GE M60 UR Series, Instruction Manual

Page: 285 / 662

GE Multilin M60 Motor Protection System 5-157
5 SETTINGS 5.6 GROUPED ELEMENTS
5
The following procedure, along with the figure above, 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
THERMAL MODEL CURVE
.
2. Determine the point of intersection between the
THERMAL MODEL CURVE
and the vertical line corresponding to the
per-unit current value of
VD ACCEL. INTERESECT @ MIN V
(see point 2).
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
VD STALL CURRENT @ MIN VOLTS
and the time value of
VD SAFE STALL TIME @ MIN V
(see point 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
setting. The acceleration time-current curve for the minimum voltage start-
ing is calculated from the following equation:
(EQ 5.26)
I is a variable multiplier of the motor rated current (values between I
1
and I
2
),
I
1
is a multiplier of the rated motor current (FLA) specified by the
VD STALL CURRENT @ MIN V
setting,
t
1
is a time value specified by the
VD SAFE STALL TIME @ MIN V
setting,
I
2
is a multiplier of the rated motor current (FLA) specified by the
VD ACCEL. INTERESECT @ MIN V
setting, and
t
2
is a time coordinate of the intersection point between the thermal model curve and the vertical line correspond-
ing to the per-unit current value of the
VD ACCEL. INTERESECT @ MIN V
setting.
5. Determine the point of intersection between the thermal model curve and the vertical line corresponding to the
multiplier of the rated current value of the
VD ACCEL. INTERESECT @ 100% V
setting (see point 4).
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
VD STALL CURRENT @ 100% V
setting and the time value of the
VD
SAFE STALL TIME @ 100% V
setting (see point 3).
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 set-
points associated with the 100% voltage starting will be 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
2
t ), but
the equivalent starting impedance at reduced voltage is greater than the impedance at full voltage. As such, the
higher terminal voltages tend to reduce
I
2
t . The rate of I
2
t reduction is dictated by the
VD STALL CURRENT
and
VD
SAFE STALL TIME
setpoints for rated and minimum voltage conditions. For voltage conditions above rated, the
locked rotor thermal limit and acceleration curve are extrapolated up to 110% of the terminal voltage. The point
coordinates (
I
s
, T
s
) for 110% are extrapolated based on the I
1
, T
1
, I
3
, and T
3
values. For starting currents at volt-
ages higher than 110%, the trip time computed from 110% V thermal limit value will be used.
The voltage dependent curve for current values above 8 times pickup (OF

FLA) will be clamped and the time
to trip will be frozen at the level calculated for the 8 times pickup current.
The following three figures illustrate the resultant overload protection curve for minimum, 100%, and maximum line
voltages. For voltages between these limits, the M60 will shift the acceleration curve linearly and constantly, based on
the measured line voltage during a motor start.
trip_time A
FACTOR
e
I
 
–

=
where

I
1
I
2
–
t
2
t
1
  
ln
-----------------------
=
and A
FACTOR
t
1
e
I
1
 

=
NOTE