Page
34
Issue 2 22/4/98
MPC2000D
15. MPC2000D Thermal Overload Protection
MPC2000D Thermal Operating Curves
The curves shown are the time/current operating
characteristics for the MPC2000D Thermal Overload
protection.
Curve 1 :Cold Curve
Overload Setting =105%
Curve 2 :Hot Curve
Overload Setting =105%
Running From 100% FLC
Hot/Cold Ratio = 80%
Curve 3 : Hot Curve
Overload Setting:=105%
Running From 100% FLC
Hot/Cold Ratio = 50%
Curve 4 : Hot Curve
Overload Setting:=105%
Running From 100% FLC
Hot/Cold Ratio = 20%
Explanation
The MPC2000D simulates the thermal condition of the motor
by means of a thermal register. The heating of the register is
related to the square of the largest of the three line currents.
The rate of cooling of the thermal register is directly related
to the rate of heating. The value of the thermal register is
called thermal capacity and it is used to simulate motor
temperature.
100 percent thermal capacity means the motor temperature has
reached the maximum allowed and is the level at which an
overload trip will occur.
When the motor is stopped for a long period of time the
thermal capacity used is zero, this is known as the 'cold
condition', and the motor has 100 percent of it's thermal
capacity available for heating before a trip will occur.
When a motor starts and is running it's temperature increases,
after running at normal FLC for a period of time the motor
will have reached a hot condition and a lower value of thermal capacity will be available.
The Hot/Cold Ratio setting determines the amount of available thermal capacity for a motor running at FLC i.e. hot
condition, this is set as a percentage ratio of the amount of thermal capacity available at the cold condition which is
always 100%.
The thermal capacity available for a motor running at normal FLC i.e. Hot condition is the Hot/Cold Ratio setting.
For example if the Hot/Cold ratio setting is 60% and the motor has been running at normal FLC for some time the
amount of thermal capacity available is 60% then the amount displayed by the MPC2000D is the amount used which
is 100 - 60 = 40%.
1
2
3
4
5
6
7
8 9
10
15
Multiples of FLC
200
100
t
10
0.01
0.1
0.2
1
2
20
0.02
