electromechanical @ zerocross

[techteam]

Hi all,

Im working on Hybrid relay, which makes use of solid state relay and electromechanical relay.

I can drive my solid state relay @ zero cross using ZeroCross Optocoupler IL410. My question is
1) Can i drive EMR (electromechanical relay) @ zerocross?
2)If yes.Then, How to achieve this?

Thanks......

 

[bassnut]

No, for 50-60hz anyway, mechanical relays are far too slow to turn off right at zero cross. Use a bigger zero crossing SSR.

 

[techteam]

k....
In my project SSR will turn on initially @ zerocross for 1 second and then EMR tuns on. After which SSR will turn off and the load current will solely be carried by EMR.
Hence I wanted EMR to turn @ zerocross. However the SSR is ON only @ zerocross.
1) Can turning ON the EMR @ zerocross be possible using PIC microcontrollers?
Or
2) can it turn on @ zerocross by connecting a zerocross optocoupler in series with EMR?

Can this happen??
pl guide.....

Thanks..

 

[Nigel Goodwin]

Mechanical relays are slow devices, it's difficult to predict how long the relay will take to close - I'm also baffled as to why you want both a relay and an SCR?.

 

[bassnut]

Ahh, I see what your doing. Use an off the shelf zerocrossing SSR (solid state relay) switched on by the PIC at any time (timing not critical). Turning off the mechanical relay would not be zerocrossing though, unless you turned on the SSR 1st just before again, and then that might be tricky because there would be no voltage across the SSR for it to turn on (shorted by the SSR). It would turn on just as the relay was opening (voltage appearing across the contacts), which might work, and then zerocross turnoff with the SSR

 

[Leftyretro]

Mechanical relays are slow devices, it's difficult to predict how long the relay will take to close - I'm also baffled as to why you want both a relay and an SCR?.

I saw this concept at work at my refinery on large industrial UPS systems of 50kw or higher. The output of the UPS wired to a large AC power switching device called a static transfer switch.

It's purpose was to have an alternate AC feed ready for the load in case the UPS failed for some reason, or for starting up the UPS with an existing power-up load, or for transferring from UPS power to the alternate bypass power. The alternate bypass AC feed came around from the input AC power of the UPS, so it could allow the UPS system to be taken out of service for repairs or PMs without disturbing the critical loads being powered. As long as there was an active input AC source the UPS would always sync it's frequency to that input AC frequency.

Anyway the use of large thyristors devices allowed the UPS to sync up (zero crossing transfer) to the alternate power prior to transferring the load to the UPS output and then opening up the contactor and removing the alternate AC feed source, then after a few seconds large contactor type relays would close across the triacs. It was like a giant SPDT switch with the common connection being the load. There was energy and heat savings from using the hard contactor contacts Vs the thyristors devices, plus the contactors were considered a more reliable component then the many thyristors for continuous use.

So while it is somewhat complex and expensive, there is ample reason to use thyristor devices backed up by EMR contacts for high current applications.

Lefty