Schweitzer Engineering Laboratories SEL-749M, Инструкция по эксплуатации

Страница: 281 / 314

F.7
Date Code 20080918 Instruction Manual SEL-749M Relay
Motor Thermal Element
Motor Running Protection
Motor Running Protection
When the motor is running, it returns heat energy to the surroundings through
radiation, conduction, convection, and, in some cases, forced cooling. The
motor running thermal element provides a path for that energy return through
the resistor, R, as shown in
Figure F.5 .
Figure F.5 Motor Running Thermal Element With Resistance and Trip Level
Undefined
To determine the value of that resistor, recall that the motor will reach an
energy level representing the motor rated operating temperature when 1 per
unit of positive-sequence current flows in the motor for a long time. Because
the positive-sequence heat factor, K
1
, is 1 in the running model, and 1 per unit
of I
1
squared equals 1, the value of resistor R equals the energy level
representing the motor rated operating temperature.
To determine the normal operating energy, recall that many motor data sheets
publish two locked rotor trip times: one longer time (referred to as Ta) when
the motor is started from ambient temperature and one shorter time (To) when
the motor is started from operating temperature.
Figure F.6 shows a graphical representation of the problem and corresponding
solution. The motor normal operating energy is the difference between the
ambient and operating temperature locked rotor times, multiplied by locked
rotor current squared. For those motors without published separate locked
rotor times, assume that the locked rotor trip energy is approximately six times
the operating energy in the relation.
Figure F.6 Calculating the Normal Operating Energy by Using Locked Rotor
Trip Times
_
+
3
I
1
2
+ 5 • I
2
2
Thermal Trip Level
U
49T
Relay
Word
Bit
I
L
2 • Ta I
L
2 • To
Normal Operating Energy
I
L
2 • (Ta – To)
Base or Ambient Energy
Locked Rotor Trip Energy
Courtesy of NationalSwitchgear.com