Start Inhibit - GE 339 Instruction Manual

CHAPTER 6: SETPOINTS

Start inhibit

NOTE:
NOTE
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
Thermal Start Inhibit
This function is provided to inhibit starting of a motor if there is insufficient thermal
capacity available for a successful start. The motor start inhibit logic algorithm is defined
by the Thermal Inhibit setpoint. If this setpoint is set to "0", starts are inhibited until thermal
capacity used decays to a level of 15%. If this setpoint is set greater than zero, starts are
inhibited while the available thermal capacity is less than the learned thermal capacity
used at start.
The margin should be set to zero if the load varies for different starts.
The learned thermal capacity used at start is the largest thermal capacity used value
calculated by the thermal model from the last five successful starts, plus a settable margin.
The margin is a percentage of this largest of five. A successful motor start is one in which
the motor reaches the Running state. See the Start/Stop section of this manual for a
description of Running state logic. When the motor information is reset, a value of 85% is
used for the learned thermal capacity used until displaced by 5 subsequent successful
starts. This 85% default requires the thermal capacity used to decay to the same 15% level
required when the margin setting is zero.
For example, if the thermal capacity used for the last 5 starts is 24, 23, 27, 26 and 20%
respectively, and the set margin is 25%, the learned starting capacity used at start is
Maximum(24%+23%+27%+26%+20%) x (1+25%/100%) = 34%. If the motor stops with a
thermal capacity used of 90%, a start inhibit will be issued until the motor cools to 100% -
34% = 66%. If the stopped cool time constant is set to 30 minutes, the inhibit time will be:
If instead the set margin is zero, the inhibit time will be:
Starts per Hour Inhibit
This element defines the number of start attempts allowed in any 60 minutes interval.
Once the set number of starts has occurred in the last 60 minutes, start controls are
inhibited until the oldest start contributing to the inhibit is more than 60 minutes old.
This element assumes a motor start is occurring when the relay measures the transition of
no motor current to some value of motor current. At this point, one of the Starts/Hour
timers is loaded with 60 minutes. Even unsuccessful start attempts will be logged as starts
for this feature. Once the motor is stopped, the number of starts within the past hour is
compared to the number of starts allowable. If the two numbers are the same, the Start
Inhibit Output Relay will be activated to block the motor start. If a block occurs, the lockout
time will be equal to the longest time elapsed since a start within the past hour, subtracted
from one hour.
For example, if STARTS/HOUR LIMIT is programmed at "2":
• One start occurs at T = 0 minutes
• A second start occurs at T = 17 minutes
• The motor is stopped at T = 33 minutes
• A block occurs
• The lockout time would be 1 hour – 33minutes = 27 minutes
Time Between Starts Inhibit
S4 CONTROLS
Eq. 23
Eq. 24
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