Schweitzer Engineering Laboratories SEL-701, Инструкция по эксплуатации

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701 Motor Protection Relay
Date Code 20010719
Motor Thermal Element
Purpose of Motor Thermal Protection
E.4
Figure E.1 Motor Thermal Limit
Characteristic Plotted With Motor Starting Current.
Figure E.1 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 due to current
unbalance. Unfortunately, neither of these elements accounts for the motor thermal
history or track temperature excursions. The SEL-701 Relay thermal element, with its
integrated design, offers distinct advantages over the use of discrete elements.
The SEL-701 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.
0 2 4 6 8 10
1
10
100
Current in Multiples of Full Load Amps
Tim
e in
S
econ
ds
1000
Overload
Running
Current
Locked Rotor
Starting
Current