GE Digital Energy Multilin 239, Instruction Manual, Page: 80 / 160

4–42 239 MOTOR PROTECTION RELAY – INSTRUCTION MANUAL
CHAPTER 4: PROGRAMMING
programmed in this setpoint while the motor is starting, the alarm relay will activate
and the “GROUND ALARM” message will be displayed.
NOTE: When the phase current increases from 0, this delay is used until the 239
determines whether the motor is RUNNING or STARTING
.
Refer to Section 5.2: A1: Status on page –61 for details on how the 239 detects a start
condition.
4.5.10 Temperature
Thermistor
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
thermal mass of the motor prevents direct detection by temperature sensors embedded in
the stator slots soon enough to prevent damage. It may take several minutes for the
temperature sensor to reach its trip temperature. Consequently, a predictive model is
required to accurately determine heat buildup within the motor. The 239 relay uses an
accurate electronic memory method based on motor currents and time based integration
algorithms. Thermal overloads rely on using motor current to heat an element with a much
smaller time constant than the motor itself to predict overheating within the motor but
these devices, although inexpensive, are subject to many limitations.
Overheating from causes other than resistive heating due to current cannot be detected
by modeling methods that only sense current. To detect the effects of motor overheating
due to blocked ventilation, high ambient temperature or other unforeseen causes, direct
temperature sensing is necessary. Since temperature rise under these conditions is much
slower, the temperature detector will accurately sense the actual temperature within the
motor which would not be true under a rapid heat buildup situation such as locked rotor
for example.
• THERMISTOR FUNCTION: Linear sensing elements such as RTDs can give an output of
actual temperature but these are expensive and unnecessary for basic protection of
small motors. Thermistors are available which give a rapid change of resistance at a
specific temperature. The 239 accepts a thermistor input and will provide a trip/
alarm/auxiliary control within 2 seconds of the thermistor threshold temperature
being matched or exceeded. Either negative temperature coefficient (NTC) or positive
temperature coefficient (PTC) thermistors can be used. The 239 assumes a PTC
thermistor connection when the
HOT RESISTANCE
is programmed >
COLD
RESISTANCE
. The 239 assumes a NTC thermistor connection when the
COLD
RESISTANCE
is programmed
≥
HOT RESISTANCE
. PTC thermistors are preferred
because three PTC thermistors can be connected in series to monitor each of the
stator phases. This is not possible with NTC thermistors because all three thermistors
must be hot to obtain a fault indication. Select OFF if no thermistor is installed. If the
motor is still overheated after a trip, the thermistor signal will prevent restarting of the
motor by tripping the 239 immediately after reset. Thermistor temperature will be
displayed as either hot or cold because the thermistor is nonlinear. If the thermistor
function is to be used for process control, assign it to the auxiliary relay in which case
the auxiliary relay will activate but no message will be displayed.