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Method of operation and control
Like overload relays operating on the bimetallic strip principle, electronic motor-protective
relays are current-dependent protective devices.
The acquisition of the actual flowing motor current in the three external
conductors of the motor connections is with motor protection system ZEV with separate push-through sensors or
a sensor belt. These are combined with an evaluation unit so that separate arrangement of
the current sensor and the evaluation unit is possible.
The current sensor is based on the Rogowski principle from the measurement technology. . The sensor belt
has no iron core, unlike a current transformer, therefore it doesn't become saturated and
can measure a very wide current range.
Due to this inductive current detection, the conductor cross-sections used in the load circuit have no
influence on the tripping accuracy. With electronic motor-protective relays, it is possible to set higher current
ranges than is possible with electromechanical thermal overload relays. In the ZEV System, the entire protected
range from 1 to 820 A is covered using only an evaluator .
The ZEV electronic motor-protective system carries out motor protection both by means of
indirect temperature measurement via the current and also by means of direct temperature
measurement in motors with thermistors.
Indirectly, the motor is monitored for overload, phase failure and unbalanced
current consumption.
With direct measurement, the temperature in the motor winding is detected by means of one or more PTC
thermistors. In the event of excessive temperature rise, the signal is passed to the tripping
unit and the auxiliary contacts are actuated. A reset is not possible until the thermistors
cool to less than the response temperature. The built-in thermistor connection allows the
relay to be used as complete motor protection.
In addition, the relay protects the motor against earth faults. Small currents flow out even in the
event of minor damage to the motor winding insulation. These earth faults currents are
registered on an external summation current transformer which adds together the currents
in the phases, evaluates them and reports earth-fault currents to the microprocessor in the
relay.
By selecting one of the eight tripping classes (CLASS) allows the motor to be protected to be adapted
from normal to extended starting conditions. This allows the thermal reserves of the motor to
be used safely.
The motor-protective relay is supplied with an auxiliary voltage. The evaluator has a
multi-voltage version, which enables all voltages between 24 V and 240 V AC or DC to be
applied as supply voltage. The devices have monostable behavior; they trip out as soon
as the supply voltage fails.
In addition to the usual normally closed contact (95-96) and the normally open contact (97-98) for
overload relays the motor protection relay ZEV is equipped with a programmable normally
open contact (07-08) and a programmable normally closed contact (05-06). The above mentioned, usual contacts react
directly via thermistors or indirectly via the current, to the detected temperature rise of
the motor, including phase-failure sensitivity.
The programmable contacts can be assigned to various signals, such as
- Earth-fault
- Pre-warning at 105 % thermal overload,
- separate indication of thermistor tripping
- internal device fault
The function assignment is menu-guided using a display. The motor current is
entered without tools using the keypad, and can be clearly
verified on the display.
In addition the display allows a differential diagnosis of tripping causes,
and therefore a faster error handling is possible.
Tripping in the event of a three-pole balanced overload at x-times the set
current takes place within the time specified by the tripping class. The tripping delay in
comparison with the cold state is reduced as a function of the preloading of the motor. Very
good tripping accuracy is achieved and the tripping delays are constant over the entire
setting range.
If the motor current imbalance exceeds 50 %, the relay trips after 2.5 s.
The accredition exists for overload protection of explosion proof motors of the
explosion protection “increased safety” EEx e to guideline 94/9/EG as well as the report of
the German Physical/Technical Bureaux (PTB report ) (EG-Prototype test certificate number PTB 01 ATEX 3233).
Further information can be found in the manual AWB2300-1433D “Motor protection system ZEV,
overload monitoring of motors in EEx e areas”.
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