temperature could be as low as 50% of the permitted level. The main reason for this is that the rotor winding is
able to dissipate the heat mor
e efficiently than the stator winding, particularly with fan-driven, air-flow machines.
During starting, the slip is low and both the stator and rotor currents are high, thereby creating heat in both
windings. However, a motor is usually designed to allow one start when it has previously been run at a stator
temperature not exceeding its rating.
From cold, the motor thermal replica characteristic is on its 'cold' curve, with the highest tripping times. As the
machine is run over a period of time, the projected thermal trip times are reduced, eventually reaching their
minimum when operating on the 'hot' curve. Unfortunately, any rapid increase in current, for example a starting
condition, may result in an unnecessary trip. This is shown in the figure below.
P24xM Chapter 6 - Current Protection Functions
P24xM-TM-EN-2.1 81
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