15/05/96
37
MPC2000 01/96B
The t6x setting must now be calculated. This is set as the time taken to trip at 6 x FLC for the Cold Starting condition
and is set in conjunctio n with the Hot/Cold Ratio setting.
For the above typical motor the Hot Withstand time is 11 seconds, the Cold Withstand is 22 seconds, therefore the
Hot/Cold ratio is calculated as 11/22 x 100 = 50%.
As a general rule the relay should be set to trip at no closer than 80% of the Thermal withstand Time, which in this
case is 80% x 22 sec = 17.6 secs. Therefore set t6x = 17 secs and set Hot/Cold Ratio = 50%
From the standard MPC2000 Thermal Operating Curves the Cold curve trip times can be calculated by mu ltiplying the
setting t6x and the factor 't' for various multiples of FLC i.e.:
Multiple of FLC
1.5
2.0
3.0
4.0
5.0
6.0
Trip time (secs)
366
175
71
39
24
17
From the Hot Curve for Hot/Cold Ratio =50% trip times can be calculated as follows:
Multiple of FLC
1.5
2.0
3.0
4.0
5.0
6.0
Trip Time (secs)
230
102
40
22
14
9.5
The curves shown on the right are the
actual curves for the calculated figures and
show that the setting has been chosen to
provide close protection to take advantage
of the motors capability and allow the
possibility of an immediate restart from the
Hot condition. However alternative
settings can be made to provide faster
tripping limited by the conditions of
ensuring the motor can run up speed
under healthy conditions without tripping.
If the Hot to Cold ratio is set to a figure
other than the 3 standard curves drawn,
then either the formula quoted below can
be used or an approximation of the Hot
curve can be drawn to calculate trip times.
(Starting/Cold Condition)
tc = 32.a.loge
( p2 )
--------
p2 - s2
where 'p' is multiple of motor FLC (per unit
value)
's' is overload setting in per unit
value e.g. 105%
'a' is setting of t6x in seconds
'tc is the operating time in
seconds
