Motor Protection Functions - ABB REF542plus Manual

1VTA100114-Rev. 04 en
Valid from Version V4D02
Multifunction Protection and Switchgear Control unit REF542plus
Manual: Motor Protection with ATEX-Certification

4 Motor protection functions

Overloading conditions cause impermissible temperature rises in motors and may re-
sult in premature fatigue and aging. If this type of condition persists, thermal destruc-
tion of the components cannot be excluded.
With regard to the protection functions, a distinction has to be made between over-
loads caused by starting processes and overloads occurring during operation. During
starting, both in the winding of the rotor and in the winding of the stator, currents may
be present that are well above regular on-load currents. A fast tripping must be gen-
erated in case a disturbance does occur during startup, for instance, in the event of a
rotor block or motor start under heavy load condition. In order to prevent overloading
conditions, also the number of starts needs to be limited.
While the motor is running, overloads may occur as a result of the working loads. For
monitoring such operating conditions, the temperature is calculated on the basis of a
thermal model.
For motor protection purposes, the REF542plus field control and protection device
therefore includes the following functions:
During the starting process
Blocking rotor protection
Motor start protection
Number of starts protection
During operation
Thermal overload protection
Unbalanced load protection or optionally a simple positive sequence monitoring
In all cases, the momentary motor temperature is crucial for the motor protection
functions. The REF542plus therefore uses a common thermal replica of the first order
for determining the motor temperature. Consequently, decisions can be taken by all
applied protection functions, depending on the momentary value of the calculated mo-
tor temperature.
The thermal replica referred to above is started immediately, as soon as the current
flows through the motor windings. Thus, the motor protection in the REF542plus is
designed as an thermal overload protection with a total memory function in accor-
dance with standard IEC 60255 – 8 [DIN1], since a previous loading condition is al-
ways taken into consideration and is being tracked by the thermal replica.
Unbalanced load protection is provided as a phase failure protection caused by an
open-circuit condition (broken phase connection). This type of protection enables
monitoring of the negative-sequence component which causes a temperature rise in
the laminated core of the rotor. Optionally, in case of small motors a simple positive
sequence monitoring function can be applied instead.
PTMV, 19.01.05 Motorprotection 10 / 66