Page
36
Issue 2 22/4/98
MPC2000D
Example Application
As an example of applying the MPC2000D Thermal Overload protection to a motor let us establish the settings for a
particular motor having the following characteristics:
Rating:
100Kw
System Volts
415 Volts
FLC:
180 Amps
CT ratio
200/5 Amps
Starting
DOL
Start Time
8 sec @ 6xFLC
Cold Withstand 22 Seconds
Hot Withstand
11 Seconds
The figures for Line Voltage, Motor FLC and CT Primary should be entered. For the Overload setting we will use the
default setting of 105%.
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 conjunction 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 MPC2000D Thermal Operating Curves the Cold curve trip times can be calculated by multiplying
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
