Multifunction Protection and Switchgear Control unit REF542plus
Manual: Motor Protection with ATEX-Certification
Where:
TMn Nominal motor temperature (permissible operating temperature)
IMn Nominal motor current referred to the nominal current of the cur-
rent transformer
Tini Initial temperature of the thermal memory after switching on the
auxiliary voltage .
TCOff Cooling-off time constant at I < 0.1 IMn (motor at standstill)
TCNormal Time constant at 0.1 IMn < I < 2 IMn (motor in normal operation)
TCOverheat Time constant at I > 2 IMn (motor during startup/accelerating)
Ttrip Temperature for tripping
Twarn Temperature for warning
Tenv Environment temperature
Trst Temperature after resetting the function
4.4.1.1 Setting the time constant:
For setting the time constant, it is assumed that the motor is rotor-critical. Further-
more, it is assumed that the preset thermal capacity has been reached if, after expira-
tion of the blocking time, the cold motor still remains rotor-locked. Therefore, equation
(9) delivers the following relationship:
2
2
MnA
2
MnA
e
)(
)(
ln
)cold(
kII
II
t
−
=
τ
(11.1)
where:
τ Heating time constant to be calculated for rotor-critical motors and which
can be equated to the setting for "TCOverheat“
te (cold) Permissible maximum blocking time for cold motor condition
IA Blocking current or maximum starting current
IMn Nominal motor current
k Overload constant which
If according to the latest standard the blocking time of the motor in warm condition is
given, then the calculation can be done according to the following equation:
2
2
MnA
2
MnA
e
)(
1)(
ln
)warm(
kII
II
t
−
−
=
τ
(11.2.)
According to the standard the warm condition is defined, if the motor reach the steady
state condition during the operation with the nominal motor current. That is why the
preload condition is equal to 1.
1VTA100114-Rev. 04 en PTMV, 19.01.05 Motorprotection 21 / 66
Valid from Version V4D02
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