P&B MPC2000D Manual page 39

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:
System Volts
FLC:
CT ratio
Starting
Start Time
Cold Withstand 22 Seconds
Hot Withstand
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
Trip time (secs)
From the Hot Curve for Hot/Cold Ratio =50% trip times can be calculated as follows:
Multiple of FLC
Trip Time (secs)
Page 36
100Kw
415 Volts
180 Amps
200/5 Amps
DOL
8 sec @ 6xFLC
11 Seconds
1.5 2.0 3.0
366 175 71
1.5 2.0 3.0
230 102 40
Issue 2 22/4/98
4.0 5.0 6.0
39 24 17
4.0 5.0 6.0
22 14 9.5
MPC2000D