4–336 869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CONTROL CHAPTER 4: SETPOINTS
Thermal Inhibit This function is provided to inhibit starting of a motor if there is insufficient thermal
capacity (TC) available for a successful start. The thermal capcity required for a successful
start or the learned thermal capacity used at start (TC
L
) is calculated even if the Thermal
Start Inhibit element is disabled.
The information about learned thermal capacity is stored in a non-volatile memory and it
is available after the power is removed from the 869 . The TC
L
is the largest thermal
capacity used value from a number (N) of most recent successful starts. “N” is set in
Setpoints > System > Motor > Setup > Number of Starts to Learn. A successful motor start is
one in which the motor reaches the Running state. See the Motor Status section of this
manual for a description of Running state logic. When the start history is not available or
“Clear Learned TCU” command (in Records > Clear Records) is executed, a value of 85% is
used for the learned thermal capacity used until displaced by the largest of the “N”
subsequent successful starts. This 85% default requires the thermal capacity used to
decay to the 15% level before the start is allowed.
Starts are inhibited while the thermal capacity is greater than the adjusted thermal
capacity (TC
ADJ
) subtracted from 100%. TC
ADJ
is equal to the learned thermal capacity
used at start (learned start TCU or TC
L
) increased by the margin (TC Used Margin). The
formula is TC
ADJ
= TC
L
+ (TC Used Margin / 100%) x TC
L
.
For example, if the thermal capacity used for the last 5 starts is 24, 23, 27, 26 and 20%
respectively, the learned starting capacity used at start is the Maximum of (24%, 23%,
27%, 26%, 20%) = 27%. If the set margin is 25%, the adjusted thermal capacity learned
value (TC
ADJ
) is calculated as 27% x (1+25%/100%) = 33.75%. A start inhibit is issued until
the motor current TCU decays to 100% - 33.75 = 66.25%. For more details, please see the
Thermal Start Inhibit logic diagram.
The Thermal Lockout Time calculation is based on the values of TCU, TC
ADJ
and the Cool
Time Constant Stopped (CTCS). The latter is set in
Setpoints > Protection > Group 1 > Motor
> Thermal Model. The formula is shown in on the Thermal Start Inhibit logic diagram:
If, for the example above, the Cool Time Constant Stopped = 30 Minutes and the motor TCU
= 90%, the lockout time is: 30 x ln (90% / 66%) = 9.3 minutes
If the start history is not available, the inhibit time is: 30 x ln (90% / 15%) = 54 minutes
The 869 constantly displays the Thermal Lockout Time in Status > Motor menu even if the
motor is neither stopped nor tripped.
If the Emergency Restart input is asserted during a Thermal Start lockout, the TCU is set to
zero and the Thermal Trip OP is reset. This causes resetting of Thermal Lockout Time to
zero and dropout of the Start Inhibit and Thermal Inhibit OP and allows a new start.
In the event of a real emergency, the Emergency Restart input operand must remain
asserted at logic 1 until the emergency is over. The Thermal Inhibit OP and Start Inhibit
operands will remain reset until the Emergency Restart Input is de-asserted. However,
calculation of Thermal Lockout Time continues after resetting to zero regardless of the
duration of the Emergency Restart input.
Path: Setpoints > Control > Motor Starting > Start Supervision > Thermal Inhibit
FUNCTION
Range: Disabled, Enabled
Default: Disabled
The element works as described at the beginning of this chapter if the Function is set to
Enabled. If the Function is set to Disabled, the element is not functional unless the motor
thermal capacity has reached 100% and the thermal model trip function is enabled. In
that case, the Thermal Start Inhibit operates and the lockout time is approximately
190% of the Cool Time Constant Stopped. After the lockout time expires, the TCU will
decay to 15% and a new start will be allowed.
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