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3.7 THERMISTOR TRIP DETECTION
Insulation breakdown of the stator windings due to overheating
is the main cause of motor failure under overload conditions.
Heat buildup in the rotor can be very rapid but the large ther-
mal mass of the motor prevents direct detection by tempera-
ture sensors embedded in the stator slots soon enough to
prevent damage. It may take several minutes for the tempera-
ture sensor to reach its trip temperature. Consequently, a pre-
dictive model is required to accurately determine heat buildup
within the motor. The Protect 4A relay uses an accurate elec-
tronic memory method based on motor currents. Thermal over-
loads rely on using motor current to heat an element with a
much smaller time constant than the motor itself to predict over-
heating within the motor but these devices, although inexpen-
sive, are subject to many limitations.
Overheating from causes other than resistive heating due to
current cannot be detected by modelling methods that only
sense current. To detect the effects of motor overheating due to
blocked ventilation, high ambient temperature or other unfore-
seen causes, direct temperature sensing is necessary. Since
temperature rise under these conditions is much slower, the
temperature detector will accurately sense the actual tempera-
ture within the motor which would not be true under a rapid
heat buildup situation such as locked rotor, for example.
Linear sensing elements such as RTDs and thermocouples can
give an output of actual temperature but these are expensive
and unnecessary for basic motor protection. Thermistors are
available which give a rapid change of resistance at a specific
temperature which is determined at the time of installation. The
Protect 4A accepts a thermistor input and will provide a trip
within 3 seconds of the thermistor threshold temperature being
exceeded. Several thermistors can be connected in series to
protect each of the stator phases. Any one of the thermistors
exceeding its threshold temperature will cause the Protect 4A
to trip. In addition, if the motor is still overheated, the thermistor
signal will prevent restarting of the motor by tripping the Protect
4A immediate after reset.
Summary of Contents for PROTECT 4A-120
Page 1: ...MOTOR PROTECTION RELAY INSTRUCTION MANUAL ...
Page 5: ...2 FIGURE1 1 PROTECT 4A TYPICAL SYSTEM CONNECTION ...
Page 11: ...8 FIGURE 2 2 PHASE CT DIMENSIONS FIGURE 2 3 GROUND FAULT CT DIMENSIONS ...
Page 21: ...18 FIGURE 3 1 FRONT PANEL LAYOUT ...
Page 35: ...32 FIGURE 4 1 SINGLE PHASE INJECTION TESTING FIGURE 4 2 100 AMP CURRENT SOURCE ...
Page 39: ...36 FIGURE 4 3 GROUND FAULT EARTH LEAKAGE TEST ...
Page 44: ...41 FIGURE 5 1 HARDWARE BLOCK DIAGRAM ...
