Starting Duty Inhibits - GE MM300 Instruction Manual

CONTROL ELEMENTS

Starting duty inhibits

NOTE:
NOTE
5–58
The following setpoint is available:
[Path: Home > Setpoints > Control > Starter]
External Stop Action
Range: Stop, Trip
Default: Stop
This setpoints selects whether an external stop is considered to be an emergency stop
(reset required to clear) or a stop control (no reset required).
The MM300 provides four elements that guard against excessive starting duty:
• Thermal start inhibit
• Starts per hour inhibit
• Time between starts inhibit
• Restart inhibit
The thermal start inhibit function inhibits 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 Start Inhibit Margin setpoint. If this value is "0", starts are inhibited until thermal
capacity used decays to a level of 15%. If this setpoint is greater than zero, starts are
inhibited while the available thermal capacity is greater 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 value of thermal capacity used
calculated by the thermal model from the last five successful starts, plus a user-defined
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. If the relay does not contain records of five successful
starts, a value of 85% is used for the learned thermal capacity used, which requires the
thermal capacity used to decay to the same 15% level required when the margin setpoint
is zero.
In addition, since a 2% safety margin is included for the relay to determine if Thermal
Capacity Used has decreased enough (ie - the motor has cooled enough) to allow another
restart, in the case where the Start Inhibit margin is set to 0%, Thermal Capacity must
actually reduce to 15% - 2% = 13%.
For example, if the thermal capacity used for the last five starts is 24, 23, 30, 22, and 21%
respectively, and the set margin is 10%, the learned starting capacity used at start is:
Maximum[24%, 23%, 30%, 22%, 21%] /5 x (1 + 10%/100%) = 33%.
If the motor stops with a thermal capacity used of 90%, a start inhibit will be issued until
the motor cools to 100% – 33% - 2% = 65%. If the stopped cool time constant is set to 30
minutes, the inhibit time will be:
30 x ln (90% / 65%) = 9.8 minutes
If the margin is set to zero instead, the inhibit time will be:
30 x ln (90% / (15% - 2%)) = 58 minutes
The starts per hour element defines the number of start attempts allowed in any 60 minute
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.
The time between starts element enforces a programmable minimum time duration
between two successive starts attempts. A time delay is initiated with every start and a
restart is not allowed until the specified interval has elapsed. This timer feature may be
useful in enforcing the duty limits of starting resistors or starting autotransformers. It may
also be used to restrict jogging.
MM300 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS