its thermal limit before the stator. Under locked rotor conditions, voltage
is induced in the rotor at line frequency, 50 or 60 Hz. This voltage causes a current to flow
in the rotor, also at line frequency, and the heat generated (I
2
R) is a function of the effective
rotor resistance. At 50/60 Hz, the rotor cage reactance causes the current to flow at the
outer edges of the rotor bars. The effective resistance of the rotor is therefore at a
maximum during a locked rotor condition as is rotor heating. When the motor is running at
rated speed, the voltage induced in the rotor is at a low frequency (approximately 1 Hz)
and therefore, the effective resistance of the rotor is reduced quite dramatically. During
running overloads, the motor thermal limit is typically dictated by stator parameters. Some
special motors might be all stator or all rotor limited. During acceleration, the dynamic
nature of the motor slip dictates that rotor impedance is also dynamic, and a third
overload thermal limit characteristic is necessary.
Typical thermal limit curves are shown below. The motor starting characteristic is shown
for a high inertia load at 80% voltage. If the motor started quicker, the distinct
characteristics of the thermal limit curves would not be required and the running overload
curve would be joined with locked rotor safe stall times to produce a single overload curve.
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