Application Guide - NP900 Series
226 (504
)
A996A
3.3.2
T
HERMAL OVERLOAD PROTECTION FOR MACHINES
T
M
>
(49M)
Thermal overload function for machines (TOLM) is used for electric machines like
synchronous and asynchronous motors and generators thermal capacity monitoring and
protection. This function can also be used for any single or multiple time constant
applications like inductor chokes, certain types of transformers and any other static units
which don’t have active cooling in addition to the cables and overhead lines.
TOLM function constantly monitors phase TRMS currents (including harmonics up to 31
st
)
instant values and calculates the set thermal replica status in 5 ms cycles. TOLM function
includes total memory function of the load-current conditions according to IEC 60255-8
TOLM function is based into thermal replica, which represents the protected object
thermal loading in relation to the effective current in the object. Thermal replica includes
the calculated thermal capacity used in the “memory” since it is integral function which
tells apart this function from normal overcurrent function operating principle for the
overload protection applications.
In heating/cooling situation the thermal image for TOLM function is calculated according to
equations described below:
Long time constant thermal image calculation:
θ
tL
=
��θ
t
-
1
-
�
I
EM
I
N
× k
SF
× k
AMB
�
2
× e
-
t
τ
1h
/
τ1c0
/
τ1cr
�
+
�
I
EM
I
N
× k
SF
× k
AMB
�
2
�
× (1-W
f
)
Short time constant thermal image calculation:
θ
tS
=
��θ
t
-
1
-
�
I
EM
I
N
× k
SF
× k
AMB
�
2
× e
-
t
τ
2h
/
τ2c
�
+
�
I
EM
I
N
× k
SF
× k
AMB
�
2
�
× W
f
, where
I
EM
=
���
1 +
��
I
2
I
1
�
2
× k
NPS
��
× I
MAX
2
�
, the effective current of the protected object including
true RMS measurement maximum phase current and possible phase current unbalance
condition.
θ
t-1
= Thermal image status in previous calculation cycle (the memory of the function)
I
MAX
= Measured maximum of the three TRMS phase currents
