Operation of Electromagnetic Relay.

[swapan]

Hi friends,

I am trying to make an automatic AC voltage stabiliser using PIC MCU. It is desirable to activate/deactivate the relays at voltage zero crossing/current zero crossing. In case of resistive load, operation of relays at voltage zero crossing will be ok since power factor of resistive load is unity. But in case of inductive load (also the auto transformer itself is an inductive load), current will lag behind voltage. In that case how the current zero crossing could be detected so that relays could be operated at zero current? As a crude way, is it wise to operate the relays using a short delay (say 2 ms) after voltage zero crossing?

Please offer your valuable comment on the issue.

swapan

 

[cl10greg]

Does it have to be a relay? You can do this easily with a Triac and an optocoupler? Triacs have built in zero crossing capabiilties and they have optocouplers made to go from microcontrollers to the Triac. In most case the Triac is able to do everything a relay does without mechanical wear (but depends on what safety requirements are).

If not, you could do some op-amp trickery to do such a comparison. I am not great with op-amps but I recall seeing and reading abound some circuits that are modified for current detection rather than voltage detection. After the conversion it should just be setting the reference close to 0 to enable and disable the output. The delay could work but it wouldn't be the most reliable way and your introducing timing errors into your circuit.

 

[swapan]

Thanks cl10greg for your comment. Actually it is not the compulsion to use relay only. I have come across circuits using TRIAC together with optocoupler for controlling AC. Being an electronics hobbyist I like to know the phenomenon of using electromagnetic relay with lesser problem while switching. I am trying to grasp the idea to detect current. Once current is detected, it will be easy to derive zero crossing pulse using op-amp. I am thinking how to detect current - using a shunt resistor or a current transformer?

 

[cl10greg]

Well to detect current you can do a bunch of different techniques that include shunting, hall effect, and comparators. To use a shunt resistor you are basically doing a difference measurement from the one side of the shunt to the other and do calculations based on the known resistance. A hall effect sensors can be either stand alone or in an IC. Basically you put the sensor right next to the current conduction path and it will translate the magnetic field to a voltage output that is correlated to the current level. There are plenty of examples that a search will yield. Then there are other routes that have built in current capability in relays, FETs, transformers, comparators, dedicated ICs. You can pretty much go to digikey and put in current sensor and it will give a bunch of devices.

 

[FvM]

Do you have an idea of the expectable switching delay and delay skew of mechanical contactors?

Review data sheets and reconsider the idea of switching electromechanical relays synchronous to zero crossing.