overload relays

Discussion in 'Homework Help' started by gjo, May 22, 2013.

  1. gjo

    Thread Starter New Member

    May 22, 2013
    I was reading basic electricity by valkenburgh and on page 5_68, he talks about overload srelay. The part i dont understand is when he says " in many circuits you will see overload relay contacts in series with the line. These contacts are normally closed, but of the temperature of the device becomes excessive, the thermal relay will close causing the overload relay to become energized and opening the line." The part i dont understand is" the thermal relay will close, causing the overload relay to become energized and opening. The line." It seems as if the thermal relay and overload relay are two separate relays, with the thermal relay closing to provide a switch to activate the elecyromechanical relay to open the overload contacts.
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    He will be talking about a Circuit breaker, there are several types and ratings depending on your Country, here is a picture of the inside of one.. Breaker

    The relay is item 7, and contacts item 3, the relay is in series and is Current activated which trips the lever item 1 and disconnects the contacts item 3.
    Last edited: May 23, 2013
  3. circuitfella11


    May 10, 2013

    maybe "thermal relay" means the instance where the relay is at "this" point in time (thermal or hot) since excessive temperature is said. then goes to the next point in time, "overload relay" where the components melt due to high temperature and produces an open..

    this is as far as i understood it..^_^

  4. panic mode

    Senior Member

    Oct 10, 2011
    no, there is no melting involved, think bi-metal. melting is irreversible, while bimetal returns to initial position after cooling off.

    circuit breakers can open by both magnetic and thermal trip. cheap products that do not have both tripping mechanisms do no qualify as circuit breakers and can be only used as a supplementary protection.

    overload relays work like circuit breakers but have different physical form and electrical characteristic (circuit breakers are designed to trip as fast as possible when overload condition is encountered, overload relays trip after overload condition lasted for specific time, this delay and sensitivity are often user adjustable). usually overloads are designed to mount directly (piggyback) onto contactor (contactors are type of large relay). another difference is type of reset (overloads can have manual, auto or both).

    the wires sticking out on the top side go into terminals of contactor of proper size:


    here is an assembled version (contactor + overload). overload is the part on the bottom: