AC relays are used as:
1. Auxiliary relays for performing auxiliary logic functions with certain number of normally closed and normally open contacts, (fan-out functions)
2. Control relays that can control some small load or variable, e.g. small fan.
3. AC protective relays:
3a. AC undervoltage or overvoltage protection
3b. AC undercurrent or overcurrent protection
3c. Etc. many others such as AC distance relays
There are essentially electromagnetic relays working on electromagnetic force principles and solid state relays working on electronic switching device principles.
Web sites may have some tutorials available under "relay", "electromagnetic relay" or "solid state relay." [sig][/sig]
I guess you are aware of the general operation of relays.
AC driven relays work almost under the same principle as dc relays, the main difference being the ratings. The main differences are:
1. Arcing on break of the relay coil supply- this is much less in AC driven relays due to the zero-crossing of the current in every half cycle
2. Heat/power dissipation- lesser, since the RMS current is lesser.
Consequently, AC relays find a lot of use in relay logic implementations, where you already have a control voltage available. It is usually easier to go for AC relay logic.
If you go for DC, then you need to have a transformer/rectifier/filter combination to feed the control. Also, if you use DC relay logic, the control relays also should be rated for DC (I mean, the contacts of the relays). [sig]<p>Narayanan UM<br><a href=mailto:umn@ieee.org>umn@ieee.org</a><br><a href= > </a><br> [/sig]
I found out elsewhere that a single turn of non ferrous magnetic material such as copper is fitted across the coil.
This holds its magnetism for a short time thus keeping the relay closed between cycles [sig][/sig]
