GE M60 Instruction Manual page 302

GROUPED ELEMENTS
This figure and the following procedure illustrate the construction of the voltage overload curves.
1. Draw a curve for the running overload thermal limit. The curve is one that has been selected in the relay as a
MODEL CURVE
2. Determine the point of intersection between the
unit current value of
3. Determine the locked rotor thermal limit point for the minimum voltage motor start. The coordinates of this point are
the per-unit current value of
1).
4. The line connecting points 1 and 2 constructs the acceleration curve for the system voltage level defined by the
VOLTAGE DEPENDENT MIN MOTOR VOLTS
is calculated from the following equation.
where
I is a variable multiplier of the motor rated current (values between I
I is a multiplier of the rated motor current (FLA) specified by the
1
t is a time value specified by the
1
I is a multiplier of the rated motor current (FLA) specified by the
2
t is a time coordinate of the intersection point between the thermal model curve and the vertical line corresponding
2
to the per-unit current value of the
5. Determine the point of intersection between the thermal model curve and the vertical line corresponding to the
5
multiplier of the rated current value of the
6. Draw the locked rotor thermal limit point for the 100% voltage motor start. The coordinates of this point are the
multiplier of the rated current value (FLA) of the
STALL TIME @ 100% V
7. The line connecting points 3 and 4 constructs the acceleration curve for the motor rated system voltage. The
acceleration time-current curve for the rated voltage starting is calculated from the same equations, but the setpoints
associated with the 100% voltage starting are applied.
8. The line connecting points 1, 3, and 5 represent the motor safe stall conditions for any system voltage from the
minimum to 110% of rated. Ideally, all the points on this line are characterized by the same thermal limit (I
equivalent starting impedance at reduced voltage is greater than the impedance at full voltage. As such, the higher
terminal voltages tend to reduce I
setpoints for rated and minimum voltage conditions. For voltage conditions above rated, the locked rotor
TIME
thermal limit and acceleration curve are extrapolated up to 110% of the terminal voltage. The point coordinates (I
for 110% are extrapolated based on the I
trip time computed from 110% V thermal limit value will be used.
The voltage dependent curve for current values above 8 times pickup (OF x FLA) are clamped and the time to
trip is frozen at the level calculated for the 8 times pickup current.
NOTE
The following three figures illustrate the resultant overload protection curve for minimum, 100%, and maximum line
voltages. For voltages between these limits, the M60 shifts the acceleration curve linearly and constantly, based on the
measured line voltage during a motor start.
5-174
.
VD ACCEL. INTERESECT @ MIN V
VD STALL CURRENT @ MIN VOLTS
setting. The acceleration time-current curve for the minimum voltage starting
trip_time = A
I
1
where 
= -------------------- -

ln
VD SAFE STALL TIME @ MIN V
VD ACCEL. INTERESECT @ MIN V
VD ACCEL. INTERESECT @ 100% V
setting (see point 3).
2
t. The rate of I
, T
1
THERMAL MODEL CURVE
(see point 2).
and the time value of
I  
–

e
FACTOR
I
–
2

and A = t e
FACTOR 1
 
t t
2 1
and I
1
VD STALL CURRENT @ MIN V
setting
VD ACCEL. INTERESECT @ MIN V
setting
VD STALL CURRENT @ 100% V
2
t reduction is dictated by the
, I , and T values. For starting currents at voltages higher than 110%, the
1 3 3
M60 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
and the vertical line corresponding to the per-
VD SAFE STALL TIME @ MIN V
I  
1
)
2
setting
setting
setting (see point 4).
setting and the time value of the
and
VD STALL CURRENT
THERMAL
(see point
Eq. 5-26
VD SAFE
2
t), but the
VD SAFE STALL
, T )
s s