voltage drop on relay

G'day all

At present I am working on a medisafe fridge which stores perishable products for a pharmacy.

The fridge is a single glass door upright commercial fridge and is controlled by a lae controller.

The chemist recently had power problems which took out the controller and because it was the weekend he didnt find out until the following monday and arrived to find a fridge full of spoiled stock.

He wanted to be able to set the controller up to trigger the shop alarm when there was a high temp condition.I set up a 240vac relay with the alarm on the NO contacts.The lae controller supplies a constant 240vac via its auxillary relay to keep my alarm relay energised.In the event of an alarm or a power failure my alarm relay de energises and triggers the shop alarm.

The chemist (who is extremely worried about the crap power we have here) has purchased a UPS that the fridge is plugged into.

The problem is this.He keeps getting high temp alarms when we have small power outages, he then has to go into his shop in the middle of the night and turn the power off to the UPS to clear the high temp alarm.

I went in to check the fridge out today and found that if you turn the fridge off at the mains the UPS keeps the it running but the voltage to the alarm relay drops to 180vac after some time running this way the shop alarm is triggered if you then turn the mains back on to the fridge the voltage at the alarm relay returns to 240vac but the alarm still remains.To clear this alarm you then must make sure that the voltage to the alarm relay is completely removed (0 volts).(Aside from this this low voltage will not do the compressor much good.)

It seems as if the alarm relay jams when it has a period of lower than normal voltage and then will not return to normal until all voltage is removed.

Has anyone experienced such a problem?

Thanks in advance.:)

I don’t quite follow what’s happening with your controller so, if what I write is obvious and you’ve already checked, my apologies! This is one of the CDC122 range of controllers? If so, the alarm relay is shown as a normally open device in the LAE literature. It is not! It is a normally closed device and if all is OK then the relay is energised and the contacts are open. If the mains supply fails, the relay contacts are closed. The alarm signal will remain and the relay remain closed, even after the fault has cleared, until one of the keys in the front panel is pressed. Does this help?

Use a solid state relay with 100-240v ac t/formed power supply. This will hold on the 180v output of the UPS and only trigger when the power fails completly. On reconnection to mains @240v the relay will reset.

would'nt a thermostat work better?

Now I dont know how fast the temperature starts rising after the power is lost tho...

Marc, that's realy a good one
Temprite, hasn't its sometimes nothing to see with the core in the relay-coil which is hanging somewhere half the coil when on 180 V, fallen/pushed not completely down by the spring. If you then trigger it back to 240 VAC, then the core is somewhat in half of it's position and there isn't enough force to overcome the spring power therefore, it stay there in it's halfway position.
Just a guess