Method of operation and control [t-head1-first]

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 seperate push-through sensors or a sensor belt. These are combined with an evaluation unit so that seperate 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, evaluats 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 behaviour; 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

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”.