Schweitzer Engineering Laboratories SEL-749M, Инструкция по эксплуатации

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4.7
Date Code 20080918 Instruction Manual SEL-749M Relay
Protection and Logic Functions
Basic Motor Protection
EXAMPLE 4.4 Acceleration Factor (TD1) Setting Calculation
In a particular application, a motor with a 10 second hot locked rotor
time always starts in 5 seconds.
Setting the TD1 equal to 0.75 causes the relay to trip in 7.5 seconds
under locked rotor conditions. This setting allows ample time for the
motor to start, but does not subject the motor to the full 10 seconds
of locked rotor current if a locked rotor start attempt takes place.
Curve Thermal Method
The Curve method is similar to the Rating method except in the following
ways:
➤
You select one of the 45 available standard motor overload/
locked rotor curves, set the motor rated Full Load Amps and
Service Factor. Based on the curve number, the relay
determines and hides LRA, LRTHOT, TD, and RTC settings
automatically.
➤
The relay does not raise the initial heat estimate at starting
(motor current > 2.5 per unit of FLA); however, the relay does
ramp up the trip value to the starting element threshold. The
locked rotor trip time, for a motor at ambient temperature, will
be 120 percent of the time for the same motor at operating
temperature.
Figure 4.2 shows several of the available curves. Refer to Equation F.10
through Equation F.17 in Appendix F: Motor Thermal Element for these and
additional curves. Be sure that the standard curve you select trips in a time less
than or equal to the motor rated locked rotor time at locked rotor current. Each
increase in the curve number yields a 2.5-second increase in the curve thermal
limit time at six times full load current. For a cold rotor, the curve 10 trip time
at six times full load current is 25 seconds.
Table 4.5 shows the thermal limit time versus current for several curves. For a
hot rotor, each increase in the curve number yields a 2.08 second increase in
the hot rotor thermal limit time at six times full load current. Continue
calculating the balance of thermal element settings with overload settings in
Table 4.6 .
EXAMPLE 4.5 Thermal Element Curve Method Setting
A 4160 V, 800 HP motor is to be protected through use of the
SEL-749M Thermal Element Curve Method. The motor data sheet
includes the following information.
Rated Horsepower (HP) = 800 HP
Rated Voltage (V) = 4160 V
Rated Full Load Current (A) = 101.0 A
Rated Locked Rotor Amps (A) = 620.4 A
Safe Stall Time, Hot = 30 seconds
Service Factor = 1.15
Each increase in generic curve number increases the hot rotor
thermal limit time by 2.08 seconds at six times full load current.
Therefore, we can select the maximum curve number by using the
following equation.
Curve
Safe Stall Time, Hot (seconds)
Locked Rotor Amps
FLA
-------------------------------------------------
⎝ ⎠
⎛ ⎞
2
•
2.08 seconds 36
•
----------------------------------------------------------------------------------------------------------------------------------------- =
Curve = 15.12; select curve 15 or less
Courtesy of NationalSwitchgear.com