Schweitzer Engineering Laboratories SEL-749M, Instruction Manual

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4.20
SEL-749M Relay Instruction Manual Date Code 20080918
Protection and Logic Functions
PTC/RTD-Based Protection
Speed Switch
(Stalling During
Start) Function
When the motor is equipped with a speed switch, you may want to provide
additional locked rotor protection by using the relay speed switch input. When
the SS TRIP DELAY is set, the relay will trip if the speed switch is not closed
SPDSDLYT seconds after the motor start begins. A separate delay,
SPDSDLYA, can also be set to provide a warning before the speed switch trip.
Figure 4.10 shows typical currents during motor start (normal and stall during
start) and the SS TRIP DELAY time setting. Figure 4.11 shows the speed
switch logic.
Figure 4.10 Stalling During Starting
Figure 4.11 Speed Switch Logic
PTC/RTD-Based Protection
Thermistor (PTC)
Input Function
You can connect up to six thermistors detectors (PTC) to the SEL-749M. The
detectors are typically embedded in the stator winding of the motor, and they
monitor the actual temperature of the winding. This function is independent of
the thermal model and accounts for conditions such as ambient temperature,
obstructed cooling, etc. The detectors and their leads are monitored for short
circuits.
Table 4.18 Speed Switch Settings
Setting Prompt Setting Range Setting Name := Factory Default
SS TRIP DELAY Off, 1–240 sec SPDSDLYT := OFF
SS WARN DELAY Off, 1–240 sec SPDSDLYA := OFF
NOTE:
In addition to setting the SS
DELAY, you must connect the speed
switch contact to an input assigned to
the speed switch (see
Table 4.32 and
Figure 2.6 , Figure 2.15 for connection
diagrams).
I
1
2
I
t
v
t
FLA
FLA
1 =
2 =
t
v
=
Normal start without hindrance
by high overload or stalling
Stalling during standing
SS TRIP DELAY time
STARTING
SPEEDSW
SPDSAL
SPDSTR
SPDSDLYx
0
Relay
Word
Bits
Relay
Word
Bits
Motor is Starting
Speed Switch Input
Speed Switch Alarm STARTING =
SPEEDSW =
SPDSAL =
Speed Switch Trip
Speed Switch Delay
A or T SPDSTR =
SPDSDLYx =
x =
Table 4.19 PTC Settings
Setting Prompt Setting Range Setting Name := Factory Default
PTC ENABLE Y, N EPTC := N
NOTE:
In addition to enabling the
PTC function, you must also connect
at least one thermistor to the relay
(see
Figure 2.6 for a connection
diagram).
Courtesy of NationalSwitchgear.com