AS-Interface power supplies / earth fault monitors - System description

  • Compliance with EN 50081 (noise emis.) & EN 50082 (noise immunity).
  • Wide input voltage range.
  • Output protected against short circuits and overload.
  • Good power reserves.
  • Robust metal housing for secure mounting.

They may be unglamorous and unobtrusive, but without them it would not be possible to operate an electronic system. Power supplies are essential. ifm offers low-cost transformer power supplies but also powerful switched-mode power supplies for different applications.

Transformer power supplies
Transformer power supplies provide a low voltage, normally 24 V DC. A transformer according to DIN 0551 ensures a safe electrical separation from mains voltage and low voltage. The output voltage can be regulated (± 5 %) or smoothed by means of capacitors. The different designs and output powers allow adaptation to diverse operating conditions.

Switched-mode power supplies
Primary switched-mode power supplies are a compact and economical solution to supply sensors and actuators. As opposed to conventional transformer power supplies with regulated output voltage they need no heavy transformers so that there are fewer iron and copper losses. They are therefore distinguished by a very high degree of efficiency of up to 92 %. Due to the operating principle by means of high frequency transformers switched-mode power supplies are much smaller and lighter than transformer power supplies with identical power. Nevertheless they guarantee an electrical separation. Furthermore, they offer a wide input voltage range as standard for worldwide use.

Power reserves
Mains fluctuations up to ± 15 % and mains interference are compensated for. Even mains voltage dips of a few milliseconds are compensated for, the output voltage is completely maintained. An active inrush current limitation reduces the inrush current by means of a fixed resistor which is bridged after start up. The outputs are protected against short circuits and overload. Special output characteristics allow a current which can be up to 1.7 higher than the nominal current without switch-off with the voltage being reduced at the same time. The dimensioning of the components allows a 20 to 25 % higher output current for a short time. This power reserve is provided by all power supplies as from 2.5 A for a period of one minute. At an operating temperature of up to 45 °C this power is available continuously.

Earth fault monitor · Introduction
Within the framework of unintentional start-up of a machine according to EN 60204 a hazardous condition such as unintentional starting or failure to stop the machine caused by earth faults or incorrect input signals is to be safely prevented. Without any protective measure this hazardous state can arise if two earth faults occur in the control circuit at the same time.

Another possibility indicated in the standard is an insulation monitoring device (IM) which either indicates an earth fault or automatically switches off the control circuit following an earth fault.

Without earth fault / insulation monitoring device (IM) it would be impossible to switch off a running engine by the EMERGENCY STOP function for example in the event of an earth fault on positions 1 and 2.

The earth fault / insulation monitoring device which can be installed in the existing AS-i system without any problems is available from ifm in two designs:
  • The earth fault monitor uses a passive measuring procedure and measures the displacement voltage between phase conductors and plant ground (earth).
    The occurrence of an asymmetrical earth fault is directly signalled on the unit by means of the permanently lit yellow LED. A potential-free signalling contact is available for further processing. The response time for detecting an asymmetrical earth fault is 5ms.
  • The insulation monitoring device detects symmetrical as well as asymmetrical earth faults or insulation faults by means of an active measuring method. A flashing yellow LED signals symmetrical, a permanently lit LED asymmetrical faults. The evaluation "earth fault / insulation fault present" can be executed via two volt-free signal contacts.