The dual pole switching of solenoid valves in the hydraulic systems of a Thyssen hot rolling mill presented a particular problem in the specification of power controllers that would meet machinery directive requirements. Since the loads are inductive, mechanical contacts experience a high contact wear. CEGELEC, (now Alstom), in Germany set E-T-A the task to develop an electronically controlled switching device in three months to satisfy the directive demands.
E-T-A’s solution was a double pole electronic power controller featuring numerous protective and control functions. It has been designed so as to meet the standard requirements for solenoids, magnetic brakes and other actuators using unearthed 24V DC supplies. The E-1072 solid state power controller, designed for 24V DC up to 3A, makes selective faultfinding possible, and corrective action can be taken in unearthed supplies without interrupting the production process.
Besides its main function – double pole switching – the E-1072 has many characteristics ideal for modern control and switching programmes used in process control. The very low internal power consumption of the unit, typically 24mA, as well as a maximum power loss of 3.5W during full load operation, means an effective limitation of additional heat generation in a control cabinet to the absolute minimum. The unit is also protected against reverse polarity of the input.
The load output, equipped with a suppression diode, is fitted with two power MOSFETs (with a very low on-resistance) and features further protective and monitoring circuits which are evaluated by a central internal electronic circuitry. Beside the inrush and short circuit current limitation, a very close meshed, overload-dependent trip curve is implemented, commencing typically 1.15 times rated current. This means that the unit can switch capacitive loads as well as the inductive loads.
The time characteristic of tripping under fault condition depends on the size of the overcurrent. The maximum short circuit current is limited so that the device is not damaged. In the event of a low-resistive short circuit, electronic tripping occurs after 50ms. After another 800ms there is a dual electromechanical disconnection of load via integral relays.
Besides wire breakage monitoring of the load circuit, the unit also features additional control of both power transistors and of the load output potential. In the event of a ground fault in the load circuit in the off-load position, the unit recognises the deviation from normal and the failure is signalled with an LED. Thus the main function of the unit – two pole switching of load – is permanently monitored.
Basically all indicated failures lead to a disconnection of load and they are electrically signalled via the auxiliary make contacts. Together with four combinations of LED indicators and a trip actuator, these offer early fault-signalling and faultfinding in systems.
E-T-A Circuit Breakers Tel: 01296 420336