1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

How is an A.C. Relay works ?!!

Discussion in 'General Electronics Chat' started by aljamri, Sep 21, 2006.

  1. aljamri

    aljamri Member

    Joined:
    Jun 12, 2006
    Messages:
    694
    Likes:
    6
    I know how DC relay works using a DC to create a temporary magnet that will attract and hold the contact.

    But what is the concept used in AC relay ? and why do we need the free-wheeling-diode in AC? :confused: !
     
  2. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    6,768
    Likes:
    72
    All relays work the same way. Current flows through the relay coils to produce an electromagnet which pulls the spring-loaded contact (which can either be open or closed in the unpowered state).or closed. The only difference between AC and DC is the type of current used to magnetize the coil.

    Where are you getting your information? Are you saying you need a diode across the AC relay coils but not DC relay coils? You need them for DC relays because the coils are inductive and when you de-energize them (stop the current from flowing through them) it creates damaging voltage spikes. The diode is used to supress this voltage spike.

    I don't think using a diode to supress the voltage spike works for AC relays because the current flow changes every half cycle causing the diode to become a short-circuit every half cycle. I don't know how it's done for AC relays. It's probably some other type of snubbing.
     
    Last edited: Sep 21, 2006
  3. Roff

    Roff Well-Known Member

    Joined:
    May 16, 2003
    Messages:
    7,757
    Likes:
    89
    Location:
    Idaho, USA
    An AC-powered relay has what is known as a "shading coil" (Google), which is a shorted turn on the relay's solenoid. A current is induced in this shorted turn which is 90 degrees out of phase with the main coil's current, and provides just enough magnetic field to keep the armature engaged during the zero crossing of the current in the main coil.
    I don't understand how the current is shifted 90 degrees. Perhaps someone else can explain this.
     
    • Like Like x 2
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    6,768
    Likes:
    72

    What is a shorted turn? You mean just a loop of wire? ?? ??

    An inductor's transfer function is jwc, so wouldn't that account for the 90 degree phase shift?

    V=IZ, Z = jwL

    So that would mean that the voltage across an inductor leads the current in the inductor by 90 degrees in the complex plane.
     
  6. Roff

    Roff Well-Known Member

    Joined:
    May 16, 2003
    Messages:
    7,757
    Likes:
    89
    Location:
    Idaho, USA
    Yes, it's a single heavy loop of wire.
    What you say is true, but, since the main coil is also an inductor, the current in it also lags the voltage by about 90 degrees. Remember that the magnetic flux is in phase with the current, not the voltage.
    It would seem that the shorted turn would be the secondary of a transformer (the main coil being the primary), and the current in both primary and secondary would be in phase.
     
  7. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    6,768
    Likes:
    72
    I found this:
    I see...so the shaded coil is ACTUALLY a loop of wire (normally a loop of wire does not meet end to end to close itself because it has + an - terminals instead). I was getting confused about the previous description of a shaded loop being a short-circuit (I was imagining the loop as completing a short-circuit whicih didn't make sense).
     
    Last edited: Sep 21, 2006
  8. Hero999

    Hero999 Banned

    Joined:
    Apr 6, 2006
    Messages:
    14,902
    Likes:
    79
    Location:
    England
    Suppression of an AC relay is normally done using a snubber network consistin of a capacitor in series with a resistor in parallel with the relay coil.
     
  9. eblc1388

    eblc1388 Active Member

    Joined:
    Jan 25, 2005
    Messages:
    2,228
    Likes:
    18
    Location:
    UK
    Now you are confused. :D

    You've imaged correctly. It is physically a completed loop joined end to end to form a perfect short-circuited arrangement. It make perfect sense.
     
  10. hawk2eye

    hawk2eye New Member

    Joined:
    Mar 14, 2005
    Messages:
    37
    Likes:
    0
    ................................................................................
    Ron H.
    relay coil AC current 90_degrees, wrt Line Voltage which produces the short circuit current

    -AC Line Voltage generates current I1 through the relay inductance, L
    I1 = Vac/wL at 90_degrees relative to Voltage

    -AC Line Voltage causes magnetic field to operate relay

    -AC Line Voltage through ' transformer", coil inductance as primary
    sc_turn as secondary, causes current I2 through sc_turn
    0_degrees relative to Voltage


    hawk2eye
     
  11. Roff

    Roff Well-Known Member

    Joined:
    May 16, 2003
    Messages:
    7,757
    Likes:
    89
    Location:
    Idaho, USA
    But the magnetic field in the relay's solenoid is in phase with the current in the "primary". Since the shorted turn is in that same field , won't the current in the shorted turn be in phase with the primary?
     
  12. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    6,768
    Likes:
    72
    What I meant as I imagined two coils in parallel inside the relay, with the shaded coil being so short as to basically form a short-circuit across the primary coil.
     
  13. zevon8

    zevon8 New Member

    Joined:
    Apr 10, 2004
    Messages:
    917
    Likes:
    2
    Location:
    Toronto, Canada, eh
    Last edited: Sep 22, 2006
  14. aljamri

    aljamri Member

    Joined:
    Jun 12, 2006
    Messages:
    694
    Likes:
    6
    Thanks for all.

    My next question is when to call it Relay and when to call it Contactor.

    Thanks
     
  15. zevon8

    zevon8 New Member

    Joined:
    Apr 10, 2004
    Messages:
    917
    Likes:
    2
    Location:
    Toronto, Canada, eh
    Generally, and this is not a perfect rule, relays are "low power" and contactors are "high power" devices. Mostly contactors are what you expect in industrial settings for operating motors, large power power loads, etc. Relays are generally everything else lower power. Often contactors have more robust mechanical designs that ensure solid and vigorus closing of the contacts, to help prevent arcing or welding of the contacts.

    It can just be a choice of words. I have used mercury wetted 3 phase devices capable of several hundred amps that were called a "relays" by the manufacturer.
     
  16. Brian Park

    Brian Park New Member

    Joined:
    Jan 18, 2010
    Messages:
    2
    Likes:
    0
    Location:
    Alexandria, Va
    The magnetic layout is slightly more complicated than a "simple" transformer (with a single magnetic path linking 2 coils). The magnetic iron from the main coil is divided into 2 parts, one goes straight to the armature, the other has to pass through the short-circuited single-turn shading coil. Because of this, the shading coil has "leakage inductance" (formed by the magnetic flux path linking the shading coil, but not the main coil.) this inductance causes the current in the shading coil to lag the current in the main coil.

    (If this lag didn't happen, the effect would be to just cancel a part of the flux linking the shading coil, causing a simple weakening or a "shading" of the flux, as described in your question.)

    The shading current, being 180 degrees out of phase, retarded more by the leakage inductance, leads to the "imperfect cancellation". The flux in the shaded part of the core is weaker and delayed from the main flux. The cross-section of the shading coil has to "be right"; fat coil, too little resistance, lots of delay, but the delayed flux then would be weak. Thin coil, lots of delayed flux, but not much delay.

    Think of an RC circuit excited by an AC source. If C and R are both large (so the time constant is much greater than the AC half-period) then nearly 90 deg delay, but almost no voltage on the cap. If C and R are small (time constant much less than AC half-period) then big voltage on C, almost in phase with applied voltage.

    So in effect we have 2 separate relay coils and magnetic-path iron, each excited by AC of different phases, acting on a common armature, both trying to attract it with 120Hz "vibratory forces" (for a 60Hz relay). The armature "sees" the sum of these forces, which always add, never subtract (due to the square-law rule of magnetic attraction of iron to magnetic flux). The phase shift guarantees that when one is zero, the other is positive. So the holding force, while being still vibratory, never goes below some value. (In an ideal world, the phase shift would be 90 degrees, and the 2 fluxes would be equal in magnitude, giving a perfect vibration-free attraction). But such perfection is rarely achieved, and is not needed, since once the armature contacts the core face, only a small fraction of the pull-in current ("holding current") is needed to prevent the relay from releasing during the zero-crossings of the current in the main coil. This is why the cross-section of the shading coil can be a small fraction of that of the main coil.
     
  17. kinarfi

    kinarfi Well-Known Member

    Joined:
    Jul 7, 2009
    Messages:
    1,514
    Likes:
    30
    Location:
    P. G., Utah, United States of America
    The purpose of the shaded pole is to create a hysteresis loop so the magnetic field switches so fast that the magnetic armature does not have a chance to open before it's pulled back in. If the shorted ring opens up, the relay or contactor will buzz like hair clippers that are adjusted too tight. If you look at the face of the pole piece, you'll see that only part of the armature has the shorted turn on.
    Kinarfi
     

    Attached Files:

    Last edited: Jan 18, 2010
  18. Brian Park

    Brian Park New Member

    Joined:
    Jan 18, 2010
    Messages:
    2
    Likes:
    0
    Location:
    Alexandria, Va
    Hysteresis is not necessary and is undesirable in a relay

    The shaded pole does not create, nor require, a hysteresis loop to function. The phenomenon involves linear circuit theory only. It is an RL circuit. In fact, the iron is doped with silicon, and carefully heat treated to minimize the amount of hysteresis (too much could cause the relay to magnetize and "stick"). If the magnetic field switched "suddenly", a huge voltage spike would be produced in the coils. Spikes can not occur while the relay is energized.

    If the shading coil opens up, you have no shading coil anymore, and as you indicate, the relay will buzz.
     
  19. aljamri

    aljamri Member

    Joined:
    Jun 12, 2006
    Messages:
    694
    Likes:
    6
    That is exactly what I'm asking about. The concept of DC coil, as a magnet is very clear to me, it is simply fixed North and South poles , but AC magnet ?

    Now the idea is a bit clearer to me.

    Thanks for all
     

Share This Page