GE 369, Быстрый старт

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4–38 369 MOTOR MANAGEMENT RELAY – QUICKSTART GUIDE
S3 OVERLOAD PROTECTION CHAPTER 4: KEYPAD SETPOINTS
Note
Above 8.0 x Pickup, the trip time for 8.0 is used. This prevents the overload curve from
acting as an instantaneous element.
The Standard Overload Curves equation is:
(EQ 4.2)
FIGURE 4–3: Custom Curve Example
Motor Cooling
The thermal capacity used quantity is reduced in an exponential manner when the motor
is stopped or current is below the overload pickup setpoint. This reduction simulates motor
cooling. The motor cooling time constants should be entered for both the stopped and
running cases. A stopped motor will normally cool significantly slower than a running
motor. Note that the cool time constant is one fifth the total cool time from 100% thermal
capacity used down to 0% thermal capacity used.
The 369 can learn and estimate the stopped and running cool time constants for a motor.
Calculation of a cool time constant is performed whenever the motor state transitions
from starting to running or from running to stopped. The learned cool times are based on
the cooling rate of the hottest stator RTD, the hot/cold ratio, the ambient temperature (40 if
no ambient RTD), the measured motor load and the programmed service factor or
overload pickup. Learned values should only be enabled for motors that have been started,
stopped and run at least five times.
Note that any learned cool time constants are mainly based on stator RTD information.
Cool time, for starting, is typically a rotor limit. The use of stator RTDs can only render an
approximation. The learned values should only be used if the real values are not available
from the motor manufacturer. Motor cooling is calculated using the following formulas:
(EQ 4.3)
(EQ 4.4)
where: TCused = thermal capacity used
TCused_start = TC used value caused by overload condition
TCused_end = TC used value set by the hot/cold curve ratio when motor is
running = '0' when motor is stopped.
t = time in minutes
= cool time constant (running or stopped)
I
eq
= equivalent motor heating current
overload_pickup = overload pickup setpoint as a multiple of FLA
hot/cold = hot/cold curve ratio
RTD Bias
The 369 thermal replica operates as a complete and independent model. The thermal
overload curves however, are based solely on measured current, assuming a normal 40°C
ambient and normal motor cooling. If there is an unusually high ambient temperature, or if
motor cooling is blocked, motor temperature will increase. If the motor stator has
embedded RTDs, the 369 RTD bias feature should be used to correct the thermal model.
time_to_trip
curve_multiplier 2.2116623 ×
0.02530337 pickup 1
–
( )
2
× 0.05054758 pickup 1
–
( ) ×
+
----------------------------------------------------------------------------------------------------------------------------------------------- =
TCused TCused_start TCused_end
–
( ) e
t
–
τ ⁄
( ) ⋅ TCused_end
+ =
TCused_end Ieq 1
hot
cold
---------- –
⎝ ⎠
⎛ ⎞
100% × ×
=
τ