P&B MPC2000+ Technical Manual page 38

MPC2000+ Technical Manual
Example Application
As an example of applying the MPC2000+ 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
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 Cold Thermal withstand Time, which in
this case is 80% x 22 sec = 17.6 secs. Therefore
From the standard MPC2000+ 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
4.0
Trip Time (secs)
22
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)
t c = 32.a.log e
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
't c is the operating time in seconds
(Running/Pre-load Condition)
Page 34
100Kw
415 Volts
180 Amps
200/5 Amps
DOL
1.5 2.0 3.0
366 175 71
1.5 2.0 3.0
5.0 6.0
230 102 40
14 9.5
( )
p 2
--------
p 2 - s 2
ISSUE 7 28/9/99
Start Time
Cold Withstand
Hot Withstand
set t6x = 17 secs and set Hot/Cold Ratio = 50%
4.0 5.0 6.0
39 24 17
8 sec @ 6xFLC
22 Seconds
11 Seconds
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