ABB REM 610 - Safe Stall Time for Hot Starts

6.1.2.2. Safe stall time for hot starts
The safe stall time setting, t
6x
, is determined according to the start-up time of the
motor. The safe stall time can easily be determined from the trip curves at prior load
1 x FLA.
Normally, the safe stall time setting is selected to allow one hot or two cold starts.
The appropriate trip curve is selected by means of the start-up current and start-up
time (and margin) of the motor. If multiple hot starts are allowed, the total start-up
time instead of a single start-up time is to be used.
The safe stall time setting can be calculated as follows:
t
t
I
FLA
p
I
FLA
I
x
start
prior
6
22
32 15
100
=
×
⎛
⎝
⎜
⎞
⎠
⎟
−×
⎛
⎝
⎜
⎞
⎠
⎟
.ln
int int
sstart
FLA
int
.
⎛
⎝
⎜
⎞
⎠
⎟
−
⎧
⎨
⎪
⎪
⎩
⎪
⎪
⎫
⎬
⎪
⎪
⎭
⎪
⎪
2
1 1025
(6)
t = required operate time (i.e the number of hot
starts x start-up time of the motor and margin)
ln = natural logarithm
I
start
= start-up current of the motor
FLA
int
= internal FLA (equals the FLA of the motor at an
ambient temperature of 40°C)
p = weighting factor
I
prior
= prior load current (normally equal to the FLA of
the motor).
Cooling between starts is not considered in the formula above.
The operate time with the selected safe stall time setting can be calculated as
follows:
operate time =××
⎛
⎝
⎜
⎞
⎠
⎟
−×
32 15
100
6
2
.ln
int
t
I
FLA
p
I
F
x
start
prior
LLA
I
FLA
start
int
int
.
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
−
⎧
⎨
⎪
⎪
⎩
⎪
⎪
⎫
⎬
⎪
⎪
⎭
⎪
⎪
2
2
1 1025
(7)
128
REM 610REM 610
Motor Protection Relay
Technical Reference Manual - ANSI Version
1MRS755537