Start Protection; Thermal Overload Curves - GE 339 Instruction Manual

S3 PROTECTION

Start protection

Thermal overload
curves
6–36
If enabled, Start Protection is used to determine the Time to Trip value while the motor
status is "starting," using the formula:
where:
I is the equivalent motor heating current in per-unit on an FLA base, which will be
eq
discussed in the unbalance biasing section.
I is the Locked rotor current in per-unit on an FLA base.
LR
t is the Safe Stall Time Cold in seconds.
LRCold
In some applications where the characteristics of the starting thermal damage curve
(locked rotor) and the running thermal damage curves fit together very smoothly, the
thermal overload curve can in these cases provide both starting and running protection, so
start protection is not required. Therefore, the start protection can be disabled or enabled
as required. When start protection is disabled, the thermal overload curve determines time
to trip during both starting and running.
The start protection is disabled by setting setpoint START PROTECTION to OFF or to any
assignable contact input that is off when start protection is not required.
The thermal overload curves can be either standard or customized. The standard overload
curves are a series of 15 curves with a common curve shape based on typical motor
thermal limit curves. The customized curve (FlexCurve) is used to more closely tailor motor
protection to the thermal limits so the motor may be started successfully and used to its
full potential without compromising protection.
THERMAL OVERLOAD STANDARD CURVE
If the motor starting times are well within the safe stall times, it is recommended that the
339 standard overload curve be used. The standard overload curves are a series of 15
curves, each a multiple from 1 to 15 of a common curve shape based on typical motor
thermal limit curves. The curve gives a Time to Trip for the equivalent motor heating
current, and incorporates hot/cold biasing, and unbalance biasing.
The standard curve is defined by the following equation, which is graphed and tabulated
below. The curve reflects the fact that under overload conditions, heating largely swamps
cooling, and that the heating is due primarily to resistive losses in the stator and rotor
windings, said losses being proportional to the square of the current.
where:
Time to Trip is the amount of time, in seconds, the relay will take to trip, given that the
motor starts cold and the current is constant.
Curve Multiplier is the value of the Curve Multiplier setpoint.
I is the equivalent motor heating current per-unit on an FLA base. However, the value of
eq
I is limited in this equation, to 8.0, in order to prevent the overload from acting as an
eq
instantaneous element, and responding to short circuits.
For example, a motor with a stall current (also known as locked rotor current) of 8 times its
FLA, with a Curve Multiplier of 7, if stalled from a cold state, would trip in:
This would respect a Safe Stall Time Cold of 10 seconds.
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS
Eq. 4
Eq. 5
Eq. 6