F.2
SEL-749M Relay
Instruction Manual
Date Code 20080918
Motor Thermal Element
Purpose of Motor Thermal Protection
➤
Provides a trip output if the present heat estimate exceeds the
present trip threshold.
➤
Provides an alarm output if the present heat estimate exceeds
the present alarm threshold (user settable as a percentage of the
trip threshold).
➤
Adjusts the present trip threshold based on RTD ambient
temperature measurement when enabled.
Purpose of Motor Thermal Protection
A typical induction motor draws six times the full load current for which it is
rated when starting. This high stator current induces a comparably high
current in the rotor. The rotor resistance at zero speed typically is three times
the rotor resistance when the motor is at rated speed. Thus, the I
2
r heating in
the rotor is approximately 6
2
• 3 or 108 times the I
2
r heating when the motor
runs normally. Consequently, the motor must tolerate an extreme temperature
for a limited time in order to start. Manufacturers communicate the motor
tolerance through the maximum locked rotor time and locked rotor amps
specifications for each motor. In a similar manner, the motor manufacturer
communicates the ability of the motor to operate under continuous heavy load
through the service factor specification.
The purpose of motor thermal protection is to allow the motor to start and run
within the manufacturer’s published guidelines but trip if the motor heat
energy exceeds those ratings because of overloads, negative-sequence current,
or locked rotor starting.
shows a typical motor thermal limit characteristic plotted with the
motor starting current. Some motor protection applications use an inverse-
time phase overcurrent element to provide locked rotor and overload
protection along with a separate negative-sequence overcurrent relay to
prevent overheating resulting from current imbalance. Unfortunately, neither
of these elements accounts for the motor thermal history or track temperature
excursions. The SEL-749M Relay thermal element, with its integrated design,
offers distinct advantages over the use of discrete elements.
The SEL-749M Relay thermal element always operates in one of two modes:
starting or running. In starting mode, the thermal element provides locked
rotor protection, allowing the motor to absorb the high energy of the I
2
t
threshold represented by the rated locked rotor current and time. In running
mode, the thermal element provides overload and unbalance protection by
limiting the motor heat energy estimate to a value represented by the service
factor and two other motor parameters.
Содержание SEL-749M
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