sepam 20 user manual - Schneider Electric

Protection functions Thermal overload
ANSI code 49RMS
Description
This function is used to protect equipment (motors,
transformers, generators, lines, capacitors) against
overloads, based on measurement of the current
consumed.
Operation curve
The protection gives a trip order when the heat rise E,
calculated according to the measurement of an
equivalent current Ieq, is greater than the set point Es.
The greatest permissible continuous current is
MT10858
The protection tripping time is set by the time
constant T.
b the calculated heat rise depends on the current
consumed and the previous heat rise state
b the cold curve defines the protection tripping time
based on zero heat rise
b the hot curve defines the protection tripping time
based on 100 % nominal heat rise.
Alarm set point, tripping set point
Two set points may be set for heat rise:
b Es1: alarm
b Es2: tripping.
"Hot state" set point
When the function is used to protect a motor, this fixed
set point is designed for detection of the hot state used
by the number of starts function.
Heat rise and cooling time constants
MT10419
I = Ib Es
10 1
10 0
10 -1
10 -2
10 -3
E
1
0,63
0
0 5 10
T1
Heat rise time constant. Cooling time constant.
PCRED301005EN - 06/2008
t
MT10420
Cold curve
⎛leq -------- ⎞
t ⎝ lb ⎠
-- L n
T
2
⎛leq -------- ⎞
⎝ lb ⎠
2
= -------------------------------
– Es
Hot curve
⎛leq -------- ⎞
t ⎝ lb ⎠
-- L n
T
2
– 1
⎛leq -------- ⎞
⎝ lb ⎠
2
= -------------------------------
– Es
E
1
0,36
0
T2
t
For self-ventilated rotating machines, cooling is more effective when the machine is
running than when it is stopped. Running and stopping of the equipment are
calculated from the value of the current:
b running if I > 0.1 Ib
b stopped if I < 0.1 Ib.
Two time constants may be set:
b T1: heat rise time constant: concerns equipment that is running
b T2: cooling time constant: concerns equipment that is stopped.
Accounting for harmonics
The current measured by the thermal protection is an RMS 3-phase current which
takes into account harmonics up to number 17.
Accounting for ambient temperature
Most machines are designed to operate at a maximum ambient temperature of 40 °C
(104 °F). The thermal overload function takes into account the ambient temperature
(Sepam equipped with the temperature sensor module option, with sensor no. 8
assigned to measurement of ambient temperature) to increase the calculated heat
rise value when the temperature measured exceeds 40 °C (104 °F).
Tmax – 40°C
Increase factor: fa=
----------------------------------------------------
Tmax – Tambient
in which T max is the equipment’s maximum temperature
(according to insulation class)
T ambient is the measured temperature.
Adaptation of the protection to motor thermal withstand
Motor thermal protection is often set based on the hot and cold curves supplied by
the machine manufacturer. To fully comply with these experimental curves, additional
parameters must be set:
b initial heat rise, Es0, is used to reduce the cold tripping time.
⎛leq -------- ⎞
t ⎝ lb ⎠
modified cold curve: -- L n
T
2
– Es0
⎛leq -------- ⎞
⎝ lb ⎠
2
= ----------------------------------
– Es
b a second group of parameters (time constants and set points) is used to take into
account thermal withstand with locked rotors. This second set of parameters is taken
into account when the current is greater than an adjustable set point Is.
Accounting for negative sequence current
In the case of motors with coiled rotors, the presence of a negative sequence
component increases the heat rise in the motor. The negative sequence component
of the current is taken into account in the protection by the equation
leq lph 2 K li 2
= + ⋅
in which Iph is the greatest phase current
Ii is the negative sequence component of
the current
K is an adjustable factor
K may have the following values: 0 - 2.25 - 4.5 - 9
For an asynchronous motor, K is determined as follows:
K 2 Cd
-------
1
Cn ld
g ⎛----- ⎞
⎝lb⎠ 2
= ⋅ ⋅ --------------------- – 1
⋅
in which Cn, Cd: rated torque and starting torque
Ib, Id: basis current and starting current
g: rated slip.
Saving of heat rise
When the protection trips, the current heat rise, increased by 10 %, is saved
(Increasing by 10 % makes it possible to take into account the average heat rise of
motors when starting). The saved value is reset to zero when the heat rise decreases
sufficiently for the time before starting to be zero. The saved value is used when the
power returns after a Sepam power failure, so as to start up again with the heat rise
that triggered tripping.
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