Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 113)

I
1,3
1,2
1,05
1,1
1,0
-5 0 20 40
[˚C]
Fig. 4.1-5
Tripping tolerances for temperature-compensated overload relays for motor protection under
IEC 60947-4-1
I
Overload as a multiple of the set current
δ Ambient temperature
Limit values under IEC 60947-4-1
Current setting
Usually, the motor protection relay should be adjusted to the rated current of the motor, for star-
delta starters to 0.58 · I
n
, as the measurement is made in series to the motor windings.
If the coolant temperature is over 40 °C, then the power of the motor should be reduced and the
current setting on the motor protection relay should be adjusted accordingly. If the motor
manufacturer does not specify otherwise, the values of
Tab. 4.1-1 can be used for correction.
Coolant temperature °C 30 35 40 45 50 55 60
Correction factor (k
1
) 1.08 1.04 1 0.95 0.9 0.85 0.8
Tab. 4.1-1
Typical values
for the correction factor for the current setting (k
1
· I
n
) on motor protection relays in relation
to the coolant temperature of the motor
At altitudes of installation over 1000 m above sea level the permissible motor loading is reduced
and hence also the setting of the motor protection relay has to be adjusted accordingly. If the
motor manufacturer does not specify otherwise, the values of
Tab. 4.1-2 can be used for
correction. If other coolant temperatures occur and at the same time the motor is operated at
high altitudes then the product of both factors should be taken into account when choosing the
current setting on the motor protection relay.
Installation altitude above
sea level [m]
≤ 1000 1500 2000 2500 3000 3500
Correction factor (k
2
) 1 0.97 0.94 0.90 0.86 0.82
Tab. 4.1-2
Typical values for the correction factors for the current setting (k
2
· I
n
) for motor protection relays in
relation to the operating altitude of the motor
Transient performance
The simulation of the thermal characteristic of the protected object at transient load conditions
by current-measuring protective devices is always an approximation and dependent on the
complexity of the protected object with respect to heating and cooling. Thus for example a
bimetal relay is much simpler in design than a motor and even complex electronic protective
devices only roughly approximate the characteristic of the protected object. Allowance is made
for this fact by precautionary making the protective device react more quickly as would be
LVSAM-WP001A-EN-P - April 2009 4-5